Simple Valve Amp
Ash Small, Mon Nov 25 2013, 07:43PMPost updated 14th Feb 16.
I've decided I probably have most of the stuff I need to build a valve amp along the lines of the 'Mighty Atom'. It will initially be mono and will be based upon this circuit diagram:
It could be powered from a transformer and bridge rectifier, with a suitable smoothing cap, but I'm planning on using a rectifier tube, etc.
The Mighty Atom is a class A single-ended amplifier, about as simple as it gets.
Suitable valves for V2, the output valve, are KT88, KT66, NT77, EL34, CV4060 AND EL37. Others could also be used, eg 805, but I'm using an 807 beam tetrode connected as a triode because I have half a dozen of them, and I may decide to add more in parallel at a later date. (I was considering valves like the 12E1, but I've mislaid it.)
Chris, who designed the basic circuit 20 years ago, says that by far the best valve to use for V1, the input stage, is a UCC82.
Two Varley 20H 120mA chokes and a Williamson push/pull output transformer.
POWER SUPPLY
(Not sure where to put fuses at the moment, see thread on page two. 23-3-16)
I'll be using a GZ37 rectifier for the power supply because I have one.
GZ37 Full Wave Rectifier
]gz37.pdf[/file]
I found what looks like a suitable transformer in the shed, Parmeko 5080/24A. I can't find any reference on the internet.
I've started plotting the terminal pins from the notes I took when removing it.
Ok, here goes, Pins (1-2) (3-4) (5-6) and (7-8) are the primaries. (5-6) and (7-8) are in parallel for 110V, and in series for 220V. Resistance is ~4 Ohm, if I remember correctly.
For 240V, (1-2) and (3-4) are also in series with (5-6) and (7-8). Resistance is ~ 0.5 Ohm. Does this make sense/fit with convention?
Pins (9-11) are the 'ends' of the centre-tapped secondary, and go to the anodes on the rectifier. Pin 10 is presumably the centre tap of the secondary. ((9-11) =106 Ohms, (9-10) =52 Ohms, (10-11) =56 ohms).
Pin 10 is shorted to Pins 20, 25, 17 and 23.
Pins (12-13) are 5V heater supply. This will do for the rectifier tube. (0.6 Ohm)
Pins (14-15) are 6.3V heater supply. This will do for the output tube. (0.3-0.4 Ohm (dmm not very accurate plus corrosion on terminals, etc))
Pins (16-18) are 6.3V heater supply. Pin 17 is a centre tap.
Pins (19-21) are 12.6 V heater supply, Pin 20 is a centre-tap.
Pins (22-24) are, I think, two 'ends' of another centre-tapped secondary, but it looks like they each powered something independantly. resistance is ~1.2 Ohms, and ~ 0.6 Ohms between them and the centre tap.
As I said previously, Pins 10, 17, 20, 23, 25 and 'SC' are all shorted together.
10H, 180mA Parmeko choke for the power supply.
Capacitors are motor run caps, I think. 2x8uF and 2x40uF in parallel to get the 16uF and 80uF required.
Re: Simple Valve Amp
Shrad, Mon Nov 25 2013, 10:23PM
beam tetrodes are said to be really nice output tubes, and to have their own sound...
Shrad, Mon Nov 25 2013, 10:23PM
beam tetrodes are said to be really nice output tubes, and to have their own sound...
Re: Simple Valve Amp
Proud Mary, Tue Nov 26 2013, 02:17PM
Mr Ash! The 6BS7 is a super little high gain pentode, robust and well designed with in-built screening, and good defences against microphony and heater hum.
12E1 is not a good choice for your circuit - not a good choice for any circuit really, except as a shunt regulator, where it works like the overflow in a bathtub, holding the level of the water at a constant height by pouring away excess water as it flows in from the taps, a costly exercise.
Proud Mary, Tue Nov 26 2013, 02:17PM
Mr Ash! The 6BS7 is a super little high gain pentode, robust and well designed with in-built screening, and good defences against microphony and heater hum.
12E1 is not a good choice for your circuit - not a good choice for any circuit really, except as a shunt regulator, where it works like the overflow in a bathtub, holding the level of the water at a constant height by pouring away excess water as it flows in from the taps, a costly exercise.
Re: Simple Valve Amp
Ash Small, Tue Nov 26 2013, 03:10PM
I recieved this reply elsewhere regarding the 12E1, PM:
"It is a series voltage regulator valve with a high output current so will double as an output valve very well
Very often power output bottles are used as Regulators
and vice versa"
I've been doing a bit of googling, and the KT88 (along with some of the other KT series) seem to be fairly highly regarded. I may try the 12E1, but maybe I'll see what else I have. I think if I decide to buy one, I'll probably look for a KT88, as it seems to be highly thought of.
Ash Small, Tue Nov 26 2013, 03:10PM
Proud Mary wrote ...
Mr Ash! The 6BS7 is a super little high gain pentode, robust and well designed with in-built screening, and good defences against microphony and heater hum.
12E1 is not a good choice for your circuit - not a good choice for any circuit really, except as a shunt regulator, where it works like the overflow in a bathtub, holding the level of the water at a constant height by pouring away excess water as it flows in from the taps, a costly exercise.
Mr Ash! The 6BS7 is a super little high gain pentode, robust and well designed with in-built screening, and good defences against microphony and heater hum.
12E1 is not a good choice for your circuit - not a good choice for any circuit really, except as a shunt regulator, where it works like the overflow in a bathtub, holding the level of the water at a constant height by pouring away excess water as it flows in from the taps, a costly exercise.
I recieved this reply elsewhere regarding the 12E1, PM:
"It is a series voltage regulator valve with a high output current so will double as an output valve very well
Very often power output bottles are used as Regulators
and vice versa"
I've been doing a bit of googling, and the KT88 (along with some of the other KT series) seem to be fairly highly regarded. I may try the 12E1, but maybe I'll see what else I have. I think if I decide to buy one, I'll probably look for a KT88, as it seems to be highly thought of.
Re: Simple Valve Amp
Proud Mary, Tue Nov 26 2013, 08:36PM
It's true that valves like 807 were often used in series regulators by radio amateurs, but if 12E1 was as interchangeable with power amplifier applications as your information suggests, why would the manufacturer have produced a valve dedicated to regulator use?
Proud Mary, Tue Nov 26 2013, 08:36PM
Ash Small wrote ...
I recieved this reply elsewhere regarding the 12E1, PM:
"It is a series voltage regulator valve with a high output current so will double as an output valve very well
Very often power output bottles are used as Regulators
and vice versa"
I've been doing a bit of googling, and the KT88 (along with some of the other KT series) seem to be fairly highly regarded. I may try the 12E1, but maybe I'll see what else I have. I think if I decide to buy one, I'll probably look for a KT88, as it seems to be highly thought of.
Proud Mary wrote ...
Mr Ash! The 6BS7 is a super little high gain pentode, robust and well designed with in-built screening, and good defences against microphony and heater hum.
12E1 is not a good choice for your circuit - not a good choice for any circuit really, except as a shunt regulator, where it works like the overflow in a bathtub, holding the level of the water at a constant height by pouring away excess water as it flows in from the taps, a costly exercise.
Mr Ash! The 6BS7 is a super little high gain pentode, robust and well designed with in-built screening, and good defences against microphony and heater hum.
12E1 is not a good choice for your circuit - not a good choice for any circuit really, except as a shunt regulator, where it works like the overflow in a bathtub, holding the level of the water at a constant height by pouring away excess water as it flows in from the taps, a costly exercise.
I recieved this reply elsewhere regarding the 12E1, PM:
"It is a series voltage regulator valve with a high output current so will double as an output valve very well
Very often power output bottles are used as Regulators
and vice versa"
I've been doing a bit of googling, and the KT88 (along with some of the other KT series) seem to be fairly highly regarded. I may try the 12E1, but maybe I'll see what else I have. I think if I decide to buy one, I'll probably look for a KT88, as it seems to be highly thought of.
It's true that valves like 807 were often used in series regulators by radio amateurs, but if 12E1 was as interchangeable with power amplifier applications as your information suggests, why would the manufacturer have produced a valve dedicated to regulator use?
Re: Simple Valve Amp
Ash Small, Tue Nov 26 2013, 09:34PM
There is an article here:
which states the following regarding replacing a KT88 with a 12E1:
"1. PIN 3 to be connected to top cap of valve 12E1. Keep lead as short as practicable. Use good quality, well insulated flex with a good high temperature stability. Use an insulated top cap as voltages in excess of 450V DC are common on this application. It is suggested that a small hole 5mm is drilled adjacent to PIN 3 and an appropriate grommet inserted to cover bare metal edges. Wire from PIN 3 to top cap passes through this hole and is soldered to PIN 3.
2. Due to higher gain of 12E1 it will be necessary to increase bias to valve. To achieve similar standing current, bias will need to be about -40V. An alternative solution is to reduce anode voltage by about 50V. This is achieved by adding a decoupling R/C network in the HT feed to the output transformer.
3. When set up correctly the 12E1 gives good base drive and excellent transient response. As the valve has lower output impedance than a KT88, it’s dynamic performance will be a noticeable improvement in the adjusted circuit.
For further information, please refer to KT88 and 12E1 data sheets available on this website."
Maybe the 12E1 was even more expensive than the KT88? I've no idea, but the article looks interesting.
Ash Small, Tue Nov 26 2013, 09:34PM
Proud Mary wrote ...
It's true that valves like 807 were often used in series regulators by radio amateurs, but if 12E1 was as interchangeable with power amplifier applications as your information suggests, why would the manufacturer have produced a valve dedicated to regulator use?
It's true that valves like 807 were often used in series regulators by radio amateurs, but if 12E1 was as interchangeable with power amplifier applications as your information suggests, why would the manufacturer have produced a valve dedicated to regulator use?
There is an article here:
which states the following regarding replacing a KT88 with a 12E1:"1. PIN 3 to be connected to top cap of valve 12E1. Keep lead as short as practicable. Use good quality, well insulated flex with a good high temperature stability. Use an insulated top cap as voltages in excess of 450V DC are common on this application. It is suggested that a small hole 5mm is drilled adjacent to PIN 3 and an appropriate grommet inserted to cover bare metal edges. Wire from PIN 3 to top cap passes through this hole and is soldered to PIN 3.
2. Due to higher gain of 12E1 it will be necessary to increase bias to valve. To achieve similar standing current, bias will need to be about -40V. An alternative solution is to reduce anode voltage by about 50V. This is achieved by adding a decoupling R/C network in the HT feed to the output transformer.
3. When set up correctly the 12E1 gives good base drive and excellent transient response. As the valve has lower output impedance than a KT88, it’s dynamic performance will be a noticeable improvement in the adjusted circuit.
For further information, please refer to KT88 and 12E1 data sheets available on this website."
Maybe the 12E1 was even more expensive than the KT88? I've no idea, but the article looks interesting.
Re: Simple Valve Amp
Shrad, Tue Nov 26 2013, 10:00PM
same with the 6080 which was intended as a shunt regulator but has proven its wide success in audio amplifiers... I still have the components for a dual 4 tube setup, for 40W per amp if I recall good enough
Shrad, Tue Nov 26 2013, 10:00PM
same with the 6080 which was intended as a shunt regulator but has proven its wide success in audio amplifiers... I still have the components for a dual 4 tube setup, for 40W per amp if I recall good enough
Re: Simple Valve Amp
Proud Mary, Tue Nov 26 2013, 10:59PM
I think perhaps my ideas about valves are too conservative, and belong in the valve era, so I have very little experience of trying to make valves do things for which they weren't originally designed. But best of luck with it all!
Proud Mary, Tue Nov 26 2013, 10:59PM
I think perhaps my ideas about valves are too conservative, and belong in the valve era, so I have very little experience of trying to make valves do things for which they weren't originally designed. But best of luck with it all!
Re: Simple Valve Amp
Ash Small, Tue Nov 26 2013, 11:46PM
Well, I do like to 'over engineer' things. I imagine it won't be particularly efficient, but I do have a 250mA rectifier tube, and class A amps are never that efficient anyway.
I have been researching power valves a bit as I'm looking for one to regulate a magnetron (5-10kV, <1kW). I like the anode cap on the 12E1 as I could fit a heatsink to it. Not sure how effective it would be, and I doubt I'll ever get this valve very hot, without blowing speakers, etc.
It's all about using what's available.
Ash Small, Tue Nov 26 2013, 11:46PM
Proud Mary wrote ...
I think perhaps my ideas about valves are too conservative, and belong in the valve era, so I have very little experience of trying to make valves do things for which they weren't originally designed. But best of luck with it all!
I think perhaps my ideas about valves are too conservative, and belong in the valve era, so I have very little experience of trying to make valves do things for which they weren't originally designed. But best of luck with it all!
Well, I do like to 'over engineer' things. I imagine it won't be particularly efficient, but I do have a 250mA rectifier tube, and class A amps are never that efficient anyway.
I have been researching power valves a bit as I'm looking for one to regulate a magnetron (5-10kV, <1kW). I like the anode cap on the 12E1 as I could fit a heatsink to it. Not sure how effective it would be, and I doubt I'll ever get this valve very hot, without blowing speakers, etc.
It's all about using what's available.
Re: Simple Valve Amp
Shrad, Fri Nov 29 2013, 10:59AM
you could use something like a FAN7530 to get a stable B+ instead of a valve rectifier and oversized inductions...
there is always the debate of using or not using the old school filtering for B+ supplies, but sincerely, why woud we still do this while we have modern solutions? there are some ways to mix designs and get the best of both worlds
Shrad, Fri Nov 29 2013, 10:59AM
you could use something like a FAN7530 to get a stable B+ instead of a valve rectifier and oversized inductions...
there is always the debate of using or not using the old school filtering for B+ supplies, but sincerely, why woud we still do this while we have modern solutions? there are some ways to mix designs and get the best of both worlds
Re: Simple Valve Amp
Ash Small, Fri Nov 29 2013, 12:07PM
Yes, I have been considering other 'solutions' as far as power supply goes.
Part of the attraction, for me, is to only use valves, with no silicon, but there are plenty of other solutions. I'm not familiar with the FAN7530 myself, but I had considered some sort of SMPS.
For me, building a power supply with no resistors seems fun, and I'm doing a lot of experiments with inductors at the moment, but the main point here is to use what's available.
I have the big rectifier tube, and a couple of smaller ones which should also be up to the job, so it makes sense to me to use them for a valve amp, in the same way I'm using valves I found in the shed.
I also have a 'big' inductor in the shed. I've not dug it out and measured the inductance yet, but 'watch this space'
Ash Small, Fri Nov 29 2013, 12:07PM
Shrad wrote ...
you could use something like a FAN7530 to get a stable B+ instead of a valve rectifier and oversized inductions...
there is always the debate of using or not using the old school filtering for B+ supplies, but sincerely, why woud we still do this while we have modern solutions? there are some ways to mix designs and get the best of both worlds
you could use something like a FAN7530 to get a stable B+ instead of a valve rectifier and oversized inductions...
there is always the debate of using or not using the old school filtering for B+ supplies, but sincerely, why woud we still do this while we have modern solutions? there are some ways to mix designs and get the best of both worlds
Yes, I have been considering other 'solutions' as far as power supply goes.
Part of the attraction, for me, is to only use valves, with no silicon, but there are plenty of other solutions. I'm not familiar with the FAN7530 myself, but I had considered some sort of SMPS.
For me, building a power supply with no resistors seems fun, and I'm doing a lot of experiments with inductors at the moment, but the main point here is to use what's available.
I have the big rectifier tube, and a couple of smaller ones which should also be up to the job, so it makes sense to me to use them for a valve amp, in the same way I'm using valves I found in the shed.
I also have a 'big' inductor in the shed. I've not dug it out and measured the inductance yet, but 'watch this space'
Re: Simple Valve Amp
Ash Small, Fri Nov 29 2013, 07:34PM
Well, I've dug out the 'big' inductor in the shed, and the plate on it says 10H 180mA. I think it should be big enough to use for a choke
Now to see what big capacitors I can turn up. I don't want to buy any 'motor run' capacitors unless I have to, and old ones would probably look 'cooler'.
EDIT: I wonder what voltage it's good for. Any ideas? It does say '463V' but that looks like a catalogue number.
Ash Small, Fri Nov 29 2013, 07:34PM
Well, I've dug out the 'big' inductor in the shed, and the plate on it says 10H 180mA. I think it should be big enough to use for a choke
Now to see what big capacitors I can turn up. I don't want to buy any 'motor run' capacitors unless I have to, and old ones would probably look 'cooler'.
EDIT: I wonder what voltage it's good for. Any ideas? It does say '463V' but that looks like a catalogue number.
Re: Simple Valve Amp
Proud Mary, Fri Nov 29 2013, 08:46PM
I think you could feel confident in that choke in a 350V supply, Ash. It's a nice one. Parmeko is the best.
Proud Mary, Fri Nov 29 2013, 08:46PM
Ash Small wrote ...
Well, I've dug out the 'big' inductor in the shed, and the plate on it says 10H 180mA. I think it should be big enough to use for a choke
Now to see what big capacitors I can turn up. I don't want to buy any 'motor run' capacitors unless I have to, and old ones would probably look 'cooler'.
EDIT: I wonder what voltage it's good for. Any ideas? It does say '463V' but that looks like a catalogue number.
Well, I've dug out the 'big' inductor in the shed, and the plate on it says 10H 180mA. I think it should be big enough to use for a choke
Now to see what big capacitors I can turn up. I don't want to buy any 'motor run' capacitors unless I have to, and old ones would probably look 'cooler'.
EDIT: I wonder what voltage it's good for. Any ideas? It does say '463V' but that looks like a catalogue number.
I think you could feel confident in that choke in a 350V supply, Ash. It's a nice one. Parmeko is the best.
Re: Simple Valve Amp
Ash Small, Fri Nov 29 2013, 09:31PM
It looks a bit dry, not too bad, though. Should I soak it in oil, for example, or anything else?
It looks like it was originally impregnated with something. A lot of the 'traditional' mixes were, I suspect, linseed oil based. Maybe I should try that?....any ideas?
Ash Small, Fri Nov 29 2013, 09:31PM
Proud Mary wrote ...
I think you could feel confident in that choke in a 350V supply, Ash. It's a nice one. Parmeko is the best.
I think you could feel confident in that choke in a 350V supply, Ash. It's a nice one. Parmeko is the best.
It looks a bit dry, not too bad, though. Should I soak it in oil, for example, or anything else?
It looks like it was originally impregnated with something. A lot of the 'traditional' mixes were, I suspect, linseed oil based. Maybe I should try that?....any ideas?
Re: Simple Valve Amp
Proud Mary, Fri Nov 29 2013, 09:56PM
It looks fine to me. I've got some very similar chokes and transformers from Parmeko, and I think that's just the normal look of the varnish. I'd give it a good wash and brush up, but I wouldn't soak it in oil or anything, as that might soften the varnish and damage the insulation.
If you've got a Megger, you could test the insulation between the windings and the core, but I wouldn't worry about it too much. If it looks in good condition, and hasn't been badly bashed or smell of burning, it's probably OK. Parmeko is the Rolls-Royce of transformers, after all.
Proud Mary, Fri Nov 29 2013, 09:56PM
Ash Small wrote ...
It looks a bit dry, not too bad, though. Should I soak it in oil, for example, or anything else?
It looks like it was originally impregnated with something. A lot of the 'traditional' mixes were, I suspect, linseed oil based. Maybe I should try that?....any ideas?
Proud Mary wrote ...
I think you could feel confident in that choke in a 350V supply, Ash. It's a nice one. Parmeko is the best.
I think you could feel confident in that choke in a 350V supply, Ash. It's a nice one. Parmeko is the best.
It looks a bit dry, not too bad, though. Should I soak it in oil, for example, or anything else?
It looks like it was originally impregnated with something. A lot of the 'traditional' mixes were, I suspect, linseed oil based. Maybe I should try that?....any ideas?
It looks fine to me. I've got some very similar chokes and transformers from Parmeko, and I think that's just the normal look of the varnish. I'd give it a good wash and brush up, but I wouldn't soak it in oil or anything, as that might soften the varnish and damage the insulation.
If you've got a Megger, you could test the insulation between the windings and the core, but I wouldn't worry about it too much. If it looks in good condition, and hasn't been badly bashed or smell of burning, it's probably OK. Parmeko is the Rolls-Royce of transformers, after all.
Re: Simple Valve Amp
Ash Small, Sun Dec 08 2013, 07:08PM
I've updated the first post in the thread with details of the mains transformer I found in the shed. I'd appreciate any relevant comments that anyone may have:
Ash Small, Sun Dec 08 2013, 07:08PM
I've updated the first post in the thread with details of the mains transformer I found in the shed. I'd appreciate any relevant comments that anyone may have:
Re: Simple Valve Amp
Shrad, Mon Dec 09 2013, 08:42AM
closely inspect the solder joints of the transformer wires to the pins, and resolder them with tin/lead if they are not good
Shrad, Mon Dec 09 2013, 08:42AM
closely inspect the solder joints of the transformer wires to the pins, and resolder them with tin/lead if they are not good
Re: Simple Valve Amp
Proud Mary, Mon Dec 09 2013, 01:29PM
23, 17, 25, C, and the two lugs second row from lower right clearly form the Earth/Chassis rail running through various centre taps.
To find out what is what: make a map of all the winding resistances between the lugs, so you can see which windings are continuous with one another, and which are not.
Windings likely to be found on this transformer could include
LT secondaries
0V-6.3V - perhaps two of these, (standard heaters)
3.15V-0V-3.15V - perhaps one of these (reduced noise 6.3V heater supply)
0V -5V - very common for full wave rectifier heaters.
0V - 4V - another common one for old rectifier heaters.
Possible oddball windings for grid bias supplies etc
One or two centre-tapped secondaries for HT windings
Primary windings could include a lineup like this 10V - 0V- 210V - 220V - 230V - 240V - 250V
Once you've made your continuity map, stick 24V or so 50Hz on the lugs you most suspect of being the primaries, and measure the outputs appearing at all the different windings.
A bit of trial and error like this and you'll get it sorted without too much difficulty.
As for condition, once you've cleaned all that gunge from around the lugs, examine the state of the insulation of the paxolin board with a hand lens. Sniff for burning, smoke smells, which can linger on a transformer for years. If there's no sign of a past disaster, and all the connections from the windings to the tag board look solid, it will probably be a runner.
Proud Mary, Mon Dec 09 2013, 01:29PM
Ash Small wrote ...
I've updated the first post in the thread with details of the mains transformer I found in the shed. I'd appreciate any relevant comments that anyone may have:
I've updated the first post in the thread with details of the mains transformer I found in the shed. I'd appreciate any relevant comments that anyone may have:
23, 17, 25, C, and the two lugs second row from lower right clearly form the Earth/Chassis rail running through various centre taps.
To find out what is what: make a map of all the winding resistances between the lugs, so you can see which windings are continuous with one another, and which are not.
Windings likely to be found on this transformer could include
LT secondaries
0V-6.3V - perhaps two of these, (standard heaters)
3.15V-0V-3.15V - perhaps one of these (reduced noise 6.3V heater supply)
0V -5V - very common for full wave rectifier heaters.
0V - 4V - another common one for old rectifier heaters.
Possible oddball windings for grid bias supplies etc
One or two centre-tapped secondaries for HT windings
Primary windings could include a lineup like this 10V - 0V- 210V - 220V - 230V - 240V - 250V
Once you've made your continuity map, stick 24V or so 50Hz on the lugs you most suspect of being the primaries, and measure the outputs appearing at all the different windings.
A bit of trial and error like this and you'll get it sorted without too much difficulty.
As for condition, once you've cleaned all that gunge from around the lugs, examine the state of the insulation of the paxolin board with a hand lens. Sniff for burning, smoke smells, which can linger on a transformer for years. If there's no sign of a past disaster, and all the connections from the windings to the tag board look solid, it will probably be a runner.
Re: Simple Valve Amp
Sulaiman, Mon Dec 09 2013, 07:44PM
Every Parmeko transformer that I have come across has been over-engineered
to be damaged it would need to have been REALLY abused
don't know what the varnish/encapsulant is but it seems to last a very long time.
Sulaiman, Mon Dec 09 2013, 07:44PM
Every Parmeko transformer that I have come across has been over-engineered
to be damaged it would need to have been REALLY abused
don't know what the varnish/encapsulant is but it seems to last a very long time.
Re: Simple Valve Amp
Ash Small, Thu Dec 12 2013, 10:32PM
Well, the voltage selector that came from the mains transformer has 110 or 240 and -20, -10, -5, 0, +5, +10, and +20, but I can only trace four windings. Two are obviously 110 (2x110=220), and I assume the others are 10V each, taking it up to 240, but I don't understand the +/-5V bit.
I found an old 24V battery charger in the shed, with a decent sized step-down transformer in it. I have a variac somewhere (~2.5 Amp, I think), but I should still use a fuse between the step-down transformer and the 'mains transformer', shouldn't I? in case I get the phases wrong when testyimng the primaries.
What sort of fuse should I use?
Ash Small, Thu Dec 12 2013, 10:32PM
Proud Mary wrote ...
Primary windings could include a lineup like this 10V - 0V- 210V - 220V - 230V - 240V - 250V
Once you've made your continuity map, stick 24V or so 50Hz on the lugs you most suspect of being the primaries, and measure the outputs appearing at all the different windings.
A bit of trial and error like this and you'll get it sorted without too much difficulty.
As for condition, once you've cleaned all that gunge from around the lugs, examine the state of the insulation of the paxolin board with a hand lens. Sniff for burning, smoke smells, which can linger on a transformer for years. If there's no sign of a past disaster, and all the connections from the windings to the tag board look solid, it will probably be a runner.
Primary windings could include a lineup like this 10V - 0V- 210V - 220V - 230V - 240V - 250V
Once you've made your continuity map, stick 24V or so 50Hz on the lugs you most suspect of being the primaries, and measure the outputs appearing at all the different windings.
A bit of trial and error like this and you'll get it sorted without too much difficulty.
As for condition, once you've cleaned all that gunge from around the lugs, examine the state of the insulation of the paxolin board with a hand lens. Sniff for burning, smoke smells, which can linger on a transformer for years. If there's no sign of a past disaster, and all the connections from the windings to the tag board look solid, it will probably be a runner.
Well, the voltage selector that came from the mains transformer has 110 or 240 and -20, -10, -5, 0, +5, +10, and +20, but I can only trace four windings. Two are obviously 110 (2x110=220), and I assume the others are 10V each, taking it up to 240, but I don't understand the +/-5V bit.
I found an old 24V battery charger in the shed, with a decent sized step-down transformer in it. I have a variac somewhere (~2.5 Amp, I think), but I should still use a fuse between the step-down transformer and the 'mains transformer', shouldn't I? in case I get the phases wrong when testyimng the primaries.
What sort of fuse should I use?
Re: Simple Valve Amp
Proud Mary, Thu Dec 12 2013, 11:06PM
Is the "-20, -10, -5, 0, +5, +10, and +20" electrically continuous? What are the DC resistances between these lugs?
Proud Mary, Thu Dec 12 2013, 11:06PM
Is the "-20, -10, -5, 0, +5, +10, and +20" electrically continuous? What are the DC resistances between these lugs?
Re: Simple Valve Amp
Ash Small, Fri Dec 13 2013, 12:10AM
I've cut it off when I stripped the transformer out, but I suppose I could re-solder it.
It only connects to four primaries, two are ~4 Ohms, I think (110?) and two are ~0.5 Ohms or less (my DMM isn't accurate at these values) (I assume these are 10V).
My plan is to use a fuse on the output of the 24V step down transformer (which will probably be ~28V, due to voltage drop of rectifier, and "24 batteries are nominally 25.2V, I think, so if I use a variac, I can adjust it to 24V, and then measure the outputs of the various secondaries.
I need to use a fuse in case I get the phases wrong, and the inductances cancel out and it's only the DC resistance that's regulating the current. It's a pretty beefy 24V transformer. I don't want to 'cook' the Parmeko.
Ash Small, Fri Dec 13 2013, 12:10AM
Proud Mary wrote ...
Is the "-20, -10, -5, 0, +5, +10, and +20" electrically continuous? What are the DC resistances between these lugs?
Is the "-20, -10, -5, 0, +5, +10, and +20" electrically continuous? What are the DC resistances between these lugs?
I've cut it off when I stripped the transformer out, but I suppose I could re-solder it.
It only connects to four primaries, two are ~4 Ohms, I think (110?) and two are ~0.5 Ohms or less (my DMM isn't accurate at these values) (I assume these are 10V).
My plan is to use a fuse on the output of the 24V step down transformer (which will probably be ~28V, due to voltage drop of rectifier, and "24 batteries are nominally 25.2V, I think, so if I use a variac, I can adjust it to 24V, and then measure the outputs of the various secondaries.
I need to use a fuse in case I get the phases wrong, and the inductances cancel out and it's only the DC resistance that's regulating the current. It's a pretty beefy 24V transformer. I don't want to 'cook' the Parmeko.
Re: Simple Valve Amp
Proud Mary, Fri Dec 13 2013, 12:37AM
1A slow blow.
If the transformer starts humming loudly, if you disconnect at once, not much harm will have been done.
Proud Mary, Fri Dec 13 2013, 12:37AM
1A slow blow.
If the transformer starts humming loudly, if you disconnect at once, not much harm will have been done.
Re: Simple Valve Amp
Ash Small, Fri Dec 13 2013, 01:44AM
Thanks. Chris also suggested 1 Amp. I think I've some in the shed somewhere
EDIT: Maybe the +/-5V is for Americans? (110 Volt land)
Ash Small, Fri Dec 13 2013, 01:44AM
Proud Mary wrote ...
1A slow blow.
If the transformer starts humming loudly, if you disconnect at once, not much harm will have been done.
1A slow blow.
If the transformer starts humming loudly, if you disconnect at once, not much harm will have been done.
Thanks. Chris also suggested 1 Amp. I think I've some in the shed somewhere
EDIT: Maybe the +/-5V is for Americans? (110 Volt land)
Re: Simple Valve Amp
Proud Mary, Fri Dec 13 2013, 12:27PM
I've just been in the shed and poked about in a box of Parmeko Neptune transformers - the cased oil-filled type - and selected four which had roughly the same physical size as yours, being perhaps a bit bigger.
Three had the 0 - 115V 0 - 115V type of primary where you have to wire them in series for UK mains, and one had strange voltages perhaps related to supplies on board ships of long ago with 100V and 107V options.
Anyway, three of these had total primary DC resistances of about 15Ω in the UK series configuration - or 7.5Ω per 115V section - and one had a DC resistance of 6Ω, or 3Ω per section.
I hope this gives you some idea about the order of DC resistance you are likely to find on your primaries.
Proud Mary, Fri Dec 13 2013, 12:27PM
I've just been in the shed and poked about in a box of Parmeko Neptune transformers - the cased oil-filled type - and selected four which had roughly the same physical size as yours, being perhaps a bit bigger.
Three had the 0 - 115V 0 - 115V type of primary where you have to wire them in series for UK mains, and one had strange voltages perhaps related to supplies on board ships of long ago with 100V and 107V options.
Anyway, three of these had total primary DC resistances of about 15Ω in the UK series configuration - or 7.5Ω per 115V section - and one had a DC resistance of 6Ω, or 3Ω per section.
I hope this gives you some idea about the order of DC resistance you are likely to find on your primaries.
Re: Simple Valve Amp
Ash Small, Fri Dec 13 2013, 12:45PM
Thanks. My reading of ~4 Ohms per section seems pretty close to your value of 3 Ohms per section.
I suppose I could check I have the phasing correct by connecting the 'four series connected primaries' in parallel with a known value capacitor, and 'sweep' to find the resonant frequency. by re-arranging the primary series connections so that the inductance is at a maximum, I'll then know I have the phasing correct?
Ash Small, Fri Dec 13 2013, 12:45PM
Thanks. My reading of ~4 Ohms per section seems pretty close to your value of 3 Ohms per section.
I suppose I could check I have the phasing correct by connecting the 'four series connected primaries' in parallel with a known value capacitor, and 'sweep' to find the resonant frequency. by re-arranging the primary series connections so that the inductance is at a maximum, I'll then know I have the phasing correct?
Re: Simple Valve Amp
Proud Mary, Fri Dec 13 2013, 01:21PM
I did the tests with a cheap multimeter, which can be about half an ohm out when you get below about 10Ω.
On the Parmekos I looked at, the four primary lugs are arranged in a row like this 115 - 0 115 - 0 and the two lugs in the centre have short bridging wires soldered across them to connect the primaries together in series. I think it likely that yours would be organised in the same way, (which I have seen on many other valve era transformers) so that only a short jumper is needed to connect the two primaries together in the correct way.
If you stick your 24V AC on what you think are the primaries, you can then measure the voltages across the other windings, which should be roughly 10% of the value you'd get if connected to the mains. (Measure the AC input voltage at the point it's connected to the transformer, as a battery charger transformer might go to 30V or so with no or only a light load, and use this voltage as the one to calculate the ratios of the other windings with)
A Parmeko transformer with no load will only warm very slightly after an hour.
Proud Mary, Fri Dec 13 2013, 01:21PM
Ash Small wrote ...
Thanks. My reading of ~4 Ohms per section seems pretty close to your value of 3 Ohms per section.
I suppose I could check I have the phasing correct by connecting the 'four series connected primaries' in parallel with a known value capacitor, and 'sweep' to find the resonant frequency. by re-arranging the primary series connections so that the inductance is at a maximum, I'll then know I have the phasing correct?
Thanks. My reading of ~4 Ohms per section seems pretty close to your value of 3 Ohms per section.
I suppose I could check I have the phasing correct by connecting the 'four series connected primaries' in parallel with a known value capacitor, and 'sweep' to find the resonant frequency. by re-arranging the primary series connections so that the inductance is at a maximum, I'll then know I have the phasing correct?
I did the tests with a cheap multimeter, which can be about half an ohm out when you get below about 10Ω.
On the Parmekos I looked at, the four primary lugs are arranged in a row like this 115 - 0 115 - 0 and the two lugs in the centre have short bridging wires soldered across them to connect the primaries together in series. I think it likely that yours would be organised in the same way, (which I have seen on many other valve era transformers) so that only a short jumper is needed to connect the two primaries together in the correct way.
If you stick your 24V AC on what you think are the primaries, you can then measure the voltages across the other windings, which should be roughly 10% of the value you'd get if connected to the mains. (Measure the AC input voltage at the point it's connected to the transformer, as a battery charger transformer might go to 30V or so with no or only a light load, and use this voltage as the one to calculate the ratios of the other windings with)
A Parmeko transformer with no load will only warm very slightly after an hour.
Re: Simple Valve Amp
Ash Small, Fri Dec 13 2013, 01:53PM
OK, so I leave all the secondaries 'open circuit', I assume?
Ash Small, Fri Dec 13 2013, 01:53PM
OK, so I leave all the secondaries 'open circuit', I assume?
Re: Simple Valve Amp
Proud Mary, Fri Dec 13 2013, 02:33PM
Yes, just measure the output voltages that appear across them.
If you are putting 24V AC across the series primaries, the output voltage across a 6.3V secondary is only going to be about 600 mV, which you may not be able to measure very accurately, but at least you'll have an indication that it is a low voltage winding.
Proud Mary, Fri Dec 13 2013, 02:33PM
Ash Small wrote ...
OK, so I leave all the secondaries 'open circuit', I assume?
OK, so I leave all the secondaries 'open circuit', I assume?
Yes, just measure the output voltages that appear across them.
If you are putting 24V AC across the series primaries, the output voltage across a 6.3V secondary is only going to be about 600 mV, which you may not be able to measure very accurately, but at least you'll have an indication that it is a low voltage winding.
Re: Simple Valve Amp
Ash Small, Fri Dec 13 2013, 04:56PM
Well, I can either 'scope it, or get the AVO out. I've plenty of moving coil meters that I can use if I bias them correctly and calibrate them. It's a lot easier to just use the DMM for 'approximate' measurements, though.
I doubt I'll get round to setting it all up until Sunday, at least. I'm off to London to get a clutch for my bike tomorrow.
Ash Small, Fri Dec 13 2013, 04:56PM
Proud Mary wrote ...
Yes, just measure the output voltages that appear across them.
If you are putting 24V AC across the series primaries, the output voltage across a 6.3V secondary is only going to be about 600 mV, which you may not be able to measure very accurately, but at least you'll have an indication that it is a low voltage winding.
Ash Small wrote ...
OK, so I leave all the secondaries 'open circuit', I assume?
OK, so I leave all the secondaries 'open circuit', I assume?
Yes, just measure the output voltages that appear across them.
If you are putting 24V AC across the series primaries, the output voltage across a 6.3V secondary is only going to be about 600 mV, which you may not be able to measure very accurately, but at least you'll have an indication that it is a low voltage winding.
Well, I can either 'scope it, or get the AVO out. I've plenty of moving coil meters that I can use if I bias them correctly and calibrate them. It's a lot easier to just use the DMM for 'approximate' measurements, though.
I doubt I'll get round to setting it all up until Sunday, at least. I'm off to London to get a clutch for my bike tomorrow.
Re: Simple Valve Amp
Proud Mary, Fri Dec 13 2013, 05:19PM
I forgot to say that the unloaded voltages on the secondaries may well be 10% or so higher than their loaded working values.
Proud Mary, Fri Dec 13 2013, 05:19PM
I forgot to say that the unloaded voltages on the secondaries may well be 10% or so higher than their loaded working values.
Re: Simple Valve Amp
Ash Small, Fri Dec 13 2013, 05:52PM
Ash Small, Fri Dec 13 2013, 05:52PM
Proud Mary wrote ...
I forgot to say that the unloaded voltages on the secondaries may well be 10% or so higher than their loaded working values.
I suspected that would probably be the case. I was thinking of possibly adding a resistor, but haven't gotten around to doing any sums yet.I forgot to say that the unloaded voltages on the secondaries may well be 10% or so higher than their loaded working values.
Re: Simple Valve Amp
Ash Small, Fri Oct 30 2015, 03:47AM
Ok, had a bit of a pause with this, and other projects. illness, another death in the 'family', more illness, a lot of upheaval, and a long recovery, I now find myself with a lot more time on my hands. It seems I can finally get back to projects.
I've now acquired a suitable stainless steel item to use as a chassis for the main amp. I think I'll keep the power supply remote, in it's own enclosure, with a multi pin connector of some sort. It's going to be heavy, so it makes sense. It should also reduce mains hum, I suppose. It also means I could use the same power supply for different applications in the future.
It looks as if, by adding a third stage, I could use it as a guitar amplifier as well. This is what my initial research suggests. Any comments will be welcome.
The main amp chassis will house three valves, I think, maybe with the option of paralleling them later, together with, hopefully, interstage transformers, output transformer(s), parafeed choke comprising, at the moment, two 20 henry 120 mA chokes in series, and parafeed capacitor. I don't yet have a suitable output transformer, but if I use parafeed I won't need a gapped one. I may double up, depending on what is available.
I've also procured some more valves. I'll have a look at them and post more details over the next few days.
Ash Small, Fri Oct 30 2015, 03:47AM
Ok, had a bit of a pause with this, and other projects. illness, another death in the 'family', more illness, a lot of upheaval, and a long recovery, I now find myself with a lot more time on my hands. It seems I can finally get back to projects.
I've now acquired a suitable stainless steel item to use as a chassis for the main amp. I think I'll keep the power supply remote, in it's own enclosure, with a multi pin connector of some sort. It's going to be heavy, so it makes sense. It should also reduce mains hum, I suppose. It also means I could use the same power supply for different applications in the future.
It looks as if, by adding a third stage, I could use it as a guitar amplifier as well. This is what my initial research suggests. Any comments will be welcome.
The main amp chassis will house three valves, I think, maybe with the option of paralleling them later, together with, hopefully, interstage transformers, output transformer(s), parafeed choke comprising, at the moment, two 20 henry 120 mA chokes in series, and parafeed capacitor. I don't yet have a suitable output transformer, but if I use parafeed I won't need a gapped one. I may double up, depending on what is available.
I've also procured some more valves. I'll have a look at them and post more details over the next few days.
Re: Simple Valve Amp
Ash Small, Thu Dec 03 2015, 06:05PM
Ok, a bit of an update, the stainless chassis thing pictured above is too small for the amp chassis, but should be ok for the power supply chassis.
I now have a Williamson output transformer and two Varley 20H, 120mA chokes, which when seriesed, will give 40 Henries @ 120mA, should be plenty for the parafeed.
The Williamson output transformer is a push/pull type with no gap, so should be ideal. I'm not familiar with push pull types, so I'm unsure at the moment how to wire it for single ended with parafeed.
Should I connect the centre tapped primary in series or parallel?
Ash Small, Thu Dec 03 2015, 06:05PM
Ok, a bit of an update, the stainless chassis thing pictured above is too small for the amp chassis, but should be ok for the power supply chassis.
I now have a Williamson output transformer and two Varley 20H, 120mA chokes, which when seriesed, will give 40 Henries @ 120mA, should be plenty for the parafeed.
The Williamson output transformer is a push/pull type with no gap, so should be ideal. I'm not familiar with push pull types, so I'm unsure at the moment how to wire it for single ended with parafeed.
Should I connect the centre tapped primary in series or parallel?
Re: Simple Valve Amp
Ash Small, Sat Feb 06 2016, 03:17AM
Ok, I'm starting to make some progress now, I think I've got everything for the power supply, although I'm still puzzling over how to connect it to the amp, I'm told I should have the amp at least a metre from the supply to eliminate mains hum.
I'm getting my head around the actual amp design now, I've had the schematic for over two years now, and I've finally had a chance to study it and redraw it for myself.
It's still work in progress, but here's my first incomplete sketch.
Ash Small, Sat Feb 06 2016, 03:17AM
Ok, I'm starting to make some progress now, I think I've got everything for the power supply, although I'm still puzzling over how to connect it to the amp, I'm told I should have the amp at least a metre from the supply to eliminate mains hum.
I'm getting my head around the actual amp design now, I've had the schematic for over two years now, and I've finally had a chance to study it and redraw it for myself.
It's still work in progress, but here's my first incomplete sketch.
Re: Simple Valve Amp
Ash Small, Mon Feb 08 2016, 02:11AM
Post deleted.
Ash Small, Mon Feb 08 2016, 02:11AM
Post deleted.
Re: Simple Valve Amp
Ash Small, Mon Feb 08 2016, 03:42PM
Ok, I've just been given the thumbs up by Chris, who designed this amp twenty years ago. The circuit below is correct, as far as he's concerned.
The schematic posted here wasn't accurate. Please see the post below.
One of the things I like about the parafeed system, and you don't need parafeed for this circuit, by the way, is that as long as the parafeed capacitor is beefy enough it protects the most difficult part of the circuit to obtain from burning out, the output transformer. Also, with the parafeed system you don't need a gapped output transformer, one designed for a push-pull amp will do nicely.
Next job is to find suitable wire wound potentiometers. I have pretty much everything else.
Ash Small, Mon Feb 08 2016, 03:42PM
Ok, I've just been given the thumbs up by Chris, who designed this amp twenty years ago. The circuit below is correct, as far as he's concerned.
The schematic posted here wasn't accurate. Please see the post below.
One of the things I like about the parafeed system, and you don't need parafeed for this circuit, by the way, is that as long as the parafeed capacitor is beefy enough it protects the most difficult part of the circuit to obtain from burning out, the output transformer. Also, with the parafeed system you don't need a gapped output transformer, one designed for a push-pull amp will do nicely.
Next job is to find suitable wire wound potentiometers. I have pretty much everything else.
Re: Simple Valve Amp
Ash Small, Fri Feb 12 2016, 10:39PM
Updated OP with finalised schematic
Ok, I finally have the schematic finalised. It's taken a while to get there, Chris is one of those chaps who, like Tesla and Einstein, ....well, you know what I mean. He's the nicest chap you could wish to meet. Sometimes it seems you don't always get a 'direct answer' to the question, but maybe that's because I'm missing something
Apparently this method of 'direct coupling' or whatever it's called is what Alan Blumlein was working on when he was killed testing the HS2 airbourne radar system in WW2.
As they say, there's a fine line between genius and insanity. .....
I'd appreciate opinions by anyone who's familiar with such circuits.
This circuit did win competitions in international magazines nearly twenty years ago. I've seen the magazines.
Ash Small, Fri Feb 12 2016, 10:39PM
Updated OP with finalised schematic
Ok, I finally have the schematic finalised. It's taken a while to get there, Chris is one of those chaps who, like Tesla and Einstein, ....well, you know what I mean. He's the nicest chap you could wish to meet. Sometimes it seems you don't always get a 'direct answer' to the question, but maybe that's because I'm missing something
Apparently this method of 'direct coupling' or whatever it's called is what Alan Blumlein was working on when he was killed testing the HS2 airbourne radar system in WW2.
As they say, there's a fine line between genius and insanity. .....
I'd appreciate opinions by anyone who's familiar with such circuits.
This circuit did win competitions in international magazines nearly twenty years ago. I've seen the magazines.
Re: Simple Valve Amp
Hazmatt_(The Underdog), Sat Feb 13 2016, 04:52AM
Ash, that design isn't really going to work.
If you think about how tubes and fets work, as the grid sees a potential, the tube conducts, causing a current to flow from plate to cathode.
Now on a small signal tube you have enough plate impedance (100K) such that when the tube is fully conducting the supply is fully dropped across the plate load, and the ensuing AC current flows out through a capacitor.
In a "single ended" circuit, which you have there, the plate load IS the OPT, thus when the tube conducts it causes the flux to change in the transformer and you have sound.
The choke is there for smoothing the rectified DC and limiting inrush current.
Your circuit does not have a low impedance path from the transformer to ground, so little current will flow in the transformer, and likely .1W of output.
In order to get the OPT working you will need to re-arrange the plate connection such that B+ flows through the transformer to the plate, only then will the system produce output.
It looks to me like this design is an attempt to use one valve as a CCS and the other valve as the amplifier, which is fine, but the OPT is still in the wrong place, but give it a shot and see what happens.
Hazmatt_(The Underdog), Sat Feb 13 2016, 04:52AM
Ash, that design isn't really going to work.
If you think about how tubes and fets work, as the grid sees a potential, the tube conducts, causing a current to flow from plate to cathode.
Now on a small signal tube you have enough plate impedance (100K) such that when the tube is fully conducting the supply is fully dropped across the plate load, and the ensuing AC current flows out through a capacitor.
In a "single ended" circuit, which you have there, the plate load IS the OPT, thus when the tube conducts it causes the flux to change in the transformer and you have sound.
The choke is there for smoothing the rectified DC and limiting inrush current.
Your circuit does not have a low impedance path from the transformer to ground, so little current will flow in the transformer, and likely .1W of output.
In order to get the OPT working you will need to re-arrange the plate connection such that B+ flows through the transformer to the plate, only then will the system produce output.
It looks to me like this design is an attempt to use one valve as a CCS and the other valve as the amplifier, which is fine, but the OPT is still in the wrong place, but give it a shot and see what happens.
Re: Simple Valve Amp
Dr. Slack, Sat Feb 13 2016, 07:58AM
Rule 1 of presenting schematics - Use component numbering. It's much easier to refer to R3, than 'the pot in the tail of the upper tube'.
Anyhow, an interesting configuration. My valve practice is pretty rusty, but I think I see how it's meant to work.
The choke does more than filter the current. It's so large that at audio frequencies it's a current source. In microwave amplifier work, I'd supply my FET drain through a few nH, for the same reason.
The current source theme is continued all the way down, though both valves self-biasing through their cathode resistors.
The two large caps (component numbering please) provide a signal ground at the top tube cathode. The top cap charges up until there's no voltage across the OPT, but conducts the signal to it from the top plate. AFAICS, the top tube + choke form a controllable bidirectional signal current source, connected to the signal grounded OPT, so plenty of power available.
The DC coupling so that the bottom tube output connects to the top tube grid is quite clever, fitting in as it does with the DC bias establishment.
I was a bit dubious about the DC stability of putting two self-biassed valves in series, how well will they share voltage? But then I remembered that triodes are not as high output impedance as pentodes, so perhaps it's not as bad as I thought. Presumably that's what the pots in the tails of the tubes are for?
Other than a certain interesting wierdness, what advantages is this configuration supposed to offer over more conventional single ended designs?
Dr. Slack, Sat Feb 13 2016, 07:58AM
Rule 1 of presenting schematics - Use component numbering. It's much easier to refer to R3, than 'the pot in the tail of the upper tube'.
Anyhow, an interesting configuration. My valve practice is pretty rusty, but I think I see how it's meant to work.
The choke does more than filter the current. It's so large that at audio frequencies it's a current source. In microwave amplifier work, I'd supply my FET drain through a few nH, for the same reason.
The current source theme is continued all the way down, though both valves self-biasing through their cathode resistors.
The two large caps (component numbering please) provide a signal ground at the top tube cathode. The top cap charges up until there's no voltage across the OPT, but conducts the signal to it from the top plate. AFAICS, the top tube + choke form a controllable bidirectional signal current source, connected to the signal grounded OPT, so plenty of power available.
The DC coupling so that the bottom tube output connects to the top tube grid is quite clever, fitting in as it does with the DC bias establishment.
I was a bit dubious about the DC stability of putting two self-biassed valves in series, how well will they share voltage? But then I remembered that triodes are not as high output impedance as pentodes, so perhaps it's not as bad as I thought. Presumably that's what the pots in the tails of the tubes are for?
Other than a certain interesting wierdness, what advantages is this configuration supposed to offer over more conventional single ended designs?
Re: Simple Valve Amp
Ash Small, Sat Feb 13 2016, 09:45AM
Hazmatt, I suggest you google 'parafeed'
The main advantage of using a parafeed choke and capacitor is you don't need a gapped O/T.
Also, the parafeed capacitor provides protection for the precious output transformer
Apparently 40H is sufficient to block down to about 8 Hertz. (If I remember correctly)
Neil, thanks for the comments. I'll re-draw it with component numbering, but there are so few components in this circuit it's possible to refer to each by it's value.
That's the clever bit of the circuit, Neil. As I said, it's apparently what Alan Blumlein was working on when he was killed.
The main advantage of this circuit is simplicity and few components. It's more efficient because it has less resistors than other circuits. It took Chris some time to come up with the direct coupling method. (Chris knew that Blumlein was working on a method of 'direct coupling', and he set out to see if he could find it)
Apparently, you adjust the two cathode pots until there is around 140V across the 3k resistor.
I'll post more details as it progresses and I'm able to find out more about it. As I mentioned previously, this design has already won magazine competitions.
EDIT:
Ash Small, Sat Feb 13 2016, 09:45AM
Hazmatt, I suggest you google 'parafeed'
The main advantage of using a parafeed choke and capacitor is you don't need a gapped O/T.
Also, the parafeed capacitor provides protection for the precious output transformer
Apparently 40H is sufficient to block down to about 8 Hertz. (If I remember correctly)
Neil, thanks for the comments. I'll re-draw it with component numbering, but there are so few components in this circuit it's possible to refer to each by it's value.
Dr. Slack wrote ...
The DC coupling so that the bottom tube output connects to the top tube grid is quite clever, fitting in as it does with the DC bias establishment.
.
The DC coupling so that the bottom tube output connects to the top tube grid is quite clever, fitting in as it does with the DC bias establishment.
.
That's the clever bit of the circuit, Neil. As I said, it's apparently what Alan Blumlein was working on when he was killed.
The main advantage of this circuit is simplicity and few components. It's more efficient because it has less resistors than other circuits. It took Chris some time to come up with the direct coupling method. (Chris knew that Blumlein was working on a method of 'direct coupling', and he set out to see if he could find it)
Apparently, you adjust the two cathode pots until there is around 140V across the 3k resistor.
I'll post more details as it progresses and I'm able to find out more about it. As I mentioned previously, this design has already won magazine competitions.
EDIT:
Re: Simple Valve Amp
Hazmatt_(The Underdog), Sat Feb 13 2016, 04:14PM
That V2 is an 807 makes more sense then an ECC83, that is a small detail that was left out!
But you are still shunting the AC current from the plate through the OPT, that will give you some power, but the additional capacitors are reactances and will cause additional signal loss because the AC signal is going through that entire branch impedance, and the OPT is only a fraction of that impedance.
What is your OPT impedance, B+, and plate current?
I could build one of these as well and test it if I can find some time, but I am doubtful I can find a 40H inductor, that is pretty big.
I would check that schematic too, there is NO WAY C1 = 4700uF. Even 100uF would make it flubby and you'd lose your high end response.
Hazmatt_(The Underdog), Sat Feb 13 2016, 04:14PM
That V2 is an 807 makes more sense then an ECC83, that is a small detail that was left out!
But you are still shunting the AC current from the plate through the OPT, that will give you some power, but the additional capacitors are reactances and will cause additional signal loss because the AC signal is going through that entire branch impedance, and the OPT is only a fraction of that impedance.
What is your OPT impedance, B+, and plate current?
I could build one of these as well and test it if I can find some time, but I am doubtful I can find a 40H inductor, that is pretty big.
I would check that schematic too, there is NO WAY C1 = 4700uF. Even 100uF would make it flubby and you'd lose your high end response.
Re: Simple Valve Amp
Dr. Slack, Sat Feb 13 2016, 07:14PM
Sorry to be picky, but the schematic needs both component numbering, so that it can be discussed in text, *and* values, so it's easy to read the diagram without having to have the eyes dart between diagram and a column of figures.
Dr. Slack, Sat Feb 13 2016, 07:14PM
Sorry to be picky, but the schematic needs both component numbering, so that it can be discussed in text, *and* values, so it's easy to read the diagram without having to have the eyes dart between diagram and a column of figures.
Re: Simple Valve Amp
Ash Small, Sat Feb 13 2016, 10:58PM
I've checked with Chris and he says "the parafeed capacitor COULD go to ground, but it's referenced to the cathode."
I've not measured O/T impedance yet, I can wire the primaries in series or parallel, though. B+ is around 300V, I've not yet put the power supply together and measured it, it's too cold to go in my workshop at the moment. The chokes (2x20H) are good for 120mA, but I doubt I'll be running any more than half that.
Four 10H chokes in series will do the job. You could use less inductance, but you'd lose a bit of bottom end response.
I checked the 4,700 capacitor with Chris the other day, he mumbled something about 5,300uF or a bit more.
He's built loads of these amps, he doesn't build many with parafeed though. He has at least a dozen in his workshop at the moment, none of them identical.
Neil, I'll post it as soon as it's done.
EDIT:
Ash Small, Sat Feb 13 2016, 10:58PM
Hazmatt_(The Underdog) wrote ...
That V2 is an 807 makes more sense then an ECC83, that is a small detail that was left out!
But you are still shunting the AC current from the plate through the OPT, that will give you some power, but the additional capacitors are reactances and will cause additional signal loss because the AC signal is going through that entire branch impedance, and the OPT is only a fraction of that impedance.
What is your OPT impedance, B+, and plate current?
I could build one of these as well and test it if I can find some time, but I am doubtful I can find a 40H inductor, that is pretty big.
I would check that schematic too, there is NO WAY C1 = 4700uF. Even 100uF would make it flubby and you'd lose your high end response.
That V2 is an 807 makes more sense then an ECC83, that is a small detail that was left out!
But you are still shunting the AC current from the plate through the OPT, that will give you some power, but the additional capacitors are reactances and will cause additional signal loss because the AC signal is going through that entire branch impedance, and the OPT is only a fraction of that impedance.
What is your OPT impedance, B+, and plate current?
I could build one of these as well and test it if I can find some time, but I am doubtful I can find a 40H inductor, that is pretty big.
I would check that schematic too, there is NO WAY C1 = 4700uF. Even 100uF would make it flubby and you'd lose your high end response.
I've checked with Chris and he says "the parafeed capacitor COULD go to ground, but it's referenced to the cathode."
I've not measured O/T impedance yet, I can wire the primaries in series or parallel, though. B+ is around 300V, I've not yet put the power supply together and measured it, it's too cold to go in my workshop at the moment. The chokes (2x20H) are good for 120mA, but I doubt I'll be running any more than half that.
Four 10H chokes in series will do the job. You could use less inductance, but you'd lose a bit of bottom end response.
I checked the 4,700 capacitor with Chris the other day, he mumbled something about 5,300uF or a bit more.
He's built loads of these amps, he doesn't build many with parafeed though. He has at least a dozen in his workshop at the moment, none of them identical.
Neil, I'll post it as soon as it's done.
EDIT:
Re: Simple Valve Amp
Ash Small, Tue Feb 16 2016, 11:21AM
OP updated with power supply schematic.
Not sure where to put fuses at the moment, see below
Ash Small, Tue Feb 16 2016, 11:21AM
OP updated with power supply schematic.
Not sure where to put fuses at the moment, see below
Re: Simple Valve Amp
Ash Small, Tue Mar 22 2016, 09:45PM
I'm hoping to make a start on the chassis soon, I've been reading up on interstage transformers, and want to leave room for 'experimentation' at a later date.
It's a compronise between leaving space for future modifications, and keeping wires as short as possible.
I suppose I should think of the first chassis as a 'prototype', and expect to replace it at some point?
Ash Small, Tue Mar 22 2016, 09:45PM
I'm hoping to make a start on the chassis soon, I've been reading up on interstage transformers, and want to leave room for 'experimentation' at a later date.
It's a compronise between leaving space for future modifications, and keeping wires as short as possible.
I suppose I should think of the first chassis as a 'prototype', and expect to replace it at some point?
Re: Simple Valve Amp
Hazmatt_(The Underdog), Wed Mar 23 2016, 03:34AM
Most Definitely!
These things go through hundreds of little tweaks and changes before they are finished.
The one on the left is powered by a SMPS
I'm getting roughly 10W out of 6V6's and it sounds great for guitar.
Hazmatt_(The Underdog), Wed Mar 23 2016, 03:34AM
Most Definitely!
These things go through hundreds of little tweaks and changes before they are finished.
The one on the left is powered by a SMPS
I'm getting roughly 10W out of 6V6's and it sounds great for guitar.
Re: Simple Valve Amp
Ash Small, Wed Mar 23 2016, 06:20PM
Ok, I've been working on the power supply layout, and I've run into a 'problem'.
I can't put a fuse in the most common place, the return path to the centre tap of the secondary, due to several other centre taps all being wired to the same point on the transformer. The fuse has to go on the positive side.
I've been looking into this, and Marshall use two fuses one on each 'leg' of the CT secondary, before the bridge rectifier. I've been advised by the amp designer to put the fuse on the output of the rectifier, but in an old thread on another forum, Steve Conner advised putting one between the resevoir capacitor and choke.
Any suggestions or comments would be appreciated.
I believe Steve Connor suggested putting a fuse where Fuse A is in the schematic below.
Most people it would appear would put a fuse in where fuse B is in the schematic below.
Fuses C in the schematic below shows how Marshall do it,
Ash Small, Wed Mar 23 2016, 06:20PM
Ok, I've been working on the power supply layout, and I've run into a 'problem'.
I can't put a fuse in the most common place, the return path to the centre tap of the secondary, due to several other centre taps all being wired to the same point on the transformer. The fuse has to go on the positive side.
I've been looking into this, and Marshall use two fuses one on each 'leg' of the CT secondary, before the bridge rectifier. I've been advised by the amp designer to put the fuse on the output of the rectifier, but in an old thread on another forum, Steve Conner advised putting one between the resevoir capacitor and choke.
Any suggestions or comments would be appreciated.
I believe Steve Connor suggested putting a fuse where Fuse A is in the schematic below.
Most people it would appear would put a fuse in where fuse B is in the schematic below.
Fuses C in the schematic below shows how Marshall do it,
Re: Simple Valve Amp
Hazmatt_(The Underdog), Wed Mar 23 2016, 09:10PM
Go ahead and call me crazy, but I do it a little bit differently.
On the amp I built for my buddy that uses a tube rectifier, I put the fuse on the AC input side, and sized it for 1A I believe. I'd have to get it back from him to verify.
Why do this? Well I'm relying on the fact that the shorted transformer secondary will reflect the load back to the primary by the reciprocal impedance squared, long story short, secondary has a problem, it opens the primary fuse instantly.
So far no problems, fuse doesn't blow on turn-on due to inrush, the rectifier doesn't arc over either, and it all works and no headaches.
But then again... I am crazy.
Hazmatt_(The Underdog), Wed Mar 23 2016, 09:10PM
Go ahead and call me crazy, but I do it a little bit differently.
On the amp I built for my buddy that uses a tube rectifier, I put the fuse on the AC input side, and sized it for 1A I believe. I'd have to get it back from him to verify.
Why do this? Well I'm relying on the fact that the shorted transformer secondary will reflect the load back to the primary by the reciprocal impedance squared, long story short, secondary has a problem, it opens the primary fuse instantly.
So far no problems, fuse doesn't blow on turn-on due to inrush, the rectifier doesn't arc over either, and it all works and no headaches.
But then again... I am crazy.
Re: Simple Valve Amp
Ash Small, Wed Mar 23 2016, 11:29PM
That's pretty much what Chris, the designer of this circuit, said when I asked him. I don't think you're crazy Hazmatt, you make some very good points. I'd nominate you as a moderator here.
I have to ask these questions though. Any serious transformer fault and the mains fuse will blow (by 'mains' I don't mean the one in the plug, I mean the one on the primary.
Almost everyone uses fuses on the secondary side though, I'm just asking questions, and hoping to get differing opinions.
Some protection on the secondary side might protect some components.
Here's the latest sketch for the power supply layout:-
Tranny on the left, choke on the right, caps top centre and rectifier tube bottom centre. This is a mirror image, it's a drilling pattern/template.
Ash Small, Wed Mar 23 2016, 11:29PM
Hazmatt_(The Underdog) wrote ...
Go ahead and call me crazy, but I do it a little bit differently.
On the amp I built for my buddy that uses a tube rectifier, I put the fuse on the AC input side, and sized it for 1A I believe. I'd have to get it back from him to verify.
Why do this? Well I'm relying on the fact that the shorted transformer secondary will reflect the load back to the primary by the reciprocal impedance squared, long story short, secondary has a problem, it opens the primary fuse instantly.
So far no problems, fuse doesn't blow on turn-on due to inrush, the rectifier doesn't arc over either, and it all works and no headaches.
But then again... I am crazy.
Go ahead and call me crazy, but I do it a little bit differently.
On the amp I built for my buddy that uses a tube rectifier, I put the fuse on the AC input side, and sized it for 1A I believe. I'd have to get it back from him to verify.
Why do this? Well I'm relying on the fact that the shorted transformer secondary will reflect the load back to the primary by the reciprocal impedance squared, long story short, secondary has a problem, it opens the primary fuse instantly.
So far no problems, fuse doesn't blow on turn-on due to inrush, the rectifier doesn't arc over either, and it all works and no headaches.
But then again... I am crazy.
That's pretty much what Chris, the designer of this circuit, said when I asked him. I don't think you're crazy Hazmatt, you make some very good points. I'd nominate you as a moderator here.
I have to ask these questions though. Any serious transformer fault and the mains fuse will blow (by 'mains' I don't mean the one in the plug, I mean the one on the primary.
Almost everyone uses fuses on the secondary side though, I'm just asking questions, and hoping to get differing opinions.
Some protection on the secondary side might protect some components.
Here's the latest sketch for the power supply layout:-
Tranny on the left, choke on the right, caps top centre and rectifier tube bottom centre. This is a mirror image, it's a drilling pattern/template.
Re: Simple Valve Amp
Ash Small, Sun Jun 05 2016, 10:49AM
I've been side tracked with making guitar parts recently, but Chris, the designer of this circuit (he had it published in various periodicals back in the nineties) has recently published the design on a Facebook Tube Amp page, which has lead to various debates, and this analysis, which is only one person's opinion (I'm not sure he's even built the circuit)
It's an interesting analysis of the circuit, and draws comparisons between it and the Loftin-White amplifier from 1930.
Ash Small, Sun Jun 05 2016, 10:49AM
I've been side tracked with making guitar parts recently, but Chris, the designer of this circuit (he had it published in various periodicals back in the nineties) has recently published the design on a Facebook Tube Amp page, which has lead to various debates, and this analysis, which is only one person's opinion (I'm not sure he's even built the circuit)
It's an interesting analysis of the circuit, and draws comparisons between it and the Loftin-White amplifier from 1930.
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