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Ash Small

Mon Nov 25 2013, 07:43PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Post 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:

1455406835 3414 FT1630 Mighty Atom With Parafeed

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.

Shrad

Mon Nov 25 2013, 10:23PM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

beam tetrodes are said to be really nice output tubes, and to have their own sound...

Proud Mary

Tue Nov 26 2013, 02:17PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

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.

Ash Small

Tue Nov 26 2013, 03:10PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

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.

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

Tue Nov 26 2013, 08:36PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Ash Small 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?

Ash Small

Tue Nov 26 2013, 09:34PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

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?

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, itâ€™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.

Shrad

Tue Nov 26 2013, 10:00PM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

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

Proud Mary

Tue Nov 26 2013, 10:59PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

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!

Ash Small

Tue Nov 26 2013, 11:46PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

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!

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.

Shrad

Fri Nov 29 2013, 10:59AM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

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

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