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Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
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.
.
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.
Registered Member #135
Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735
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.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
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.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
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.
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.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
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?
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
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,
Registered Member #135
Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735
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.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
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.
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.
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Simple Valve Amp
