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