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drago 12th April 2006 09:12 AM

Tubes driven by BJT's
2 Attachment(s)
This really confuses me where shall I post it. I got this schematic from a magazine a few years ago here in HKG. I'm planning to build it because the prices of tubes and outputs are lowering a little these days. Does
anyone ever build this amplifier before and or where is it originated?
There are many schematics of tubes driving BJT's and FET's on the web, but this is the other way around. Any comments on it? Shall I
make a regulator for the +/-60V rails, or perhaps a constant current
source for the +60V end. Any suggestion is welcome. Thanks, guys.....

EC8010 12th April 2006 11:10 AM

The expensive bit of a valve amplifier is the output transformer - closely followed by the HT transformer, rectification and smoothing arrangements. Unless you get really keen, the driver circuitry is comparatively cheap. It's actually possible that providing those +/-60V supplies might make semiconductor drive more expensive than valve drive...

Geek 12th April 2006 02:23 PM

IMO the only advantage is if you require insanely now input noise figures, or your using a power tranny with a nice winding suitable for such. They seem to have been made, since I have one...

bigwill 12th April 2006 02:40 PM

I know next to nothing about valve amp design but that posted schematic looks almost too simple to work - can anyone see any problems with it? The design LOOKS alright to me, but it almost looks like an oversimplified circuit to explain amp concepts or something. It looks tempting to build but my intuition says that it would oscillate or blow up or something :D

drago 12th April 2006 03:07 PM

I copied this schematic by hand drawing at first, I've lost the original magazine. I think they tranlated that article from an European magazine, because they used to do this in Hong Kong. I remember the article includes a PCB layout, so I'm quite sure it works.

kevinkr 12th April 2006 04:30 PM

No, Bigwill has a point, there is no bias supply or cathode bias shown for the output tubes so in fact as drawn it won't work. Otherwise the design is workable, but having heard something quite similar I question the wisdom of building it at all. There are better designs out there.

EC8010 12th April 2006 04:34 PM

Bigwill, your intuition is correct. It will blow up. There is nothing to limit the current in the output valves. If the 330k resistors were removed and the 220nF capacitors replaced by short circuits, then the -30V on the collectors would be about right to bias the valves but it would be a bit precarious.

drago 12th April 2006 06:01 PM

The Circuit
2 Attachment(s)
I really made a terrible and careless mistake when drawing the circuit. Here is the correct one. The tubes are biased by the collectors of the MPSA92's at -30V. That's why I am thinking of a regualtor for +/-60V rails thus the bias can be trimmed.

EC8010 12th April 2006 06:17 PM

You don't need a +60V regulator, but a -60V regulator and replacing the 30k emitter resistor with an adjustable CCS feeding the wiper of a 100R variable resistor connected between the emitters would be a good thing.

drago 12th April 2006 06:48 PM


Thanks a lot for your good suggestion. Would it be good to ground the tubes through a 10 ohm resistor each so I can easily monitor their current and adjust them after a period of time (months)?

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