Re: Circuit Topology Question

Elmo wrote
>
> http://gstube.com/schemes/scheme/Guitar.gif
>
> Anyone care to guess what's going on with the switches between
> preamp and power amp in this Russian guitar amp?
>
> (I suspect a switched tone control network).

There are heaps of switches! I assume you mean the ones in what
looks like a Baxandall filter. See

http://www.20sim.com/webhelp4/library/iconic_diagrams/Electric/Examples/Baxandall.htm

just the first example from a google search.

> There are a few other odd things happening...
>
> The OPT is wired for UltraLinear operation.
>
> The power tube on the left is operating normally,
> but the grid on the right-hand power tube is grounded.
>
> The power tube cathodes don't go to ground,
> but to the NFB connection from the speaker.
>
> The more I look at this, the more it confuses me!

If you look back a month or so ago there was another post on a
similar grounded grid power stage. Essentially, one output valve is
driven from its grid, the other from its cathode. In addition, yours
has feedback applied to the cathode, which appears as positive to
one valve, and negative to the other. Pretty clever, not seen that
before. Attaching the speaker lead to the cathode of a valve isn't
very safe though, and combining so many forms of feedback (four
around the output stage) makes it hard to analyse.

The key to understanding the stage is to remember that the circuit
aims to achieve the same Vgk signal to both valves. The signal
appearing across the common cathode resistor acts as negative
feedback to the grid-driven valve, and positive to the
cathode-driven side. That signal is combined with feedback from the
transformer secondary, handy because it is a low-impedance source.
This is clever because it is voltage derived, voltage applied
feedback, so it decreases output impedance of the cathode-driven
valve. The feedback from the cathode resistor is current derived,
and increases output impedance. Possibly the valves end up pretty
much equal...and possibly not...too complicated to tell without
screeds of calculation.

Other parts of the circuit I wonder about...

What's going on in the section at bottom left, with the handwritten
scrawl where feedback arrives from the speaker to earlier stages?
What are all those switches for?

What is the range of resistance of the LDR, if that's what it is?
When that is switched in it appears across the two opposite outputs
of what is more or less a concertina. Where is the light source that
goes with it to make it into a compressor?

cheers, Ian


.

Relevant Pages

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