Transistor Substitution Question

[Eeyore]

Are pretty much all modern amps DC coupled, then, as opposed to capacitance
coupled? My solid state amps are from the golden era high-powered SS audio,
mid-70's to early 80's and both amps and my preamp have capacitor-coupled
inputs.

I was referring to the output actually, where the elimination of a coupling cap
has great benefits.

Graham

 

[Eeyore]

Any replacement, you have to deal with possible instabilities. You can make a darlington
with two transistors and a resistor. You basically multiply the two gains
to get one.

The stability issue will relate to the fT of the transistors.

Graham

 

[Chris F.]

I might have found the cause of the problem. With the output transistors
removed, I was measuring a bias voltage of +/- 64V on their bases -
something obviously wasn't right. Putting a 1k resistor across the B-E of
the output transistor in the good stage made this drop to 0, but doing the
same on the bad stage instantly activated the protection mode. The bias
voltage problem seemed to go quite far back into the circuit. After some
very laborous voltage tracing, I discovered two transistors at the very
start of the preamp stage (one each for inverting and non-inverting halves
of that channel) with a small DC voltage - about 80mV, on their bases - the
manual said there should be none. Checking the same two transistors on the
good channel, I found they had about 60mV on their bases, again they should
have read zero. The only possible source for this small DC voltage was
either through the collectors or emitters of the transistors themselves.
With these two transistors removed, the bias voltage on both these preamp
transistors and all following transistors in the circuit returned to normal,
dead on to the manuals specs. The transistors in question test good with a
diode check function of a DMM, but at this point I'd have to say they are
leaky and are most likely the cause of the whole problem.
I still don't have 100% faith in my diagnosis, but that's the best theory
I can come up with. I'll replace all transistors in question and see if that
works, if not this thing goes in the junkpile. I normally wouldn't have gone
to this much trouble for an item worth $100 at most, but I'm a very stubborn
person and I often don't quit when I should.

 

[Arfa Daily]

I might have found the cause of the problem. With the output transistors
removed, I was measuring a bias voltage of +/- 64V on their bases -
something obviously wasn't right. Putting a 1k resistor across the B-E of
the output transistor in the good stage made this drop to 0, but doing the
same on the bad stage instantly activated the protection mode. The bias
voltage problem seemed to go quite far back into the circuit. After some
very laborous voltage tracing, I discovered two transistors at the very
start of the preamp stage (one each for inverting and non-inverting halves
of that channel) with a small DC voltage - about 80mV, on their bases -
the manual said there should be none. Checking the same two transistors on
the good channel, I found they had about 60mV on their bases, again they
should have read zero. The only possible source for this small DC voltage
was either through the collectors or emitters of the transistors
themselves. With these two transistors removed, the bias voltage on both
these preamp transistors and all following transistors in the circuit
returned to normal, dead on to the manuals specs. The transistors in
question test good with a diode check function of a DMM, but at this point
I'd have to say they are leaky and are most likely the cause of the whole
problem.
I still don't have 100% faith in my diagnosis, but that's the best theory
I can come up with. I'll replace all transistors in question and see if
that works, if not this thing goes in the junkpile. I normally wouldn't
have gone to this much trouble for an item worth $100 at most, but I'm a
very stubborn person and I often don't quit when I should.

Aha ! See my reply above about problems back in the preamps of DC coupled
amps ...

Arfa