Or people who've worked on the 911? I'm trying to find out what the correct bias setting should be. Apparently, there were some circuit tweaks over the years this was made, and the version on my bench most closely matches the attached schematic, courtesy of dangus.
Ok, I haven't gotten any traction from looking for owners of this amp, but I'm still in the dark regarding a bias setting. The outputs are 2N3773's. I'll take any educated guesses folks are willing to proffer.
My guess is room temp bias is zero, set just before they start to conduct. No thermal tracking at all, and very large emitter resistors.
Interesting. At first I had thought those emitter resistors were 0.6 ohms, but now that I look at them again, I can see that you're right, they're 6 ohms.
Which emitter resistor(s) should I measure across for the bias?
Which emitter resistor(s) should I measure across for the bias?
Now that I look more closely at the schematic, the bias is actually specified. 50 mV at the emitter of Q11. That’s higher than I thought it would be considering that the bias stack is just an adjustable resistor. They are hoping that 3 ohms of effective emitter resistance (an ohm and a half for the whole stage) is enough to prevent runaway. It might be. That’s a lot of emitter resistance.
I don't know how I missed that, but thanks for catching it.
Yeah, I hope that it's enough resistance. I've read some reports of these amps going into runaway.
Yeah, I hope that it's enough resistance. I've read some reports of these amps going into runaway.
One could add a proper bias stack, like the Dynaco ST120 upgrade. Probably DON’T want to replace the drivers with TIPs, though.
for images and schematic check out last posts (#126/127) under
what-was-the-first-Complementary AB Bipolar Amplifiers
what-was-the-first-Complementary AB Bipolar Amplifiers