So that characteristic obviously makes it a very good limiter for testing purposes since it has a low resistance to drop only a small voltage at low currents but rapidly goes up in resistance at higher currents to limit the current at a much lower level then if just a fixed resistor were used.The incandescent light bulb is basically a current dependent resistor and it's really based on the resistance temperature curve of Tungsten. Tungsten has a resistance about 12x lower at room temperature.
Well, some of this riddle may be solved. I've been assuming I'm supposed to see 120VAC at the AC side of the rectifier on the Sunn because my brain was thinking this was the line, but it's not. It's the secondary of the transformer. Looking at the schematic, I'm only *supposed* to be seeing 66VAC. The GOOD news is that I *have* been running the amp at pretty much full voltage! Which means it only draws 220mA at the outlet.
That out of the way, here are my findings with a 1KHz 240mVp-p sine wave injected directly into the power amp (bypassing the preamp board for now, which has its own problems), and the master volume turned up to about 3 or 4 (hard to tell as it's not mounted) we get a 5Vp-p sine wave at the output at full power. Actually, we get that even at around half power, where it starts to drop off as I turn the variac down.
At full power, there is detectable crossover distortion, but pretty tiny--might not see it if you're not looking for it. Around half power the output wave begins to lose amplitude and the crossover distortion grows more severe as we get down to 1/4 power, 20% power, etc.
At 10% power the amp starts singing Elvis Presley tunes. Not sure what that's about.
Other finding: Sometimes I had trouble getting the signal to appear at the output. I had to reset the function generator a couple times and/or disconnect the signal wire and reconnect. I remembered you had said to the inject the signal as I was turning up the variac, so maybe this is normal? Next time I run tests I'll have a 2nd probe at the input to see where the signal might be getting clamped down, if it is.
No other distortion of the sine wave as I was turning it up.
Thoughts?
Q15/Q6 etc is actually the VI limiter.
I think I'll pull IC1 right now to test that theory . . . . NOT!
Still, good theory?
* Get it just below clipping. It should almost reach the power rails. i.e. A few volts less.
* Take a peak at the -15 V supply with a signal applied.
* Take a peak at the DC output voltage (speaker terminals) with a signal applied.
Symmetry. One you have a nice sine wave at close to the rails, use GND reference and check symmetry.
Thanks for keeping me moving along.
Has that OP amp been replaced?
Valve/Tube amps MUST have a load.
You will need to consider replacing Q13 and moving R42 to minimum in the future.
Do you think I should remove the jumper across C13 then retest the transistor? For this application, do you think a regular ol' 2N3906 could be dropped in there? Or do you see some characteristic of the 2n4250 that might be required for this part of the circuit to function properly? The two devices seem fairly close when I look at the datasheets.
I remember testing the pot for resistance, but not for varying resistance. Might be time to peel back the RTV, yes?
You had mentioned a reading of > 90mV across any emitter resistor would not be a good thing. Any other precautions before I attempt this?
Thanks for the morale support. I always thought peeling off RTV and tweaking scary-looking esoteric adjustment pots was only for the big league techs
Yes, I was wondering about that myself.
It was listed on the schematic, but nit where to measure it, right? You have to set it in volts, so why not use volts. It really is the "Idle current ADJUSTMENT". e.g. Adjust to 90 mV
ikewise, with the some of the AC test points. I had gone thru and verified voltages at the DC test points. But when I got to the AC ones I held off because nowhere on the schematic does it say what the input signal should be. So my measured output signal would be meaningless. I've watched a bunch of amp repair vids online and I see the techs typically input a 1KHz sine wave. But at what voltage? This, too, seems to be an unspoken and understood thing.
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