I did not get as much time as I could have used last night but I did manage to run the sine wave through the unit and observe the voltage on the 1uf cap with respect to common.Was this with continuous sinewave input, and what input signal level and R1 setting were you using?
What were the corresponding generated control voltages (top of the 1uF cap)?
Does that come down to the same as the 1uF being on the wrong side of the attack pot (with pad resistor)? If so, then the 6AL5 anodes would be connected directly to the 1uF, which could produce very wrong results.Looking the photos just now I think I have wiring error on the attack pot...I think its wired after the 1uf not before it? I will confirm tonight.
From the picture I believe the 1uF is connected directly to pins 2and 7....which I will confirm tonight and fix. Not sure what I was thinking when that happened?Does that come down to the same as the 1uF being on the wrong side of the attack pot (with pad resistor)? If so, then the 6AL5 anodes would be connected directly to the 1uF, which could produce very wrong results.
Thanks should be C8 and C9...You show the input signal (100Hz, 1V rms), but do you know what waveform you have at the 6AL5 side of the 100nF feedback caps (both shown as C8)? It should be the full output transformer primary amplitude (one side of it) offset by the threshold pot voltage.
Yes I think it's getting closer, but if I have the cap wired the on wrong side of the attack pot, that could explain some of my grief.A control voltage of -12V d.c. (or rms!) could well be about right for that input, as the 6386 cathode voltage (and hence current through the valve) is about 20% of the quiescent. The curves for the 6386 show something of the order of a 10-fold decrease in transconductance going from a grid-cathode volts of -2 to -12.
The output measurement is right on the the terminal strip, so on the secondary of the output transformer after H pad attenuator.. The input measurement was taken right on the output of the function generator.When I first saw the 100Hz scope picture, I was puzzled by the apparent phase advance shown in the output vs. the input trace. Then I saw the 5V offset between them, and the distortion in the output waveform. I'm assuming you are taking the output waveform from the secondary of the output transformer, so where's the d.c. offset coming from?
I can do a frequency sweep on my scope. Coupled with the function generator it has built in BODE function. That would highlight the harmonics, but I don't have proper audio analyzer like a Audio Precision.Have you got any way of measuring distortion?
That apart, it would be good to see a long timebase scope picture of what the control volts do when the sinewave input amplitude is changed quickly from a low value to a high value, and a second picture doing the inverse. Medium settings on attack and release.
I don't know what you mean by the "ac signal". The meter should show the response of the Audimax to whatever signal is being applied and not the actual amplitude of the input signal. The d.c. offset is there to set the quiescent position to the 0dB mark on the meter. The deviations about that should be the amount of applied expansion or compression around that quiescent point.The meter is thing that bothers me, the ac signal is so small compared to the dc offset. The needle doesn‘t move with with audio. I am thinking about somehow amplifing the ratio of the ac signal to the dc signal on the meter ...