Princeton Reverb circuit refinements

[stratomaster]

Yes. I run a 1M input resistor and a 1.2k bias resistor. Its a much smoother overall sound to my ear, but I don't have an A/B comparison.

[shooter]

a 'side kink' that moves up and down the wave with volume setting

those can be "transients", an AC signal riding on your fundamental Frequency
does your scope have a FFT/spectrometer mode?
IF so;
look for a spike that comes n goes, usually it's a harmonic of the fundamental you're using for testing

[johnfromcyrene]

Yes. I run a 1M input resistor and a 1.2k bias resistor. Its a much smoother overall sound to my ear, but I don't have an A/B comparison.

do you have a pi grid stop resistor?

[tristanc]

Can you temporarily solder a 3.3k resistor in parallel with the 1.5k cathode resistor on your cathodyne?

There's a thread on TDPRI where the role of this resistor was investigated thoroughly. The conclusion was that the 5E3 value of 1k gave a much better PI balance than the 1.5k used on the 1164.

Thanks - I think I've just found that thread. In the spirit of trying to alter the bias point, I quickly swapped in a 12AT7. Messes up the tremolo but the distortion / fuzz is still there. I'll have a think about what that could mean. Next time I've the chassis out I'll clip in the 3.3k and report back.

a 'side kink' that moves up and down the wave with volume setting

those can be "transients", an AC signal riding on your fundamental Frequency
does your scope have a FFT/spectrometer mode?
IF so; look for a spike that comes n goes, usually it's a harmonic of the fundamental you're using for testing

The FFTs are in the PDF along with some harmonic values. I've pulled the graph out (attached).

I also spotted another thread on here from a few years ago. Still wading through it, but it was suggested issues with the OT could explain things like I'm hearing. I may have a spare OT so temporarily hooking that up may be another option to try.

[shooter]

looks like there's a low freq oscillation riding on your fundamental or something is breaking down with signal load.  since it's affected by volume, it's probably creeping in before the PA section.