Square wave testing guitar amp OT

[Malcolm Irving]

Another possibility is that the asymmetry is coming from differences between the two halves of the OT primary. They generally have different resistances, but I expect they also have different leakage inductances and inter-winding capacitances.

It would be interesting to swap the two connections between the PI outputs and output tube grids. If the ringing stays at the bottom of the wave – the asymmetry is coming from the OT primary. If the ringing now occurs at the top of the wave – the asymmetry is coming from the PI.

[GainFreak]

Can you measure the level of feedback - eg. constant input level, and difference in output voltage in dB when comparing no FB to full FB.

No FB - 17.2V, with FB - 12.2V which is like 3.3dB.

Another possibility is that the asymmetry is coming from differences between the two halves of the OT primary. They generally have different resistances, but I expect they also have different leakage inductances and inter-winding capacitances

The two halves measure 54/46 Ohms and 2.23H/2.21H of inductance.

This is how some high gain material recorded from another amp's preamp Send looks like when pushed to max through this amp (the oscillation at the bottom is most pronounced when Presence is past 2 o'clock). At lower input levels the signal is oscillation free:





I also did some listening tests and I can't hear anything unusual.

[trobbins]

Is the oscillation frequency the same for those two plots - both for bottom and top sections of 'square' waveform, and is that about the same frequency as shown in the 5kHz squarewave with F/B plot in post #14 (ie. about 33kHz) ?

[GainFreak]

I spent the whole afternoon trying to get rid of that bump at the bottom of the "square" wave. Because I'm not that good in theory and in interpreting scope waves I resorted to the let's-stick-various-pf-caps-around-the-circuit highly sophisticated scientific approach while running in the same time some high gain material (close to square wave as seen from the screenshots) pushing the amp into overdrive and watching if I can kill the ringing at the bottom.
Long story short I managed to kill it 95% with only occasional ringing here and there (all this at max power and power amp into overdrive) by using a 180pF cap across the 47k FB resistor (~19kHz) and one more 330pF cap in parallel with the Volume pot (~8kHz at max) which is shunt type - 220k resistor to 50k pot (for 0dB input sensitivity). Since the the 330p is in parallel with the 50k pot it's actually a variable filter which at low volumes audibly doesn't do anything but at higher volumes where top end is plenty it does it's job. I'll have to do some more listening tests though.