• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Oscillation in tube amps

For [relatively] wideband FM modulation, Slope detection works fairly well.
Not to mention that there can be some very low % AM modulation out of an FM transmitter.

Your De-modulation Results May Vary.
A lot of FM stations take advantage of the extra 9% modulation allowance by carrying Additional content at 67kHz and 92kHz. RDS happens at 57Khz.


Paid Member
2020-02-10 2:22 am
Long Island
Can oscillation sound something like cicadas but slightly more empty glass bottle like in the distance or would that more likely be a blown tweeter? Have been hearing this from my right channel on and off and seams like its more pronounced when my amp (pp) has been on for an hour or two. Guess i need to feed some tones and watch whats happening on the scope. I guess a faster check would be to simply swap right and left.. ugh.. guess its time to get the se 300b off the bench, trying to track an imbalance problem with that one..
Wish i was as capable as many here, i enjoy the learning but now feel pressure to resolve asap... and, im also wondering if a capacitor in my xover could be an issue..

Any advice on a methodical search path would be greatly appreciated!
Yes, swap the L and R speaker leads at the amplifier L and R outputs, to see if there is anything related to a speaker problem.

If the problem is still in the same amplifier channel, then swap the L and R signal source outputs to the amplifier inputs to be sure the signal source is not the problem.

By then, you will be well on your way to finding the source of the problem . . .
Signal source,
or Speaker.