What would cause an amp to get distorted on only one note?

[Mastervolume]

What would cause an amp to get distorted on only one note?

It seems that when I hit an A I get a funky distortion almost like a bad ground. It happens on different cabs and guitars so I know it is the amp.

the amp is a 15W cathode biased EL84.

[mlp-mx6]

120Hz hum is close to Bb (A#) so it could be a filter and/or ground issue.

[d2camero]

I had this on a few amps of my own design. They were always close to oscillation, so just a touch of the wrong lead dress and then hitting a low note would cause an oscillation at the leading edge of the waveform. To a hell of a long time to track down, but boiled down to a combination of too much gain going into the power tubes, and lead dress for grid wires need a little clean up.

[Phil_S]

Try changing the power tubes. I have a set that give a buzz on A. Loose plates?

[Andy Le Blanc]

I completed a repair on an old princeton reverb that was brought out of storage in the worst conditions....after a some obvious fixes and eviction of insects it did a very similar problem....the fix was to clean the input jack..i saw another with that issue where the cause was one tube pin that was slightly thinner than the others new out of the box.....