Modded Bandmaster Clone Bullhorn Noise

[Thisismyname]

A modded AB763 Bandmaster I built for someone recently had a very loud bullhorn-type noise occur at gig level. When testing it myself, it only occurs at slightly higher volume, about 9 o'clock, so it seems relative to gain.
The tone breaks up a little bit if I hit a chunky chord. If I do that a few times, it eventually starts to oscillate on a very loud note, F#.
It is not related to the negative feedback loop because it occurs with and without NFB. I've previously turned the amp up to about 2 o'clock without any problems.

Another interesting problem that appeared at the same time is that the volume control doesn't fully eliminate sound. It doesn't crackle or anything, it's like the control won't go any lower than 1-2 on the volume control.

Since I can't test it in my apartment and the nearest place I can test it is 20 miles away and I have to navigate 4 flights of stairs round trip, I haven't done any testing on my bench. The amp looks ok inside.
One of the new JJ 6L6s went microphonic since it last left my bench.

Does anybody have any ideas what it could be?

[shooter]

a very loud bullhorn-type noise

speaker flabbing out?
happens with "both" channels?


which pot are you calling Vol?

[Esquirefreak]

Could it possibly be a "ringing" OT? Just thinking out loud.

/Max

[PRR]

> I can't test it in my apartment

They don't test rockets in outer space, either.

Get a dummy load, attenuator, monitor speaker. Since this is a ~~30W amp, a 50W resistor will dummy for many minutes. A 1K 2W resistor in series with a moderate size speaker will make a small sound when the amp is roaring.

This won't bullhorn if the cause is acoustic feedback howl-round from the full-strength main speaker back into the chassis.

[Thisismyname]

By using an attenuator (with a headphone jack) I found no problem with the amp at any level of gain, so it's probably some acoustic vibration that somehow produces it's own positive feedback or something.

I know it can't be the speaker since it had a different speaker in it when it first occured and I got the same result after swapping to something else.

Any clues?

[Thisismyname]

Anybody have any ideas of what it could be or could not be?

Acoustic vibration from playing loud that induces a tone into the circuit that produces positive feedback and sustains itself without any further guitar input. The oscillating pitch is a lower F#, either 87 Hz or 175 Hz, don't remember what octave.

[shooter]

acoustic vibration

so put the speaker cab on a guitar cable, set it 15 feet away from the bench where the amp sits  move the cab 45 degrees, repeat.

[PRR]

If it is acoustic feedback, ANY part can be the "microphone". Tubes are notorious, but caps and resistors can be good microphones; also bad joints.

Lots of careful chopsticking. Everything is a little microphonic, but if you can change the howl by pressing on something, there's a clue.

Don't get  shocked.

[Thisismyname]

I found the problem. It was not acoustic feedback.
The + lead for the preamp filter cap broke right at the turret, causing an intermittent connection at high volumes. I replaced the cap and will test at gig volume tomorrow.

Could anything be damaged from intermittent filtering in the preamp?

[sluckey]

Probably not.