Very weird symptom on my amp: high pitched beep

[Joost]

Hi everyone,
I have really been enjoying my amp for the past eight months. However, for the past four of those, I have been getting a weird high pitched noise when I play.

The strange thing is that it goes away as soon as I turn down the volume control on the guitar. However, it does so on all of my guitars, so it can't be the guitar. Also, sometimes it goes away on its own

I thought it might've been the AC we installed this summer, which is when I think it started (it didn't do this at first), but it still continues and we haven't had the AC on for months.

What can this be? Any help or tips would be great!!
thank you

[ToneMerc]

Parasitic oscillation maybe.

TM

[Structo]

Cell phone?

[Bob-I]

I had something similar happen, turned out to only happen with one guitar cable. I found it when I disconnected the pedal board to see if it was related. I spent hours troubleshooting the board only to find it was the cable from guitar to board.

If that doesn't help, check your lead dress by shifting wires around with a chopstick as it's making the noise. The character of the noise will change when you hit the area causing the problem.

Good luck.

[Luthierwnc]

My first read was the same as ToneMerc's -- parasitic. Those can be hard to track. I made a gizmo with an alligator clip on one end of a half meter of wire and a .001 630v cap on the other. The other end of the cap has a little hook test lead (the spring loaded kind) Everything is double shrink tubed. There's nothing sacred about .001 but parasitics are very high and a bigger cap will also shunt some of the good signals. Respect the voltage rating.

Clip the clip to ground and just touch various points on the signal path. The cap will bleed the offending frequency to ground (along with some of the high end). I don't have a good methodology for where to start. If the shunt does kill the weird pitch, it doesn't mean you've found the problem. You'll need to go towards the input to see where it starts. Once you've got a general area, check the lead dress and solder connections.

Course, that may not be it but that's a tool you can make from stuff laying around the bench and every shop should have one. sh

[Structo]

Try a 15pf-25pf ceramic on the input grid to ground to kill anything getting in on the input.