Amp circuit singing!

[Stretto]

I am trying to diagnose a problem with a LSC amplifier. I was using a freq. sweep input and noticed than when I turned the gain up the I was hearing the sound. There is no speakers hooked up and the sound seems to be an exact replica. I did not try other sounds as it seems pretty obvious that something is making reproducing this sound and it is not coincidence.

I guess I could make some type of acoustical amplifier to probe around the circuit to find where it is coming from but besides that is anything I should look for? Maybe this is a natural thing? I suppose there is the possibility of a resistor or cap producing the sound due to some strange effect or it could be the sum of all the minute effects from each component but most likely it is something pretty straight forward(bad tube)?

Any ideas where I should start looking?

 

[screamingdaisy]

The singing is the sound is the sound of electricity flowing through the power tubes. The reason you normally can't hear it is because the sound from the speakers drowns it out.

I hope you had a load box on your speaker output, otherwise you may have just damaged your amp....

 

[Stretto]

Yep, I didn't have a load on it as I forgot a tube amp always requires a load. After a resistor started smoking I quickly shut down the amp. Seems no major damage occurred. Surprised there is fail-safe for a disconnected load. Seems pretty dangerous to not have something just in case. I guess this is one way to make money on repairs/parts. Hopefully there wasn't enough heat to damage the output transformer but as far as I can tell everything sounds ok(the same as it did before at least).

 

[Scary]

take your amp to the shop, before you kill it!

 

[mikey383]

Yep, I didn't have a load on it as I forgot a tube amp always requires a load. After a resistor started smoking I quickly shut down the amp. Seems no major damage occurred. Surprised there is fail-safe for a disconnected load. Seems pretty dangerous to not have something just in case. I guess this is one way to make money on repairs/parts. Hopefully there wasn't enough heat to damage the output transformer but as far as I can tell everything sounds ok(the same as it did before at least).

There is. It's called an output transformer.


I'd check the screen and grid resistors if I were you.