Bias adjustment on Bassman 100

[fredricken]

hi! Has anybody had thunder noises in their Bassman 100 and found out what it was? I have a early 70s silverface bassman 100 and when it gets warm it starts making 'thunder' like noise. Could it be the bias of the power tubes? I'm trying to find out where those bias things are but I can't find them

My first post here but its a great forum! so thanks for any advice on the subject.

Fredrik

[Enzo]

Isolate the problem.

When it makes the noise, do ANY of the controls have ANY effect on it? That is, can you turn the noise down with a volume control or affect the tone of the noise with the EQ?

No, it won;t be the bias.

Tap on each tube with your fingernail of a pencil or something. Also wiggle each tube in its socket. ANy response or noise?

[fredricken]

thanks a lot for reply. yes if I turn the volume control down the rumbling/thunder noise goes away. no noise when I tap the tubes.

[Enzo]

Then the noise is coming from before the volume control. Most likely you have a noisy resistor.