Laney LC15R - have I got sympathetic oscillation ?

[chazpope]

no they don't, because had I moved the bias pot. I meant that even after swapping I could not get a closer I ratio than 26/18. But do you also get a different to ground voltage at P1/P2, not sure if I got that right? I am still to try it at full blast (it was late last night) and compare. Will be looking for valves too, it is academic but do some valve amps have a balance adj ?

[Mick Bailey]

You will get different voltages to ground at P1 and P2 because the voltage drop across each winding is not usually identical.

Plenty of older Fenders have a bias balance adjustment which balances the current draw between each half of the OT primary. You can hear when the output is balanced - the power amp hum drops. Not to be confused with the heater hum balance, which is also often fitted to the same amps.