Swap phaseinverter valve

[Mats A]

Can i swap the phaseinverter valve with another without adjusting bias?

[Anitoli]

Yes. Assuming they are the same tube type such as 12AX7A ECT....

[dukealexan]

I don't know if I would. Are the power tubes and PI tube balanced. If not and the amp uses 2 bias controls then you could be out of wack

[Anitoli]

The PI on dsl's ( i'm assuming that's what the op is referring to) is not connected to the power amps bias circuitry. The two bias pots are there because the power tubes are biased in pairs or one each side, not all 4 at the same time as in other Marshall setups. You can swap out the PI all day long with no ill effects. Actually you might find some tubes are better there than elsewhere in the circuit.

http://www.drtube.com/schematics/marsha ... ctions.gif

[Cipher]

^^ This. The PI has nothing to do with the power tube bias on these amps. DSL's really like an NOS, high gain, unbalanced 12AX7 in the PI slot. Early 1960's Sylvania's & GE's work well in the PI.