vintage Ampeg SVT - 12AU7 in place of 12BH7

[Reidk]

That section serves as a cathodyne (aka concertina) PI. The result of using a different tube would be assymetry in the output feeding the 12BH7 predrivers, but VR3, the "balance" control, can be adjusted to compensate. See under "Calibration Procedure". No one ever seems to mention that when they say you can sub a 12AX7 without changes.

Reid

PS I've never owned or worked on an SVT, but this seems obvious from looking at the circuit.

[boroman]

That section serves as a cathodyne (aka concertina) PI. The result of using a different tube would be assymetry in the output feeding the 12BH7 predrivers, but VR3, the "balance" control, can be adjusted to compensate. See under "Calibration Procedure". No one ever seems to mention that when they say you can sub a 12AX7 without changes.

Reid

PS I've never owned or worked on an SVT, but this seems obvious from looking at the circuit.

They are actually saying in a service bulletin that you CAN replace 12DW7 with 12AX7 without any changes.

[g1]

They are actually saying in a service bulletin that you CAN replace 12DW7 with 12AX7 without any changes.

Without any circuit changes, yes (only for power amp V1 position). But the calibration must still be completed, which was part of any servicing of the SVT.

[Helmholtz]

The balance of a cathodyne PI only depends on its plate and cathode resistors - not on the tube.

[boroman]

Without any circuit changes, yes (only for power amp V1 position). But the calibration must still be completed, which was part of any servicing of the SVT.

Yes, of course, so replacing 12DW7 with AX7 i poweramp (V1) poistion + bias + side balance calibration. Okay - that will lead to THE SAME sound? I mean, same clean power, same characteristics? That was my inital question.

[g1]

Yes, it should still sound the same. If it changed the sound, they would have recommended some circuit changes like they did for the preamp section.