Tube Amplifier: where does the preamp end and power-amp begin?

Thread Starter

jaydnul

Joined Apr 2, 2015
151
Hi! I am working on a tube amplifier (guitar amp) and am unfamiliar with tube topologies in general. The basic idea is i want to add an effects loop connection, which for non guitar people is basically rerouting the output of the preamp to some other external electronics before returning the signal back to the power-amp.

I uploaded the schematic with the red highlight where i think this point might be. But I thought the 12ax7 tubes were the preamp tubes, however the output of the preamp is then fed into more 12ax7's (assuming where I highlighted is correct). Am I right in this assumption?

Thanks!
 

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crutschow

Joined Mar 14, 2008
23,141
Yes, V1a and V1b appear to be the preamp tubes.
Taking the loop from the point you show should be okay. The signal there is about 200mVac.

V2a and V2b are used as an amplifier and phase-splitter for the push-pull output.
 

Thread Starter

jaydnul

Joined Apr 2, 2015
151
Cool. So what does a phase splitter do? (It splits the phase but to what end?) It looks like the bottom tube is serving as negative feedback when connected to the negative terminal of the output transformer.
 

crutschow

Joined Mar 14, 2008
23,141
So what does a phase splitter do?
A push-pull output requires the audio signal be split into two signals, one of them shifted 180° in phase from the other (so one can "push" while the other "pulls").
The phase splitter circuit does that with the two cathodes connected together and driving one grid with the signal while AC grounding the other grid.
 
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