Building a push-pull - what's different?

[bluefoxicy]

What's different in a push-pull amp? I'm assuming the preamp and power amp stages work the same way -- resistors here, caps there, and signal. It can't be that different...

My best guess, going on absolutely nothing, is somewhere there's a circuit that somehow splits the upper part of the incoming signal with the lower part and sends them off. I don't know how to do this; maybe run the signal to two imbalanced triodes? A 12AX7 dual triode with one triode biased way low (so it overdrives the entire bottom half of the wave to flat) and one biased way too high (so it overdrives the entire top half of the wave to flat)?

Does that set the mid point too so that the center of the (now deformed) output wave is where the would-be center of the input wave should land a properly amplified wave? If so I can send that to two amps and mix their outputs...

[Enzo]

The phase inverter, also called phase splitter, stage is usually the tube just before the power tubes. It sends th same signal but at opposite polarity to the two sides of the push pull.

Get a copy of the RCA tube manual, there are very nice tutorial chapters in the front on how circuits work, plus you could then look up the specs on any tube. Reprints of RCA RC-30 are available from many sources. The older RC-10 does not include a lot of the newer more modern tube we sometimes use.

As to how to do it, look at the schematic for one of the many amps that use it. Most Fender amps, for example.

[drlowlow]

the simplest method for this (used on many early push pull amps) is a center tapped secondary transformer. The next simplest is a concentina splitter as found on the 5e3 where one tap is taken from plate and the other from the cathode to get the two respective phases. There are several other more ellaberate options found on different amps. The different methods will have a marked effect on the tone.

The output signals from the phase splitter should be class A. Some executions of these circuits might cause one phase to have a larger signal. I suppose it would be possible to send to multiple amps but having enough control to make that work well might be difficult.

[d95err]

What's different in a push-pull amp? I'm assuming the preamp and power amp stages work the same way -- resistors here, caps there, and signal. It can't be that different...

My best guess, going on absolutely nothing, is somewhere there's a circuit that somehow splits the upper part of the incoming signal with the lower part and sends them off. I don't know how to do this; maybe run the signal to two imbalanced triodes? A 12AX7 dual triode with one triode biased way low (so it overdrives the entire bottom half of the wave to flat) and one biased way too high (so it overdrives the entire top half of the wave to flat)?

Does that set the mid point too so that the center of the (now deformed) output wave is where the would-be center of the input wave should land a properly amplified wave? If so I can send that to two amps and mix their outputs...

No need for guessing. Probably more than 95% of all tube guitar amps are push-pull. Just go over to www.schematicheaven.com and checkout some schematics.

This site is also good for understanding the different parts of a guitar amp:
http://www.valvewizard.co.uk/

[MWJB]

"the simplest method for this (used on many early push pull amps) is a center tapped secondary transformer. The next simplest is a concentina splitter as found on the 5e3 where one tap is taken from plate and the other from the cathode to get the two respective phases." Simpler still is "self-split" push pull, not commonly used in guitar amps but there are some out there.