Transformer based Phase inverter for a small PP amp

[trevorus]

What would be the advantages/disadvantages of using a transformer for a phase inverter, like in a balanced/unbalanced transformer for feeding 2 EL84's in push pull? I know power saving on the PT would be one advantage, but is there any real advantage beyond that?

[Jazz P Bass]

Transformer PI

Other than costing more, weighing more & being bigger, a center tapped phase inverter works very well.
You need a well made transformer. Both sides should have relatively equal output.
You may have to design in a compensatation circuit for the correct frequency response.
Hey, it is a transformer after all.
John G

[octal]

I've built a 2X6v6 amp with a transformer PI- my guitarist friend digs the sound.

Pluses: Very good balance/match between the two outputs.
Simplicity/parts count.
Can drive the tubes class AB2 for more output if desired.
No bias shift issues.

Minuses: PI transformer can pick up hum from PT, feedback from OT.
Really need a driver tube which can push some current- a 12AU7 or 6CG7 is good.
Slightly more expensive and heavy.
Difficult or impossible to apply negative feedback across entire power amp stage due to phase shifts in the PI x-mer. (I don't care for NFB, so it's a non issue for me.)
Driver stage is usually single ended and needs a well filtered power supply.

You can check out the Gibson GA-20 RVT and the Fender Musicmaster Bass amo (I think that's the right model on the Fender) for examples of commercial amps using transformer PIs.

Good luck with your project!