• Phase inverters

I've run across 7 names for different phase inverter circuits, but have only found 4 in the guitar amp schematics that I've looked at. The purpose of a phase inverter is to provide 2 signals that are 180 degrees out of phase with each other. These signals drive each half of a push-pull power tube pair.

Longtail pair

Longtail pair

The long-tailed pair is perhaps the most common, and was used in many Fender, Marshall, and Vox models. It uses 2 tube sections, and can provide gain. Randall Aiken has two great articles on in the Tech Info-Advanced section of his site if you want a thorough description and info on design.Some notes on the above diagram--it's a silverface Fender design. (I've got a bit on the difference from a blackface design.) Although both the plate resistors are the same here, they are often different--82K/100K was a common combination (upper/lower), and some say that 91K/100K is a better match. The difference is an attempt to balance the outputs of both halves.Negative feedback can be part of this circuit, as well as a presence control.

Cathodyne or split load

The split load or cathodyne phase inverter only uses one tube stage, and has unity (or less) gain. It's probably the second most common PI I've found. It's used in Fender Princetons and some Gibson amp models.Dan Torres makes a point about making the values of the plate (56K) resistor and the total cathode resistance (56K + 1K = 57K) equal. Others say the tolerance of the resistors is a bigger factor, don't bother. I haven't tried this to have an opinion.Gibson amp style

Transformer

Interstage transformers can be used to invert the phase. The Fender Musicmaster and an Epiphone EA-15RVT amp are the only ones I've seen with this. I've read that the cost of the transformer and a phasing problem when negative feedback is used are reasons it's not too common.

This one had me going for a minute. V5A is a split load phase inverter, and V6A and V6B are amplifier stages to provide more drive to the power tubes. This is a Gibson GA45-RVT amp. This is the first circuit that I've come across that has a gain stage after the PI. This provides some possibilities if you consider adding a master volume.

Gibson amps commonly used this circuit, although I don't know its name. It's one of the easier ones to understand how it works. A voltage divider (220K and 6.8K resistors) is used to feed a portion of V1's plate signal to the grid of V2. This makes the output of V2's plate out of phase with V1's signal. Gibson amps are reputed to sound good, although this circuit is not supposed to be one of the better ones from what I've read. I don't think I'd use this in one of my homebrews.