What do these amps have in common?

Fender Princeton and Princeton Reverb, blackface and silverface
Fender Super Champ
Peavey Classic series
Peavey Delta Blues
many classic Orange models

Seems to me that they have a sweet clean sound which develops into a good power amp crunch when you turn them up.

The other thing they have in common is an unusual phase inverter design. The phase inverter is the circuit stage that comes just before the power valves. Its function is to split the signal waveform into two halves for the paired power valves to handle in ‘push-pull’ – because this is a very efficient way of using valves.

The majority of amps use what is called a ‘long-tailed pair’ design of phase inverter, typically using the two halves of a standard preamp valve (ECC83, 82 and 81 valves have two separate triodes inside). This design has the advantage of good balance and some inherent gain.

The above amps, however, use a design which uses only one triode – one half of an ECC83/81. The split halves of the waveform are tapped from the ‘top’ and ‘bottom’ (plate and cathode) of the triode. It is called a cathodyne phase invertor, or more memorably a ‘concertina splitter’. This design has no inherent gain. It is well-balanced in the signals it puts out under normal conditions, but the impedances of each half are different and when you push it a bit this produces imbalance. And that imbalance seems to sound good. It produces the kind of harmonic distortion that gives that nice up-an-octave singing sustain.

If you get the chance, give one of the above amp types a try – turn it up and see if you agree with me.