The "ultra-linear" configuration was developed by Mullard to sell their EL34 pentode. It generally differs from a typical SET in that the SET usually has two tubes per channel - driver (eg 6SN7, 6922) and output (eg 2A3, 45, 300b). An ul amp will usually have four tubes per channel - an input pentode (EF86), a triode phase splitter (ECC83) and two output EL34 tubes - and it splits the signal. This is a push-pull type confuration whereas the SET is a straight-thru amplifier. The difference is that the SET sees fewer signal manipulation than the ul, similar to Class A transistor vs Class A/B push-pull ss amps. The "ultra-linear" name does not describe signal linearity but instead describes the way the EL34's are connected to the transformer (both the anode and the screen grid are connected - to stabilize the whole setup).
The trade off is power for the signal linearity of the SET (I agree with Marakanetz, SET signal path is usually shorter). Of course, it does not mean one is better than the other - there are excellent ultra-linear designs (Mullard 5-20) that can give the best of both worlds.
The trade off is power for the signal linearity of the SET (I agree with Marakanetz, SET signal path is usually shorter). Of course, it does not mean one is better than the other - there are excellent ultra-linear designs (Mullard 5-20) that can give the best of both worlds.