Integrated Amplifiers

Tube amps are not only becoming more sophisticated, but also more user-friendly. Auto biasing keeps the output bottles in optimum fettle – also opening the door to tube rolling – while switching between ultralinear (UL) and triode modes is now often made available 'on the fly'. That's certainly the case with PrimaLuna's EVO 400 tube amp [HFN Apr '20] and also the physically much larger EAT E-Glo i featured on these pages.

Triode operation certainly has a cult fan base. Technically, distortion can be reduced by switching from an UL to triode configuration, but gain is almost halved so, in practice, negative feedback is typically reduced to boost gain at the expense of distortion. That's the case here where distortion is actually slightly higher, not lower, in triode mode [red, with UL in black, inset Graph]. So EAT has 'squared the circle' – there's only 1dB less gain in triode mode (so UL will also sound that bit louder if the volume knob is left untouched), and a 1dB improvement in S/N ratio, but power output drops from 35W to 19W. Levelling the field still further, neither is there a significant difference in output impedance or frequency response between modes. PM