Pre/Power Amplifiers

With praise for the 'sweet sound' of pure triode amplification reaching near-religious zeal in some quarters, some of the smarter tube amp brands have been offering switchable ultralinear (UL)/triode modes on their integrated and power amplifiers. With its 'AutoBias' facility and ability to accommodate almost any choice of pentode or tetrode, PrimaLuna's EVO series is one of the most flexible in this regard. Historically, and partly by design, the EL34s fitted here are perfectly suited to triode adaption – the control and suppressor grids are disconnected (or linked to the cathode) while the screen grid is coupled to the anode (plate) typically via a 100ohm resistor. So the screen grid no longer has a fixed DC potential but swings with the audio on the anode.

Distortion is reduced in this triode configuration, but gain is almost halved so, in practice, negative feedback is typically reduced to boost the gain at the expense of distortion. PrimaLuna walks a sensible compromise – there's 2dB less gain, but also 2dB less noise, in the EVO 400's triode mode and maximum power is 45W in place of UL's 75W. So direct A/B comparisons may fool your ears if you do not tweak up the volume control when switching to triode... The reduction in output impedance from 4.8ohm (UL) to 3.9ohm in triode mode confers better bass control and a more uniform frequency response. As for distortion, within the more limited dynamic range of triode mode it still has a slight advantage offering, at best, a x1.5 reduction in THD [black, with UL in red, inset Graph]. PM