Integrated Amplifiers

Under the bonnet, the P50A turns out to be a moderate-gain (+34.6dB) power amp with a variable-law volume control rather than a fully-fledged preamp. Most integrated amps (pre and power amp combined) offer another 10dB or so extra gain but in practice this is quite unnecessary as +35dB is more than sufficient for a modern 2V line source. Otherwise the P50A is still a multi-stage amp with triode phase-splitter and driver stages feeding a push-pull pair of KT90 pentodes or, more correctly, 'Kinkless Tetrodes'.

These are claimed to operate in Class A which, if we are being very strict about the definition, means that the plate current is maintained in both tubes across the full positive- and negative-going audio cycle and over the full power output of the amplifier. With no audio signal present this means the tubes are dissipating this maximum bias current as heat, and the P50A does get very hot indeed – I measured over 70oC on the top plate, which is too hot to touch and means the amp needs to be sited in a well-ventilated rack!

Maximum power output may also be reduced in a Class A tube amp if the plate voltage is dialled back to ensure optimum symmetry between the negativeand positive-going tubes. Certainly I have measured greater than 25W/8ohm from a pair of KT90s [the Ming Da Dynasty Cadenza achieved 70W/8ohm under the same conditions, see HFN Oct '16], suggesting Pegaso is indeed walking the plate current/voltage tightrope with appropriate caution. Finally, a Class A amp's AC consumption will be high and unchanged at low and high power – in practice the P50A idles at 250W but draws a higher 280W at full output. PM