John Heath wrote:



** Feedback comes from the 8ohm tap on the output transformer to the cathode of V3A. The phase splitter cannot accept feedback since it uses the "concertina" circuit rather than the more common cathode coupled pair.

Tube matching is normally refers to nominally identical tubes that show the same plate current under class A conditions. For pentode and beam power tubes, actual plate and screen voltages are critical so the best test conditions are those found in the amp itself.



Yes , now I see it. It is marked with a big sign saying FEEDBACK -- , ha.. So who cares about matching tubes if it has feedback. The feedback should compensate for that yes / no ?



** Feedback is used to mainly reduce the 3rd harmonic distortion of a push-pull valve power stage - particularly class AB ones as used in most guitar amps.

What it does not do however, is adjust the bias current balance so there is no net magnetisation in the output transformer. Few amps have an adjustment for this so the valves need to be matched, at least in pairs fitted to each side.

Using poorly matched valves often results in a large increase in 2nd harmonic (ie bad waveform symmetry ) at low frequencies, since the iron core of the transformer goes into saturation much earlier in one polarity.


BTW:

With transistor amps, the same idle current always flows in both output devices cos they are connected in series.



..... Phil