[electrogram]

I think there are minor differences, on the 15 the pentode anodes are fed directly from rect. cathode - via output transformer, and the cap associated with the bass control in the feedback loop has a different value, otherwise pretty much the same. Distortion may be due to grid coupling caps to the pentode sections being leaky. On the 42 these are always the reliable Mullard mustards but on most of the 15s I've seen they are Supamoulds and must be replaced