Plate characteristic of triode.

[loudthud]

Loudthud, a change in gain from 57.5 to 55 is only about 0.5dB, I don't believe it is an audible effect.

I calculated 0.39dB, but remember that's only for a single stage. The B+ and amount of sag is just something I pulled out of the air. The experiment I want to perform is to measure one input channel of a 5F6A with the volume control all the way down and play through the other channel. The re-issue amps have more filtering than the vintage amps, that will probably change the results.

Other factors that contribute to the compression effect are the B+ sag effect on clipping level and bias shift throughout the amp.