JimN wrote: American-built and American-based amplifiers tend to quote 20w+ for a pair of EL84s and 50w for a quartet of the same valves:
Both the Vox AC15 (2xEL84)and AC30 (4xEL84) use cathode bias for the EL84s (sometimes called self bias). This inevitably limits the power available. The bias changes as the current through the valves changes. With a separate fixed negative bias supply, a useful amount of extra power is available.
Vox even used cathode bias for their 4xEL34 AC100, which meant the name was a bit of an exaggeration. I had one and the true power was no more than about 80 watts. By contrast, a Marshall 100 watt head with 4xEL34 and fixed negative bias gives a true 100 watts with a bit to spare. Marshall of course based their design on the Fender amplifiers, which also used fixed bias.


