Nope. You can bias the amp hotter or warmer, but it will work equally fine with Mullards or JJs. The real issue is the plate and grid resistors, but adjusting these doesn't affect the bias. The stock 100 ohm and 1.5K values should be bumped up to 470 ohm and 2.2K for longer tube life, particularly with modern tubes (not as rugged as Mullards, which barely clung to life in an AC30 anyway). This should be done for Mullards, too, as the amp will run cooler with longer tube life with no tonal tradeoff. It was, frankly, a mistake made by Jennings back in the day.µ¿ z3®ø;3708697 said:well yeah, of course.
however, the mullard quad that i have are very robust and can take a very high bias and sound spectacular when used that way. having the amp tweaked to provide a higher bias would move the amp, marginally, in a direction that i would prefer. now, it's likely that if i were to put a modern quad of EL84s in the hayseed after having it biased for those mullards, that the modern valves would not last nearly as long and may even suffer failure.