I think the ST70's EL34s are biased at around 50mA per tube at a plate voltage of around 410V. You can find these at the ST70's manual.
Most PP cathode bias output sections I've seen have their EL34s at around 390V with a bias current of 70mA per tube, while fixed bias have the opposite, higher plate voltage and lower bias currents. This probably shows the inherent efficiency of fixed bias topologies in Class AB1.
Anyway, I doubt you'll find the plate/supply voltage stamped at the back of the amp, the manual of the amp may have this though. Opening the amp may answer your question, though care must be practiced, the caps may still be charged and maybe enough to injure you. Safety first dude.
Cheers,
JojoD