Mark wrote:

Its a push-pull amplifier

I see. So the cathodes of two EL34's are connected together, and
is grounded through a resistor, over which you notice the change
of voltage from 0.75[V] to 0.65[V] after a while.

Yes there's a grid bias (negative) voltage applied. The cathod
resistor is connected to ground and it was biased at 0.75V
(to determine current through the tube).
Nope the resistor doesn't get hot at all.

Sounds like the operating current for the output stage actually
decreased after 1.5 hours, mean while the supply voltage also
dropped from 480 to 440v.

I would then check the followings over the 1.5 hours
of operation as the plate supply voltage changes.

o If the primary line voltage(AC 110v or whatever) measured at
the power transformer changes.
o If the secondary high voltage from the power transformer
changes.
o If you are using rectifier tube(s), see if the voltage
drop(s) across your rectifier tube(s) change.

Atsunori