F.H.Chan wrote:

after 1.5hours, the supply voltage remains at 440VDC. When the supply
voltage drops, the bias voltage across the cathod resistor for the EL34
drops from 0.75VDC to 0.65VDC and stays there.

Hi, quick questions.
o What is the topology of your output circuit,
single, push-pull, triode-connection or pentode-connection?
o What is the exact biasing method for the EL34?
o Is your cathode register very hot?

I was kinda confused for the cathode resistor voltage of 0.75-0.65[V].
If this is the actual bias voltage, it is too low for EL34 with plate
supply voltage of over 400[V]. So I assume that you are also applying
grid bias voltage somehow, but I don't know for sure.

Atsunori