Hello Gurus!

I've completed my amplifier testing (EL34 based) and noted that the voltage
supply drops from 480VDC to 440VDC over a period of 1.5hours. Of course the
power transformer was taking quite long to get hot (still touchable) and
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.

Is this normal for a tube amplifier to be behaving this way when the output
tranformer gets hot?

Is it possible to keep everything stable using a LM317 floating voltage
regulator in place of a choke?

Regards,
Mark