ST-70 Bias Drift

Group,
I have a question on bias voltage that I have always been curious about, but it has never bothered me enough to ask about it, until now. I have two ST-70s, both with VTA boards. One has 12AU7 tubes, and the other has the octal board (6SN7 tubes). Both exhibit a slight bias drift downward as they warm up. For example, if I turn an amp on and wait 5 minutes, then take a bias voltage reading, it will read .42VDC (which is where I set it). Then as the amp operates for say two hours the bias voltage will drift downwards to .40VDC, which is the target bias voltage for EL34 tubes. My dumb question is, is this normal or should I be looking for a problem? I thank you in advance for your input.

Chuck

justin_122 wrote:Group,
I have a question on bias voltage that I have always been curious about, but it has never bothered me enough to ask about it, until now.  I have two ST-70s, both with VTA boards.  One has 12AU7 tubes, and the other has the octal board (6SN7 tubes).  Both exhibit a slight bias drift downward as they warm up.  For example, if I turn an amp on and wait 5 minutes, then take a bias voltage reading, it will read .42VDC (which is where I set it).  Then as the amp operates for say two hours the bias voltage will drift downwards to .40VDC, which is the target bias voltage for EL34 tubes.  My dumb question is, is this normal or should I be looking for a problem?  I thank you in advance for your input.  

Chuck

It seems perfectly reasonable. Don't worry !

most amps I've seen the bias actually drifts up slightly when warm, which is why on a new amp you want to set the bias, then check it about half hour later.
Then it should be fine for a couple weeks. Unless you home AC power drifts more than 1v per day. Mine is dead steady at 122-123vac all the time.
And I run my amps with a variac set at 117vac