bias drift

[nyazzip]

i assembled my DIYTube ST-35 about 10 months ago; the tubes were brand new @ assembly. it has probably operated only for maybe two or three weeks straight since then(@ 24 hours a day). i have been wondering, 1)how often does one need to monitor bias settings on a tube amp, or more specifucally, a DIYTube ST-35?
also, i biased it using line voltage, which at the time averaged around 124VAC( i checked line voltage once this summer on a very hot day and it was back down around 117-119).
anyway, since biasing the st-35 for the first and only time, i got a hold of a variac, which i now use to run a "power strip" which feeds all my tube gear: 2) is it advantageous to try to run the amp at 117 or 120VAC using the variac, if i rebias at that baseline, or is it ok to run it at 124VAC, if biased at 124?
my hunch is that it must be best to try to duplicate the power transformer input spec as closely as possible if you have the luxury of doing so... or am i obsessing?
it is weird, because even on contemporary equipment, the advertised power transformer spec still varies- some say 115VAC, some say 117VAC, and yet others say 120VAC....but i swear sometimes i can hear a difference of 3 or 4 volts on my guitar amps when using a variac

[EWBrown]

The AC primary side of the St35 power trannie gets a little built-in voltage drop from the CL-90 inrush cirrent limiter, so the primary doesn't see the full AC line voltage. I measure around 3 to 4V drop.

The bias settings aren't all that critical, a few mA "drift" up or down is not a real problem. If you want to set it up most "conservatively", set up the bias witn the highest (124VAC) line voltage, then it will run somewhat lower than 35 mA when the line voltage drifts downward during heavy usage periods. The power tubes and power transformer will thank you...

There are constant current regulator schemes, using the LM317, one per tube's cathode. THis works, but the jury is still out on whether this is the best method for best audio quality. CCS works best with full Class A operation, and begins to introduce some distortion in Class AB mode.

Most guitar amps are running at the upper "razor's edge" of their performance envelope, so they would tend to be more sensitive to line voltage variations.

/ed B