"William Sommerwerck"

Tubes deteriorate slowly, so there's no point in continuously checking the
tube bias. It should be sufficient when the amplifier is turned on, or the
user initiates a rebias cycle.

** Oh, but tubes often DO draw more current as they get hotter and this is a
sample to sample thing too.

PLUS the AC supply voltage has a big effect as well.

Imagine a band using a basic generator or on board a party boat - the
Bugera servo could be lethal.


And, as Phil points out, how do you keep the biasing system from being
"confused" by the audio signal?

** Well, that one is just quietly swept under the carpet .............


I'm not an expert on tube output-stage design. But it should be obvious
that the correct idle current for one tube type is not necessarily optimum
for another. The promo indirectly acknowledges this by pointing out the
mixed tubes will produce "interesting" harmonic and IM distortion. And if
the tubes' characteristics are sufficiently different, you might even wind
up with a "healthy" (not!) level of DC through the output transformer's
primary.

** Yep, poor symmetry in a push-pull stage does just that.

The Bugera servo only sets the DC balance condition at idle, which is gonna
be OK at higher current levels ONLY if the tubes are all closely similar.

Same type number, maker and age etc.

I tried one EL34 in place of a 5881 and the DSP servo bias worked as
intended - but the output sine wave became was asymmetrical at high
levels.

But a pair of EL34s, one on each side of the push pull tranny, would have
been pretty much OK.



.... Phil