On Sun, 15 Mar 2015, Mike Wald wrote:
> Hello Ron,
> I hope you could answer a couple of questions for me concerning the Alembic
> Zener Bias Regulation?
> First, why was it used?
Stabilize the bias voltage on Fender amps and McIntosh amps. not important
for everyday use on well-regulated power line voltages. but on generators
or some overloaded stage circuits, if the line voltage goes down below 90VAC
or less, then you overheat the tubes and they will come close to failing, because
the bias voltage is reduced and if you check the tube specifications for
6L6 then you see that it's already operating out of the "safe operating area"
in the Fender amplifier anyway, so you're just pushing it with lower bias
voltage. As you know this amplifier topology is called "fixed" bias as
opposed to cathode bias or automatic bias and you can get more power out
in the fixed bias mode. But as the negative bias voltage on the tube
is reduced, the tubes overheat and instead of the plates of the output tubes
glowing dull orange (only seen looking in a dark room), they glow orange
and are visible in a lighted room.
things were much worse with the MC3500 amplifiers. they would overheat
when the line voltage went down to even 105 volts and if you were outdoors
in the sun on a hot day, the glass envelopes of the output tubes would
melt and a small hole dimpled in the side of the tube and the vacuum was
> Second, is it still a relevant modification?
the newer design output tubes are more rugged (as the main market is for
this slightly beyond the ratings use) so not quite as critical, and not
needed at all if you don't expect to encounter low line voltages.
> And lastly anything more you care to elaborate on concerning this mod? Like
> i have heard it may have incorporated a potentiometer, but I was under the
> impression that this mod was developed and used to eliminate any possible
> movement of the stock pot.
no, just for the stabilization of the current drawn from low line voltage.
the pots didn't change from vibration or anything like that. some people
would set the tubes by the plate color, so if you got tubes out on the road
and wanted to make them do all they could do with those tubes, then you
could set the bias visually, turning the bias down (less negative - toward
zero volts) until they glowed in the dark, and then raising the bias (more
negative) until the color went away, or perhaps just barely perceptible
plate glow in a totally dark room. probably not needed with more modern
on the MC3500, it was very bad and i tried to convince the designer at
McIntosh to change the circuit for future production with no luck. but
we had to do it for using them on the road.