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POWER TUBE BIAS
POWER TUBE BIAS:
What is bias? Simply put, it is a circuit
inside the power amplifier section that con-
trols the “idle current” that flows through
the power tubes. Much like the idle speed
on a car. There is an optimum setting
where the engine (amplifier) is running
(idling) fast (hot) enough to keep it from
stalling (distorting) but not too fast (hot)
to cause excessive wear and overheating.
Get it?
WHY DON’T ALL AMPLIFIERS
HAVE BIAS OR IDLE CURRENT AD-
JUSTMENTS?
Most do have some provision for that but
typically involve removing the amp chassis
from the box, exposing you to very danger-
ous high voltage. Special test equipment
and knowledge of amp circuits and tubes
is also needed. Not a skill most musicians
possess and shouldn’t need to.
WHY WOULD I WANT TO ADJUST
THE BIAS?
All power tubes are different. They each
have unique sonic and electrical charac-
teristics. The Armageddon amplifier is
shipped with a quad matched set of 6L6
tubes, but is designed to accept a variety
of different tube types. EL34, 6L6, 5881,
6CA7, 6550, KT66 and KT77 are among
the many possible choices. Because they
are all different, each requires different
bias settings for safety, reliability and opti-
mum performance.
Please read the following instructions on
how to use this cool feature…….
You will need a decent quality digital
voltmeter capable of measuring in the 100
to 200 millivolts DC range. This is a very
basic type of meter available at any elec-
tronic supply house or Radio Shack. They
typically cost anywhere from $10 to $25,
about the cost of one bias adjustment from
your local amp tech.
You will also need a small, flat blade
screwdriver to turn the adjustment controls
that are recessed inside the grommets be-
low each tube label on the rear panel.
1) Turn the amp on, standby switch in
the PLAY position. All controls all the
way down. Turn the meter on and set for
reading DC millivolts. Consult the meter
instructions for how to do this properly.
Since all meters are different, it is extreme-
ly important that you thoroughly under-
stand what you are looking at on the meter
display.
1) Insert the black (negative) test lead
into the panel hole labeled (-) COMMON.
2) Notice there are two identical sections
to the left and right of the common termi-
nal.
3) First step is to insert the red (positive)
meter lead into the left test point (+) hole.
4) With your flat blade screwdriver, turn
the BIAS ADJUST control to obtain a cor-
rect reading from the chart below.
5) Repeat this procedure for the right
tubes.
6) Now allow the amp to warm up for
about 10 minutes and retweak so the read-
ings are within the range of the recom-
mended reading.
115
Vengeance 7-5-1 1
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