POWER TUBE BIAS
WHAT IS BIAS?
Simply put, it is a circuit inside the power
amplifier section that controls the “idle cur-
rent” that flows through the power tubes.
Much like the idle speed on a car. There is
an optimum setting where the engine (am-
plifier) 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
ADJUSTMENTS?
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 character-
istics. The Vengeance amplifier is shipped
with a matched quartet of EL34 tubes, but
is designed to accept a variety of differ-
ent 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 optimum
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 con-
trols that are recessed inside the grommets
below each tube label on the rear panel.
1) Turn the amp on, standby switch
in the 120W/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 extremely important that you thoroughly
understand 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 sec-
tions to the left and right of the common
terminal labeled 1&4 and 2&3. These
correspond to the power tube numbers
designated on the rear panel.
3) First step is to insert the red (posi-
tive) meter lead into the left test point (+)
hole.
4) With your flat blade screwdriver,
turn the BIAS ADJUST control to obtain a
correct reading from the chart below.
5) Repeat this procedure for the other
pair of tubes.
6) Now allow the amp to warm up
for about 10 minutes and retweak so the
readings are within the range of the recom-
mended reading.
115
Vengeance 7-5-1 1
S
er
ia
l n
u
m
b
er
s
ti
ck
er
1
2
3 4 5
6
7
8
9
10
11
12 13 14
17
18
19
20
21
22
23
24
25
26
15
16
1
2
3
4
5
6
7
8
9
a
b
c
d
12
