TOURMASTER
TM
trols all the way down.
POWER gRID slide switches to
100 watts and the FULL/HALF
power switch to FULL power. 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.
a)
Insert the black (negative)
test lead into the panel hole
labeled gROUND.
b)
Insert the red (positive)
test lead into the panel hole
labeled HOT.
c)
with your flat blade screw
driver, turn the BIAS ADJUST
control to obtain a
reading between 115mVDC
and 130mVDC.
IMPORTANT:
The bias adjust-
ment control is active ONLY with
the POWER gRID switches in the
100 watt settings and the
FULL/HALF power switch in the
FULL position.
4)
When making these measure-
ments, be aware you are seeing
the sum current of all four power
tubes. It is recommended that pow-
er tubes be replaced in matched
sets. Let’s say you have a set of
new tubes and you set the BIAS
control for a reading of 120mVDC.
If all the tubes are matched and
drawing the same current, each
one is idling at 30mADC, which is
quite comfortable. Now consider
if your tubes are very mismatched.
One tube could be drawing a
very low 10mADC for example,
while another might be idling at
50mADC, which will not make for
a long lasting, happy tube. In
addition, your tone will suffer
because your amp can only per-
form as well as the weakest link.
It’s kind of like having one low tire
on your car. Of course it will still
drive but the performance will be
adversely affected.
Displays differ from one meter to
the next. Some may indicate, for
example, 120.0 for 120 millivolts.
Others may show .120 for 120 mil-
livolts. Knowing how your meter
works if of the utmost importance.
You should always check the bias
readings whenever you replace
output tubes and readjust if need-
ed.
ADVANCED THEORY
(FOR THOSE WHO CARE):
Those authorized service techni-
cians with electronic knowledge
may notice we are referring to
current draw but are making
measurements in millivolts. Ohms
law states that I=E/R or current
(I) equals voltage (E) divided by
resistance (R). Inside the amp is a
one ohm resistor in the cathodes of
the output tubes. The external test
point allows access to this resis-
tor. When you measure across the
resistor at the rear panel test points,
you are reading the DC voltage
drop across a one ohm resistor.
Referring to ohms law, if R=1 in
the formula, then I = E or current
equals voltage. So when you read
for example, 120mV you are also
seeing the equivalent value of cur-
rent or 120mA.
WARNING:
DO NOT be tempted
to run your tubes hotter than the
maximum recommended value.
You may find it sounds really cool
as you destroy your expensive
tubes and possibly damage your
amp, of course voiding your war-
ranty! Also, in case you haven’t
found out the hard way yet, power
tubes get extremely hot (as high as
800 degrees)!!!!
WARNING: Do not remove rear
metal grill.
WARNING: Do not attempt
to check or replace tubes by
yourself. Refer to authorized
service center.
WARNING: Never touch the
tubes while the amplifier is
turned ON. You can severely
burn your fingers and hands!
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