•
An electronic volt meter (a digital model is preferred) with a millivolt setting. These can
be found inexpensively and just about any one will do.
•
A small to small flathead screw driver to adjust the bias control.
•
A speaker cabinet to plug into.
Biasing Procedure
(Please reference the tube diagram on pg. 12)
Measuring & Adjusting Output Tube Bias:
1. Use a phillips head screw driver to remove the rear panel of the amplifier.
2. With a speaker cabinet plugged in, turn the amp to Standby, let it warm up for about 1
minute, then turn ON.
3. Set your multimeter to DC millivolts (mV).
4. Insert the negative/common test probe into the black test point (ground). On 100W
and 200W models use the black test point that adjacent to the tubes you are
measuring.
5. Plug the positive probe into the red test point behind the tube you are measuring.
6. Take note of the reading on the meter.
7. Remove the probe from the red point, and repeat the procedure for each tube. On
100W and 200W models, move the negative probe to the other black test point nearest
the tubes you are measuring.
Your readings should be the same within 10 millivolts, which means the tubes are
reasonably matched. If a reading on a particular tube is below the safe/suggested
ranges (see below) by more than 10-15mV, then it's possible a tube has gone bad and
needs to be replaced.
Here are the bias settings/ranges for each tube type and model. Biasing on the lower,
“cooler” side will slightly increase headroom and tube life. Biasing higher, or “hotter”
will encourage quicker power tube distortion, but slightly decrease tube life. These
differences are very subtle, however.
8
