288 Monaural Amplifier Rev 1.2
11
3.3
Biasing
Your amplifier has been biased from the factory. However, the bias setting should be checked on
first startup and periodically thereafter to maintain performance and longevity of the power tubes.
Note:
preamp tubes are automatically biased. No adjustment is needed
A. Checking the Bias:
To check the bias, power the amp on and let it warm up for at least 10 minutes. The bias reading
for the power tubes can made off of the two analog ammeters located on the back face of the
amplifier.
Bias Set Point: 60 to 65mA
With the volume turned all the way down, the bias should be set between 60 - 65mA.
To adjust, use the screwdriver-adjustment potentiometer labeled
“Bias”
on the back of the
amplifier. This range is based on sound preference as well as desired output tube life. The lower
the bias, the longer the tube life with the expense of higher distortion.
When using a matched pair of KT88 tubes,
each meter should read +/- 7mA or less for best
performance.
A well-matched quad of tubes will measure much better than this.
If the bias variation between meters e/- 7mA, try switching the position of the V3 and V4
power tubes. If the current variation follows the position of the power tubes, then the tubes have
become unmatched. If the tubes are new and the meters read significantly differently as described,
then the set is not correctly matched.
