•
V7, Red: Plug in positive multimeter probe here to measure V7 output tube bias.
•
V8, Red: Plug in positive multimeter probe to measure V8 output tube bias.
Bias Measuring & Biasing Procedure
What is bias?
Simply put, bias refers to the idle current flowing through the output tubes. In fact, all tubes
need to bias, but in guitar amps the bias of the preamp tubes is permanently set and does not
need adjustment. Here we will be only discussing the measurement and adjustment of the
output tubes’ bias current.
CAUTION: Maladjustment of the bias controls can lead to output tube failure. The bias
adjustment is not fool-proof, and output tubes can be under-biased (i.e. too “hot”). The bias
control needs a wide enough range to accommodate different tube types and sets (tubes
vary from set to set), and therefore it is possible to under-bias the output tubes in some
cases. If under-biased for a sustained period, the output tubes will likely fail. Science
Amplification is not responsible for output tube failure due to maladjustment of bias
controls.
Please familiarize yourself with the bias procedure before making bias adjustments. If you
feel uncomfortable making adjustments, take the amp to a qualified technician. We do,
however, encourage you to bias yourself. It is easy once you get the hang of it, and can save
you quite a bit of cash!
WARNING: Adjusting the bias requires the amplifier to be ON. The output tubes adjacent to
the test points and adjustment control get very hot during normal operation and there is a
potential burn hazard. Be careful near those hot tubes!
Here are some situations where you may need to check and/or adjust the bias:
•
You are changing tubes.
•
You are changing tube type from EL34 to 6L6 or vice versa (applies to 50W and 100W
models only).
•
The amp is quieter than normal.
•
The amp has a hum that is not affected by the setting of the Loudness control.
What you’ll need:
•
A phillips screwdriver to remove the amp’s back panel.
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