6
Tube Layout.
Custom 22 has 5 12AX7 Preamp tubes and 2 EL84 Power tubes.
The 12AX7 tubes each have unique functions in the circuit, a
brief overview of which is provided here. Counting from the
first 12AX7 which is nearest to the input Jack:
V1
Gain
V2
Overdrive gain
V3
Tone controls
V4
Effects loop
V5
Phase inverter
When changing the Custom 22’s tubes – either preamp or
power tubes – it is not necessary to adjust the bias. This amp
does have a bias adjustment control on the circuit board which
is used for fine-tuning the bias at the factory, and should not be
adjusted by anyone other than a qualified service technician.
Here are some signs that you need to change your tubes:
Excessive noise like squealing, humming, or hiss
Little or no high end/treble
Muddy low end or loss of clarity; amp sounds “muddy”
Mysterious fluctuations in the amp’s overall volume
Low, or no output volume