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5) TUBE COMPLEMENT AND BIAS ADJUSTMENT
V1
V2
From left to right:
V1: 12AX7/ECC83 (Input stage Bright and Normal channels)
V2: 12AX7/ECC83 (stage 2 and tonestack cathode follower)
V3L: 12AX7/ECC83 (phase inverter for power amplifier)
V4-V5: EL34 (power tubes)
V6: GZ34 (tube rectifier)
BIAS ADJUSTMENT CONTROL IS INDICATED BY THE RED ARROW
To measure your power tube bias, carefully follow these steps with the amplifier in
OPERATE, MASTER at minimum, and connected to a speaker load (not doing so may
damage your amplifier!):
1) Turn on a digital multimeter (DMM), and set it to read millivolts (mV) in the 100mV
range (this will vary from DMM to DMM)
2) Plug a black probe into the color-coded jack on your DMM, and do the same for a
red probe
3) Insert the black probe tip into the black probe jack (red arrow). This is GROUND in
the amplifier.
4) Insert the red probe tip into the V4 red probe jack (green arrow) on the left. This
measures bias for V4. Write down your measurement
V3 BIAS POT V4
V5
V6
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