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7
Calibration Procedure
Left Channel
This procedure sets the 2A3 plate current to 60 mA and the 6DJ8 between 8 and 12 mA. There may be
one or two combinations of switch and trimpot settings to achieve this goal. If possible, avoid currents
higher than 60 mA during the procedure.
1) Locate the RUN/CAL switch behind the power switch. Change the RUN/CAL switch to the “CAL”
position.
2) Locate the selector switch below the current meter. Change the switch to the “A” position. (This
monitors the current through the left 2A3 output tube.)
3) On the Left heat sink, there are three small holes in a vertical line. These set the bias points and
operating currents of the tubes on the left channel of the amplifier. (The holes on the Right heat sink
have the corresponding functions for the right channel.)
4) Put on the white gloves, and insert the adjustment tool into the top hole. Rotate the trimpot so the
meter reads a low value around 10 mA. (In most cases these will be counter-clockwise turns.) This is
a 25-turn trimpot, so it will take several rotations.
5) Insert the tool in the bottom hole (2A3 bias, medium adjust switch). Rotate until the meter falls in
the 40 to 60 mA range.
6) If there are not enough steps, back off two steps, move the tool to the middle hole (6DJ8 bias,
coarse adjust switch) and increase by another step.
7) If necessary, move the tool to the top hole (trimpot) and rotate until the current reads 60 mA. (In
most cases, this is a clockwise rotation.) Please turn slowly so the circuit can stabilize and give an
accurate reading.
8) Move the selector switch for the current meter to the “B” position. This monitors the current
through the left side of the 6DJ8 input tube. Check to see the current falls between 8 and 12 mA
(lower green zone on the meter). A 6DJ8 is still considered serviceable (and listenable) even if it’s a
little out of this range.
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