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Hmm, the PI trimmer doesn’t sound as fun. Oh well, I still want to play with it! Is that okay?
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Again, before we continue, realize that you are adjusting PLATE VOLTAGE. In this case, your screwdriver is
about a millimeter away from about 300VDC. DO NOT adjust this trimmer with the amplifier on!!!!
Furthermore, measurement of the progress of the adjustment is tricky, because you are measuring near
300VDC!
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We highly recommend the use of a non-conductive rubber screwdriver, and only adjust the trimpot when the
amplifier is not powered and disconnected from the wall outlet. DO NOT ADJUST THE PI TRIMMER WHILE
THE AMPLIFIER IS ON! Note that the power supply capacitors in the amplifier can/will still store charge, and
you can still be bitten by 450VDC even though the amp isn’t connected to the wall! Adjust the PI TRIMMER
only at your own risk!
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On the next page, there are three arrows. The ORANGE arrow is the PI trimmer. The GREEN arrow is the
test point for measuring the plate voltage at V3A. The BLUE arrow is the test point for measuring the plate
voltage at V3B.
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To measure the plate voltage at V3, your amplifier must be in OPERATE mode and connected to a dummy
(preferable) or a speaker load. Turn the MASTER down to minimum! Set your DMM to the VDC
measurement (between 200VDC and 1000VDC), and plug the black probe into the black bias probe jack.
Place the red probe on the bare wire at the tip of the GREEN arrow. Write this value down – this is your V3A
plate voltage. Now plate the red probe on the bare wire at the tip of the blue arrow. Write this value down –
this is your V3B plate voltage.
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Best results are typically with V3A being about 6VDC higher than V3B. To increase the voltage to V3A or
V3B, turn the trimmer with a non-conducting screwdriver toward the test point you want more voltage on.
ONLY ADJUST WHEN THE AMPLIFIER IS OFF AND DISCONNECTED FROM THE WALL!
