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JDeV 7th April 2006 05:10 PM

Fender Adjustments
Bussy repairing a Studio Bass for a friend. There are 2 POT's at the rear: HUM adjust and TUBE balance adjust.
What is the correct procedure for setting this up?

Thanx for any assistance.

blacklight 7th April 2006 07:26 PM

Bottom of the page.

testlab 7th April 2006 07:40 PM

You may want to convert the bias adjustment from a balance type. This adjustment merely divides the bias evenly between the tubes instead of accurately setting the idle current.

JDeV 7th April 2006 08:03 PM


Originally posted by blacklight

Bottom of the page.

Thanx for the link. I also fond this one, very good quality, still need to know how to adjust these POT's for best results.

Netlist 7th April 2006 09:06 PM

I'm sure you'll find the answer in this link:


blacklight 7th April 2006 09:13 PM

Ach, sorry. I thought you meant how to install new pots. Well, cheers anyway.

poobah 7th April 2006 09:17 PM

With a shorting on the input... or no input at all; connect a voltmeter from plate to plate (pin 3) on the output tubes. With the amp warmed up well, adjust the matching pot for zero volts. This matches the idle current (basically) through the tubes.

If the tubes are new, you may want to do this again after tubes have some hours on them.

With that done... simply adjust the hum balance for minimum audible hum at the speaker.

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