5F6H
1444 posts
Dec 06, 2012
1:56 AM
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Call/e-mail Kendrick, Gerald uses a panel mount, multi turn cermet pot in his GWSA signature amp, looks like a 1W unit (or try googling "Bourns 50K 1W multiturn cermet pot").
However, there may be a simpler solution with no drilling required - your "ground switch" is not actually connected to anything, have a look. You could remove the switch and put a regular SF/BF Fender style 10K bias pot in there (look at how it is wired on a BF/AB763 amp, only use the middle tab & one other, more ohms = less bias current for when you set the direction of adjustment), in series to ground with a 33K or 39K resistor. The feed wire to the pot comes from the end of R40 farthest from the bias diode. The 33K/39K resistor could be mounted on the pot body and the pot & resistor combo would replace R41 in the amp (which you would remove). Tweak R40 to give the desired sweep (say 10-30mA, +/-5mA). A possible down side is that the 10K pot may not give you a great deal of sweep...you could also try a regular 25KL amp pot for more sweep, but I would be concerned that you would accidentally knock it in use/transit and change the setting.
I generally find that I spend a few minutes setting the bias & listening when installing new tubes, then leave it where it sounds best, it's not something that you typically re-do on a regular basis with a given pair of tubes...so really, having to remove the back panel to adjust is not a big deal.
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Last Edited by on Dec 06, 2012 2:33 AM
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