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Old 4th February 2009, 09:46 PM   #1
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Cool Dirty Cheap Vintage Pot Fix - Your Opinion Please

Hi!

I have a problem with Noble vintage pots in tube amp - they went from 500K to 800K - 1M (ALL of them). Cleaning will not help - resistance increased due to aging of carbon film, not usage or dirty contacts. They are quite unusual - 6mm split long shaft + threading = 30 mm, and so far I found no replacement!

1) For 2-gang 500K linear balance pot (had to be replaced because one gang become broken) - I found a way to build "new" from skeleton of old one and new 2-gang guitar pot.

2) For bass/treble controls I think to use dirty cheap trick - 2 shunt 680K resistors as shown on the schematic. R=(R1*R2)/(R1+R2), resulting approx 500K. However, this pot will not be linear anymore, but rather something really odd. (Trick #1 will not work, no physical space available inside amplifier).

3) I found 250K dual-gang log pot from Marantz vintage amp with tap from the middle for loudness control (its shaft is 5mm shorter then required but I can work around it). Since the resistance decreased twice, capacitance of C13/C14 must be increased by a factor of two. The question is - can be 250K too low for output impedance of V3a?

Everyone is welcome to share his/her opinion

PS. If someone has such pots, please drop a message
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Old 5th February 2009, 06:57 PM   #2
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Unfortunately, I have overlooked one simple thing with shunting 800K pot with 2 resistors. Its a 2 (not 1) group of parallel-connected resistors. And the resistance between points a and b WILL CHANGE :thumbsdn: when pot is rotating. With 2 x 680K shunting resistors resistance between points a and b will change in range of 503.70K (middle pot position) to 367.57K (left or right pot position). With ordinary pot resistance between points a and b is constant.
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