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been working on this 299A the second version with 6BQ5s lined up parallel to the back of the chassis, I was changing up the bias resistors (4 of them 150K each in each channel) when I decided to put the pot and clean it. Whoa, the pot is listed as 50K in the first version schematic, this one measured 26K! Do the values of these pots change much over time? I had heard that bias pots should be checked out and cleaned. The centralab part number is 50103-2D (outside of the 1346338, CRL 1963 38th month) does that number jibe with being a 50K pot? The other pot measures 60K, how important is it that the bias pot be at least close, and how close is close? thanks!
Follow Ups:
Randy
The balance pot is usually 50k and the bias pot is 10k as I recall. Look at the numbers on your pot and see what they say..Yes,they can short and read less than rated resistance.
"If it measures good and sounds bad, it is bad; if it measures bad and sounds good, you have measured the wrong thing."
- Daniel R. von Recklinghausen
thanks, Michael, that pot reads 24K, the other pot is reading 60K, how much variance can I have from the 50K that the pots should be? I have a couple of pots that are 50K but measure 58K, is that too far off to use? I have some brand new 100K pots that measure 88K, are these pots made to be the correct value or what? The 2 50K (measure 58K) pots have a milky plastic for the carbon track support rather than the usual brown bakelite, is that anything to be concerned about? thanks!
Randy
Believe it or not,those out of tolerance will work with no issues for the simple reason that the bias network draws very little current and you can adjust them down to where they need to be..When I said look at the pot,I meant read the actual numbers printed on the pot and you can probably see if they are close to the measured values.
Your pot that measures 58k is fine..Most of those pots were 20% tolerance anyway.
"If it measures good and sounds bad, it is bad; if it measures bad and sounds good, you have measured the wrong thing."
- Daniel R. von Recklinghausen
and it measured 75K, but printed on the pot is clearly "100K" (its also an older looking pot, FWIW). So it appears that there are two widely different value of pots in the DC balance circuit, shouldn't they be the same values, or does that not matter? Any suggestions on what value of pot I should use for the second pot? 50K or 100K, or just leave the out-of-value one in there?I am planning on using the Scott method of balancing the bias in the amp via the 1st gen instruction in the Sams, even though it is a second gen version of the 299, do you thin that will be OK? thanks!
Randy
Those pot values are not that critical simply because of the minimal current draw in the circuit.The 100k pot you are simply using as a balance pot to even the cathode voltage between each pair of output tubes..I would use what the schematic calls for if you have them but it's not a big deal. If you can set the bias voltage and balance,you are good to go..
"If it measures good and sounds bad, it is bad; if it measures bad and sounds good, you have measured the wrong thing."
- Daniel R. von Recklinghausen
Edits: 02/01/15
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