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Discussion in 'Amp Tech Center' started by zachlovescoffee, May 9, 2021.
Does anyone know where you can get these? The ones that have a sunken adjustment screw?
You mean like this, but in a 25KL version? Not sure. Mouser?
BTW, what are you using it for?
bias adjustment pot for a Princeton reverb to convert from fixed to fixed adjustable.
RobRob’s layout has a 25k linear pot but. Cant find them anywhere.
A 10K pot in series with a 22K resistor adjusts fro 22K TO 32K. This is the typical solution.
He's totally right about the 10KL, and in fact that's why I asked.
The classic, time-tested PR setup was described by Doug Hoffman back in the dot-matrix era -- using a 10 KL pot. (Note in the PR I think it's most common and works well to go with his 27K tail resistor, even though that misses the 'split the difference' approach that would suggest 22K.) Also nowadays most of us ground that resistor to the nearby power amp/bias ground instead of the pot.
As a side trip, it may seem obvious that a bigger pot, like 25 or 50K, will give you a wider range, but this is only true *if* you keep the head and tail resistors in the same relationship. Merlin notes a 10KL pot is adequate to span *any* range, if you adjust the 'range' and 'tail' resistors appropriately, and I built a calculator to test the idea. Guess what, Merlin's right.
Now Rob is right, too; a 25 KL pot here will give you a wider range if you keep the same resistors, but over the years tons of folks have found that in a PR (and similar amps, like the 5F11 and 6G2) the 10KL spans all the range you need. Note this is not the standard ~50V bias tap, so that big ol' 100K 'range' resistor and stock 27K tail give you a nice wide range when you throw in the 10KL.
Further detour: In the 5F11 and 6G2, especially with lower HT voltages, you may need a smaller tail / shunt resistor to get the center point right, but as I say, 27K seems to work well in most PRs (with their fairly hot HT voltages feeding the bias tap).
Last fall when I built my BF PR clone I used the 10k-L pot with a 22k tail resistor. PD at about 8.4w with pot set almost dead center.
20k and 30k here
Three that use a 10K pot.
Potentiometer axles are so soft aluminium that it is quite easy to saw them shorter and saw to take Phillips driver which is almost as easy to use as flat driver in a hole. Use tape to keep saw particles out.