Dual, single, triple, or quadruple 220K log pot is not going to be a
catalog item. 250K, maybe, but 500K has the best odds at a shop
specializing in fixing guitar amps. There are some of those on line with
pots at reasonable prices. 500K was the standard value for an audio gain
pot with tubes. Guitar amps like to use tubes yet. Now to make a 500K
linear pot do 220K log curve, put the pot in parallel with a 390K. If it
was a gain pot I'd put that from wiper to ground, but in this
application its a rheostat. This resistor will make the 500K linear pot
function close to a log curve and when the pot goes open will still keep
audio working.
211K is closer to 200K, pots never did pick values on the RETMA 10%
series, but round numbers like 100K, 250K, 500K, and 25K.
There is logic behind the 10K, 12K, 15K, 18K,... sequence for fixed
resistors. If you want to use every value resistor made and decide on
powers of 10 being desired, taking bands 10% wide with those values the
edges of the bands touch, so with the carbon composition process making
a considerable distribution of values, selection rejects none, then they
are marked after selection. Same thing for 5% and 20% sequences. I
suspect when demand for 5% resistors is high that 10% and 20% values are
missing the middle 5% and 20% may be missing the middle 10%. That the
wider tolerance resistors may only have the closer tolerance values when
demand for those closer tolerance resistors are lower than the demand
for the cheaper wide tolerance resistors. Today, making laser trimmed
film resistors, that continuous distribution isn't so prominent. There's
tolerance buts its more from trimming machinery slop than process
variations because the trimming or spiral cutting allows for variation
in film thickness. But that number sequence didn't get applied to pots
because they were never offered in as many values as fixed resistors.
73, Jerry, K0CQ
On 2/7/2011 9:06 AM, Steve Hunt wrote:
The potentiometer wasn't marked other than with a manufacturer's number.
It measured 211k across the ends, and mid-way on the rotor the
resistance from the slider to either side was 176k/35k, so I think it's
safe to call it a "220k log".
73,
Steve G3TXQ
On 07/02/2011 14:34, NL7VL wrote:
Hi Steve,
So you're saying that the double-gang notch pot on the Corsair I is a
220K? That's about how it measured out with the ohm meter, but it does
say 500K on the schematic. Jerry's right about things showing up on the
case of the pots, but all I saw was what looked to be an OEM part
number or something like it. Please let us know how it goes.
There are a few DDS mods for the oldies out there - like N4YG's mod. I
have a Corsair II PTO that I will be investigating this week. It's kind
of a crap-shoot with those things: The Century 21 I have stays within
50hz after warm-up, but it was rebuilt no so long ago. Going with the
DDS + display combo heads off future problems with the original display,
too. Not a bad idea. I had thought about using a "Huff-and-Puff" or some
kind of fast response phase lock loop on the PTO, but maybe that's not
the way to go.
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