[sdiy] Dead Unobtainable Concentric Pot, What to do?

[Steve Lenham]

On 19/01/2021 19:26, James Coplin wrote:
> This was my plan re the curve but I have not been able to find any 
> concentric pots except those for guitars which are the wrong size (I 
> think I can work with that) and none in the 10k value. 500k seems avail 
> but its the 10k I'm stumped by at the moment.

Hi James,

It's terribly difficult to find much in the way of dual-concentric pots 
nowadays, doubly so for lower and/or non-matching values. I feel your 
pain, having had to do it myself for the Lexicon Prime Time delay which 
has one 10K linear section and one 500K log section from the old A-B 
ModPot range.

So I looked up the schematic for your Orban 111B.

Firstly, I believe the level controls actually have standard log tapers. 
  It's the BANDWIDTH controls that are dual 100K CCW LOG (or 
reverse-log). So that makes things a bit easier from the off.

Secondly, there is an easy way out. The input level pot section has been 
made 10K in order to make the input impedance of the unit 10K "for 
free". The pot wiper actually drives the very-high-impedance 
non-inverting input of a TL072, so could be of a much higher value.

I suggest that you simply fit an available dual-concentric 500K log pot, 
then add a 10K resistor across the outer pins of the input pot to keep 
the input impedance unchanged. The TL072 will be perfectly happy with 
its increased source impedance. How do I know? Because the existing 500K 
log output section feeds a TL072 in exactly the same way (the output pot 
is fed from a passive EQ network, so has been chosen to be a 
high-impedance load).

Hope this helps!

Cheers,

Steve L.
Benden Sound Technology