Strange Pots

[daveq]

I'm looking for a way to accomplish the following:

I'd like to only have one knob on the front of a guitar but I need to adjust the volume and adjust the gain of an existing (not making my own) onboard preamp. I need to verify this but I think the preamp uses a 25K pot. I could either use a EMG-AB or a Seymour Duncan booster (they are not currently available though).

I've seen plenty of dual concentric pots out there but none that have either 250K/25K or 500K/25K. I also found a company that will custom build them but for $100, I don't think so.

If by any chance someone knows where I can get one of these - please let me know. I'd guess that it would be too difficult to take one apart and rebuild one with my own 25K, right?

I've also considered modifying the preamps but I don't know what's involved with doing that yet. I'd either have to find a schematic or reverse engineer it - doesn't seem worth the time/effort.

One last possibility might be to have the gain for the preamp be adjustable from the back or side (recess mount with a tiny knob or adjust with a pick) - don't know if I like that solution either though.

Any ideas on how to solve this one other than biting the bullet and using two knobs?

[Devon Headen]

I really know nothing about electronics at all unless I'm staring at a diagram, so I don't think this is possible or you would've thought of it. Can you use some kind of capacitor to cut out the frequncies the 25K pot would eliminate, and use a regular concentric pot? I'm not even sure that's what a 25K pot does, but I know that 250k is supposed to be darker than a 500k, and a 1meg is almost completely true bypass for a guitars signal.

[aeli]

Mybe you could do so by soldering a resistor to the dual concentric pot?

tutorial

[KeithHowell]

Are both Pots in the circuit of the Preamp or is one (the 500k) across the pickup and the 25k in the preamp circuit? If so you could just go with the gain adjust pot (25k) and make sure your input signal on the preamp is at a suitable level using some resistors to replace the 500k pot so as not to overload the input then do all your adjusting with the gain pot.

Keith

[daveq]

Thanks for the suggestions guys -

The 250K or 500K would be the volume for the pickup - the 25K would be for the gain for the preamp.

aeli - the link didn't work for me - I'm guessing that it shows how to wire a resistor in parallel or series with the pot? That may work to a certain degree but I'd certainly lose the taper - I don't know - maybe I'll look into that if I can't find anything else. Thanks for the suggestion.

StonesCreed - I think the only option that I'd have in terms of adding components might be either what aeli suggested or examining the preamp and see if it is possible to substitute a larger value without losing much performance. Thanks.

KeithHowell - That's certainly a good idea but I'm going to be a real pain in the butt on this one - I really want to be able to adjust the gain in case I use the guitar on a different amp, ... Thanks for the suggestion though - for most people, that probably would have been the thing to do.

I guess I'll look into the preamp circuit design and see if I can change the pot value without messing things up too much.

Thanks,

Dave

[psw]

I'm not sure if this is of any help.

I've taken apart double gang pots (one shaft turns both pots at once) and replaced the guts of one with that of the value that I want.

Most pots are a standard size. Many are held together by bent strips on the outside case. Bend them up and insert the resistance 'circle' of the odd value that you want.

Are dual concentric the ones with two knobs on the one shaft? If so I have had no experience but a similar approach might be possible.

Good luck!

[lovekraft]

The quick and easy solution is the one aeli referred to - get a 250K or 500K Dual Concentric Pot, and strap a 27K 1/4w resistor across the fixed (outside) terminals. It changes the taper of the pot, but you can probably live with it - you'll just have to try it. The other option is to find a preamp that was designed to work with guitar volume pots. You might want to give Ansil a shout, he's probably got something that'll work. BTW, the tutorial that aeli tried to link to is here.