Secret life of Dual concentric pots - mixing wafers?

[axg20202]

So, after doing a lot of research and then reading RG's Secret Life of Pots article, I've decided that the only way to get the two different values of dual concentric pots I need is to mix wafers. Firstly, does anyone know if this is possible with dual conentric pots (e.g. those made by CTS), and secondly, the values I need are going to be tricky. I need one dual-c pot that is 50K linear on both shafts (the easier of the two I guess) and a second daul-c pot with 100k reverse audio on one shaft and 25K reverse audio on the other. Help! I can't obtain these off the shelf, so any help appreciated.   

Cheers,

Andy.

[GREEN FUZ]

Good question. I don`t have a straight answer, though I would guess, having read the same article, that one could theoretically and with a bit of care disassemble the dual pot and replace the existent wafers with the required replacement. Sounds like a lot of work. What are you building?

[Processaurus]

Good thinking, being able to make specialized pots is a good skill to have.  This isn't for sure, but some manufacturers you can switch the wafers from their single gang pots into their dual gangs.  CTS is a good choice because they're probably easier to dissasemble + get back together than the cheaper alphas, which I think might get wrecked in the process, if i remember correctly that the shaft is kind of smashed on.

[SonicVI]

I've recently made a custom CTS triple ganged. They're pretty easy to get apart and back together.      You can actually swap wafers between Alpha's 16mm dual pcb mount pots. well, you can swap the bottom wafer one anyway.

[axg20202]

I've recently made a custom CTS triple ganged. They're pretty easy to get apart and back together.      You can actually swap wafers between Alpha's 16mm dual pcb mount pots. well, you can swap the bottom wafer one anyway.

Cool. So it's been done before. So for the CTS triple ganged you managed to change the wafers in all three positions or just the back one?

My other problem is finding single CTS pots of the values I need. The 50K linear pots should be available, but I can't find 25K reverse audio or 100K reverse audio.  Hmmm.

[Mark Hammer]

Strikes me as being a bit risky.  The chief obstacle is that the resistive wafers are often held in place via crimping, and the crimping itself is the sort of thing that can easily crack the strip/wafer.

[SonicVI]

Yes, you can access all of the wafers in the CTS pots.   

You do have to be careful when reassmebling, especially if you're using older pots. The pots I had were 70's-80's surplus pieces I found at local electronics shop.   I broke a wafer when I was crimping the last little piece of metal, and on the bottom wafer of course, so I had to disassemble the whole thing again. D'oh!    The small Alpha wafers are even more fragile as they're a bit thinner than CTS.

[SonicVI]

My intent with the custom triple (50K/50K/25K) btw is to have a variable twin-t filter in a brassmaster clone rather than switching between two sets of resistors. It was just luck that the triples and duals that the shop had were 25 and 50K's.  I haven't tried it out yet. Hopefully it'll work.

[axg20202]

Strikes me as being a bit risky.  The chief obstacle is that the resistive wafers are often held in place via crimping, and the crimping itself is the sort of thing that can easily crack the strip/wafer.

You're probably right Mark. Still, nothing ventured, nothing gained.... I'll be using new CTS concenctric pots and single CTS pots as lambs to the slaughter. Now then, if someone could hook me up with a supplier for single CTS reverse log pots in 25K and 100K values I'll have what I need. Do they even exist? I've looked all over the place.

Andy.

[SonicVI]

Doing a quick google search for CTS reverse log I found them in 50K and 3M.  Maybe you could use those with resistors in parallel?

[axg20202]

Thanks.

By chance, I just found a CTS pot in my parts bins and it came apart very easily with no danger of damage. The design is such that the wafter just slips off the end and the bushing just pops off it, leaving a clean wafer that I've barely touched in the process. So, hopefully this is going to work out. I've just ordered a bunch of CTS pots to mangle so fingers crossed....

[axg20202]

Thought I'd report back on this. I received a couple of CTS dual-concentric pots (both 250kA/500kA) in the mail along with 4 donor single gang CTS pots of the various values I needed . Basically, swapping out the wafers was what we Brits like to call "a piece of piss" (easy, in other words  ). The only mildly tricky bit was drilling out the central holes in the doner wafers to allow for the slightly larger spindle diameter in the dual-concentric pots. I used an 8mm dill bit for this, but would have used a 9mm bit if I'd had one. I used a cordless drill set with low torque and speed so that the drilling wasn't too aggressive. A drill press would have been very handy for this but I managed fine with a handheld. Other than this detail, the wafers from the single gang pots were identical in construction to the dual-c wafers. End result: two dual-concentric pots with values that are no longer commercially available or at least very hard to come by (one is a 100k log/100k log and another is a 50K rev log/500k rev log - rare as rocking horse sh*t).

Bootiful.

Andy.

PS. what puzzled me is why CTS makes wafers with different sized holes in the centre in the first place. One size fits all as I used the wafers from the dual-c pots to make perfectly functional single gangs out of the donor carcasses. But then, who cares......

PPS. here's a top tip (from the school of the bleeding obvious) if you fancy doing this yourself: before you start, clearly label the actual wafers (right above the solder tags) with the pot values using a permanent marker before disassembly.