Alpha Pot mods

[NateS]

I know (now) that Alphas are not the best pots to use for mods due to the deformation of the shaft during manufacture. None the less, I have a pair of ganged 250k pots, one linear pair, one audio pair. If I'm willing to accept destruction of one as a donor pot, can I still salvage one ganged A250K/B250K pot out of the two? The way the ganged Alphas are made, it looks like it would be difficult to separate the upper pot from the lower pot. But what about the plastic tab on the bottom pot? Can it be removed, and then an alternate retainer fashioned?

BTW - this is for a ganged magnetic/MIDI volume for the Graphtech Hexpander. If this doesn't work, next idea up is to use a TBX B250k/A1M pot.

[David Schwab]

I've taken Alpha pots apart with no problem. All I had to do was bend the tabs that hold the top and bottom pots together.

I've also never seen any with deformed shafts.

But why do you really need audio and linear taper? I've used linear taper for years for volume pots with no problems.

[Mark Hammer]

I make it a standard practice to remove the backs off any new Alphas I buy and pre-treat them with Stabilant, before placing the backs on again. The wipers on the larger (24mm) pots are of much higher quality than those on the 16mm pots.

The dual-ganged types do not always lend themselves well to "borrowing" parts from others, since the shaft is rivetted in place. That said, if the pot is of the type where the back plate is the phenolic disc where the resistive strip is, you actually can simply pop that disc off and replace it with one of another value or taper, provided, of course, that it is exactly the same pot form factor. Here is what you should see.

The internal architecture of the pot has one wiper facing front and one facing back, so what you'll see when the back section is removed is essentially a non-committed wiper that can now be "assigned" to whatever is attached as the back section. If you need to butcher several pots to get what you need, Tayda is a good place to order from. Sometimes slow, but always cheap.

[NateS]

The reason I want the Audio/Linear mix is that the MIDI volume signal expects linear, and that's what Graphtech recommends. It's an an effort to keep guitar and MIDI more or less matched while doing volume swells.

Unfortunately, these pots have the wafer on the top. Like so:


When I pop the back off, I see something like


I also tried removing the top section, since it would appear to be more accessible, but I cannot remove the threaded collar - it comes up maybe 0.050" and just stops, with nothing visible in the gap that might be retaining it. (RG also mentioned in Secret Life of Pots that the shaft was deformed to hold it together.) I guess it looks like I'm in search of another donor.

[David Schwab]

The reason I want the Audio/Linear mix is that the MIDI volume signal expects linear, and that's what Graphtech recommends.

Right, and as I said, you can use a linear taper pot for your magnetic pickups with no problem. Often times they work better than audio taper pots and are more even.

[NateS]

I understood, but I'd like the magnetic volume to more or less match the MIDI volume message, which is logarithmic. Most people just use their volume as a rotary mute knob anyway.

[greekie]

I smell a resistor across the a linear magnetic volume pot :-)

[David Schwab]

I understood, but I'd like the magnetic volume to more or less match the MIDI volume message, which is logarithmic.

So get a dual ganged linear pot. They are easy to find. Check out Mouser.

Most people just use their volume as a rotary mute knob anyway.

That's what I do!

[Jazz P Bass]

The reason I want the Audio/Linear mix is that the MIDI volume signal expects linear, and that's what Graphtech recommends. It's an an effort to keep guitar and MIDI more or less matched while doing volume swells.

Unfortunately, these pots have the wafer on the top. Like so:
[ATTACH=CONFIG]16594[/ATTACH]

When I pop the back off, I see something like
[ATTACH=CONFIG]16595[/ATTACH]


I also tried removing the top section, since it would appear to be more accessible, but I cannot remove the threaded collar - it comes up maybe 0.050" and just stops, with nothing visible in the gap that might be retaining it. (RG also mentioned in Secret Life of Pots that the shaft was deformed to hold it together.) I guess it looks like I'm in search of another donor.

If you want to give it a go, that back plate is what is holding everything together.
If you look at the rear of it, you can see where the end shaft was splayed to hold on the disc.
Squeeze it, file it. Get rid of the splayed part & it will all come apart.

[jrfrond]

I'm still trying to figure out why you need to disassemble the pot to apply Stabilant. Mark?

I've used sh*tloads of Alpha pots, with few things to bitch about. Have never seen any shaft deformation.

For you older dudes here: I really miss the old Centralab pots that you could purchase and stack yourself,plus pick the shaft type. Anyone remember these?

[Mark Hammer]

I'm still trying to figure out why you need to disassemble the pot to apply Stabilant. Mark?

I just put a dab in as a kind of insurance on the resistive strip. Most of my pots end up in pedals where they are going to be a real bear to get at if any sort of rehabilitation is necessary. I realize Stabilant is essentially for post-hoc repair purposes, but figure I'll just do the work up front so I won't have to deal with it later under less hospitable circumstances.

[Steve Conner]

The volume control voltage gets a logarithmic mapping applied to it by the MIDI synth, so it is technically correct to use one linear and one log pot.

The log versions of the miniature Alphas have one log wafer and one reverse log, because the wafers face opposite ways. Always made me dubious about channel tracking.

A dual linear pot with a law-bending resistor would work. A real geek would use a single 10k linear pot and control the pickup volume with a Blackmer VCA, as these have an exponential control law, measured in dB per millivolt.

(of course to a mathematician all of these "log" tapers are actually exponential)

[jrfrond]

I just put a dab in as a kind of insurance on the resistive strip. Most of my pots end up in pedals where they are going to be a real bear to get at if any sort of rehabilitation is necessary. I realize Stabilant is essentially for post-hoc repair purposes, but figure I'll just do the work up front so I won't have to deal with it later under less hospitable circumstances.

I totally get the Stabilant thing. We go through a lot of it. I was just wondering why you take the pot apart. You can get to the carbon track with a needle-applicator. That's what we use anyway.

[David Schwab]

The volume control voltage gets a logarithmic mapping applied to it by the MIDI synth, so it is technically correct to use one linear and one log pot.

Except that the MIDI volume isn't the same as a pickup being loaded down. Log taper pots don't generally work all that well for volume control on passive pickups. They drop right out after you hit 8.

Also, are you really sure the MIDI is being mapped as log?

The log versions of the miniature Alphas have one log wafer and one reverse log, because the wafers face opposite ways. Always made me dubious about channel tracking.

That's an A/B taper pan pot. You can get them running in the same direction.

A dual linear pot with a law-bending resistor would work.

I agree.

(of course to a mathematician all of these "log" tapers are actually exponential)

Well commercial log or audio pots use two linear tapers to get an approximation, which is part of the reason they often suck.

[Mark Hammer]

Ah, okay I see your point. I don't use enough of it to have a needle tip applicator. I just have the little 0.5ml tester vials with a plastic applicator that a place near me was selling for 50 cents a pop. The only way I can apply it is by popping the back off, locating a small exposed area of the resistive strip, plunking down a little sesame-seed sized droplet with the plastic applicator, and spreading it around with the pot wiper. I usually take the opportunity, while the back is off and space is available, to get my bent needle-nose pliers and give the pot rivets a gentle squeeze for extra insurance, since cheap pots can sometimes make intermittent contact between the resistive strip and the solder lugs.

I guess that's what happens when one works in non-commercial low volume contexts. Now you know how the other half lives!