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Custom Reverse Audio Push-Pull Pot

I'm going to put a Stratoblaster preamp in the yellow Danelectro DC-2. I've never had one in any guitar, and for some reason it seemed like an interesting thing to try.

I've always been curious to try the Stratoblaster. One of my old guitar heroes, John Lees, had Alembic do some mods many years ago on his famous gold sparkle 1961 Stratocaster, including the Stratoblaster. After all these years, I figured, why not try one myself?

I have the kit from General Guitar Gadgets. There are some mods J.D. Sleep made to the circuit, including the use of a 50K reverse audio taper pot instead of an audio taper pot, which he says provides a better taper.

Now, the original Alembic circuit had a trimpot on the PCB, but it seems to be to be a better idea to put the pot where it's easily accessible on the fly - on the pickguard of the guitar. I also want to include a switch to turn the preamp on or off, again, while playing.

Which brings me to the point of this post. I don't want a separate switch - I'd like to have a push-pull pot to switch it on, and also to control the gain. However, try finding a push-pull pot with a 50K reverse audio taper! Does not exist.

Unless, that is, you modify a standard push-pull switch/pot to use the carbon track from a 50K reverse audio taper pot! Ha!

On the left you see a B250K ('B' meaning linear taper) switch/pot, and our victim donor C50K ('C' is reverse audio, aka reverse log) pot on the right.

Here are the backsides (hee hee) of the two controls. Note the DPDT switch on the left.

Trust me, this is not rocket surgery.

First we take apart the push-pull pot. Pry back the tabs on the pot section.

Then pry back the tabs for the switch section.

Not too hard, huh? Even I can do this!

Then disassemble the whole thing. It all just comes right apart.

I laid the parts out in order and I'm glad I took this picture. I needed it later.

We're going to swap out the B250K carbon track wafer that's the second piece on the right with the C50K one. Easy.

Take the back cover off the C50K pot.

You could do this with any value or taper you'd like - I see 250K, 500K and 1 Meg push-pull pots out there, but you could easily make a custom pot to whatever value and taper you need.

On this pot though, unlike the other pot and unlike a CTS pot, there is a tab at the bottom of the shaft that holds the wiper in place.

Originally I tried to drill out the tab, with no luck. Then I tried to cut off the wiper. I had the right idea, but I cut the wrong side (!), destroyed the carbon track, and had to get a new pot.

The things we sacrifice in the name of science.

Here's the right way to cut the shaft to get the wiper off.

Put your saw between the carbon track's wafer and the body of the pot. If you put it on the wiper side, you will destroy the carbon track. That's what happened to me, and held me up for a few days while I procured a new pot (I got three from Small Bear in case I wrecked another one...).

Here's what we wind up with. The wiper (on the right) is not needed.

What we need is the C50K wafer on the left. Now we'll transplant that into our push-pull switch! Bwahaha!

Here's all the parts about to go back together. Note the old 250K track in there - that will get thrown into my bin of 16mm pots - not sure why but there you have it.

Note there are some scrape marks on the 50K wafer - no big thing. It doesn't affect the performance at all.

I'll use the white wiper from the push-pull switch since the control shaft can go right through it.

I figured I should test the resistance of the "new" pot before it all went together to make sure I had the wiper the right way round. Worked fine. I just had to hold the wiper up against the carbon track for it to work.

There's a little pin on the bottom of the control shaft that engages with a slot on the switch half of the control (the black plastic piece on the bottom). We just need to make sure the tab is engaged in the slot when the two halves are put together

Pulling the shaft up then moves the switch up, and pushing it down moves the switch down. It contacts the respective contacts as it is moved.

Simple and effective.

An electro-mechanical device at work.

Reassemble the switch to the pot and push the tabs back down. Make sure everything is seated together before the tabs go back down.

Note that the switch terminals should face in the same direction as the pot terminals. It will sort of work the other way around, but it won't fully 'click' into place.

Don't ask me how I know this.

There you have it.

A custom value push-pull pot! Works perfectly.

Now to install it on the guitar.


Post a Comment 1 comments:

  • October 2, 2017 at 7:26 AM
    Hello, can you send me schematic of how to reassemble Alpha push/pull volume of this product:

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