Potting Pickups with something other than wax.

[nicholaspaul]

I have built two humbuckers with ABS sheet plastic that are very microphonic, probably due to the winding not being overly tight. I'm a little afraid that the ABS will melt in a pot of 140 degree wax so I was wondering if there is something else I can use to pot the pickups. Could I soak them in thin polyurethane? Would a coating of superglue help? I'm not worried about taking them apart or rewinding once I pot them.

Cheers

[big_teee]

There are several threads around on all types of potting.
I would not be afraid to pot with wax if you use a thermometer, and keep the temp down.
These two threads discuss a bit of everything.
Look for the Bruce Johnson posts, he uses a special epoxy, and I think applies as he goes?
http://music-electronics-forum.com/t31683/
http://music-electronics-forum.com/t36769/

[David Schwab]

You can test a piece of the ABS to see if it deforms.

Before I switched to wax I used vanish and then later polyurethane. They both work, but they take a while to dry, and it appears the varnish often never dries fully inside the coil. Neither worked as well as wax.

You can also try thin epoxy or superglue. But don't expect to be able to rewind those bobbins!

[nicholaspaul]

I like the thin epoxy idea. Bruce does it when the pickups are built and drips it on. Great idea!
I have a double boiler and have used wax, but the 90degree melting point of ABS scares me. Testing is in order methinks.

So how long, David, is a while? Are we talking days or weeks?

[David Schwab]

ABS does not have a melting point. It's glass transition (when it gets amorphous, or turns into a soft blob) temperature is ~221°F (105°C). So I think you should be fine dropping them in hot wax. I leave my bobbins in for 10 minutes.

I've had bobbins 3D printed and they are ABS.

But test on scrap to be sure.

[copperheadroads]

What's wrong with wax ?

[Bruce Johnson]

Yeah, the wax vs. epoxy debate has been a friendly rivalry here for a long time. Either one will work. As David says, you shouldn't have any problem with wax dipping the ABS. Wax dipping has its own tricks and tips that you can read about extensively on this forum. Or, the CPES epoxy that I use is an alternate long term technique with its own pros and cons. I personally like the epoxy for various reasons.

Now, if you are just messing around building some experimental pickups for yourself, thin superglue will work for potting. You have to use the really thin superglue that soaks all the way through the coil. That's the point of potting. The superglue is quick and simple, and will eliminate the microphonics and allow you to evaluate the pickup without that distraction. Most superglue dries hard and brittle, which may result in coil failures over time. So it's risky to use for production pickups that you are going to sell to people. But it's fine for prototypes.

As I'm sure you've read on here, pickup making involves a lot of experimenting. You don't need to use the "perfect" production process for prototypes.

[bbsailor]

The dielectric constant of the space between wire turns affects the amount of capacitance seen between the coil windings. This space consists of (1) the thickness of the wire insulation, (2) the insulation material itself (see web link)Dielectric Constants of various materials, and (3) the amount of air between windings.

When potting pickups you are replacing the dry air with a dielectric constant of 1 with a material with a higher dielectric constant value, thus introducing more capacitance between the coil turns and also lowering the resonant frequency somewhat. How much depends on the amount of air space being replaced with another material.

PVC insulated wire has a dielectric constant that varies between 4 and 6 while Teflon has a dielectric constant near 2. Pay attention to the type and thickness of the insulation you are using on your pickup wire as well as how compactly your coils are wound. Typical machine wound coils have less air between the turns and potting them will have less of an effect on the resonant frequency than a hand wound coil with more air space between the turns and layers.

Joseph Rogowski

[nicholaspaul]

What's wrong with wax ?

I'm a little afraid that the ABS will melt in a pot of 140 degree wax so I was wondering if there is something else I can use to pot the pickups.

I gave it a shot today and the ABS was just fine in 145 degree wax. Thanks big tee and David S for your helpful guidelines.

[nicholaspaul]

Yeah, the wax vs. epoxy debate has been a friendly rivalry here for a long time. Either one will work. As David says, you shouldn't have any problem with wax dipping the ABS. Wax dipping has its own tricks and tips that you can read about extensively on this forum. Or, the CPES epoxy that I use is an alternate long term technique with its own pros and cons. I personally like the epoxy for various reasons.

Now, if you are just messing around building some experimental pickups for yourself, thin superglue will work for potting. You have to use the really thin superglue that soaks all the way through the coil. That's the point of potting. The superglue is quick and simple, and will eliminate the microphonics and allow you to evaluate the pickup without that distraction. Most superglue dries hard and brittle, which may result in coil failures over time. So it's risky to use for production pickups that you are going to sell to people. But it's fine for prototypes.

As I'm sure you've read on here, pickup making involves a lot of experimenting. You don't need to use the "perfect" production process for prototypes.

Thanks Bruce. I read your method on using epoxy with interest. I didn't want to splurge on some more product but I like the idea.
Yes lots of experimenting! Once I get a method for making my own bobbins I'll chart some results and figure out what I'm doing. You guessed that I'm just among a few here and there, and for now it's just for my own built guitars, so superglue may be an answer too. Thanks for your input! Much appreciated!

[chad h]

I've been wondering why potting might change the sound of a pickup - it looks like you've just explained it for me. Thanks.

Still, I'm interested in how much of an impact potting is likely to have. I'm going to run some experiments some time soon, using my CGM-101 oscilloscope to see how the resonant frequency and the frequency response curve for some of my pickups respond to potting. Seems like a pretty simple "before" and "after" comparison.

[Bruce Johnson]

In addition the the shift in capacitance that Joe explained, potting can (or, should) eliminate Funny Background Noise. That's a technical term. Microphonics can happen in all different degrees with different results. When internal parts of the pickup can wiggle in relation to each other, they can create little unintended signals that can add to, or subtract from, the main signal. Very unpredictable in frequency and amplitude. It can change what you are hearing and disguise the pickup's sound. You may like the resulting sound of a particular pickup that's slightly microphonic, but it will be difficult to replicate that sound, and it may even change over time.

To me, that's the biggest reason for thoroughly potting the coils and the pickup assembly: Get rid of the FBN so that you can hear the actual tone of the pickup, consistently. Eliminate that variable, and then work with the other variables to shape the tone. Trying to deliberately incorporate microphonics into the sound of your pickup, in a measured repeatable amount, is going to be tough and frustrating. Not impossible, but tough.

[Beef Coon]

In addition the the shift in capacitance that Joe explained, potting can (or, should) eliminate Funny Background Noise. That's a technical term. Microphonics can happen in all different degrees with different results. When internal parts of the pickup can wiggle in relation to each other, they can create little unintended signals that can add to, or subtract from, the main signal. Very unpredictable in frequency and amplitude. It can change what you are hearing and disguise the pickup's sound. You may like the resulting sound of a particular pickup that's slightly microphonic, but it will be difficult to replicate that sound, and it may even change over time.

To me, that's the biggest reason for thoroughly potting the coils and the pickup assembly: Get rid of the FBN so that you can hear the actual tone of the pickup, consistently. Eliminate that variable, and then work with the other variables to shape the tone. Trying to deliberately incorporate microphonics into the sound of your pickup, in a measured repeatable amount, is going to be tough and frustrating. Not impossible, but tough.

I've been experimenting with different size components & "fitting" in the assembly of a couple of my pickups to try and come up with a consistent formula to have "just enough" mechanical resonance to have some microphonics and "liveliness." I personally like my stuff to be right on the edge - actually a bit over the edge, but lots of people def. don't! When I first started making pickups I had friends play my guitars and for most of them the pickups were too out of control and it was a really good learning lesson.

[SonnyW]

I've been wondering why potting might change the sound of a pickup - it looks like you've just explained it for me. Thanks.

Still, I'm interested in how much of an impact potting is likely to have. I'm going to run some experiments some time soon, using my CGM-101 oscilloscope to see how the resonant frequency and the frequency response curve for some of my pickups respond to potting. Seems like a pretty simple "before" and "after" comparison.

I encourage you to try measuring this. I have measured the resonant peak before and after wax potting many times using my Tek scope on the x-y setting to plot inductive reactance vs capacitive reactance, using the method that Joe Gwinn has detailed several times, and have always seen a drop in the resonant frequency, sometimes as much as 1 kHz. I'm confident this is due to the different dielectric constants of potting wax vs air as Joseph R. has explained earlier. Also this effect could explain why some of the early Fender pickups have a unique tone that might be partially due to the kind of potting wax that they used. The formula they used contained a small amount of carbon, as in lampblack (i.e. the wax was somewhat more conductive than waxes used these days) I have seen this kind of wax in rewinds that I have done.

On another topic, I have also seen somewhat of a small drop in DC resistance which happens after potting that I cannot logically understand the cause for, but it has happened many times.

[charrich56]

I have had pretty good luck using vinyl-based sanding sealer for potting pickups. It was talked about and recommended on the TalkBass forum some time back. I tried it and like the results.

It does a good job potting the coil and also allows rework since you can peel wire off the coil even when potted.

Here's the one that was recommended to me and that I use:

Sherwin-Williams
Wood Classics
Interior Wood
Fast Dry Sanding Sealer
Clear
6403-15891

I am going to try this with a vacuum potting setup one of these days and see if it pots faster or better, but am pretty satisfied overall. Just make sure you move the bobbin to different angles while potting to release trapped air bubbles just as you would do for wax. You can clean up drips etc. with mineral spirits when wet. Only drawback is drying time but overnight usually works OK.

Might be worth a try. I can't comment on the effect on capacitance or sonics versus wax but it does do a fine job with microphonics. Also a lot less hassle and no risk of melting bobbin material. You could even use paper/cardboard bobbins and it would serve to reinforce the paper.

-Charlie