Emergency potting

[J M Fahey]

Old style wire enamel was strong, quite chemical resistant and you had to burn it first, then scratch/sand it away, go figure.

I deplore modern reliance on sissy snowflake "self soldering" enamel, which evaporates leaving behind shiny copper perfect for soldering but is atoms thick and resists no solvents (except, maybe, alcohol or mineral spirits and not too sure about the second) and statistically what´s most probably in your pickup, but you can safely use paraffin wax which melts around 68C so a tin floating in hot water is fine, just leave pickup inside long enough (1 hour?) so its deepest core reaches melting temperature.

I much prefer wax potting to anything else because when wax cools, it "pulls" very little and in any case is way softer than anything else there.

[Chuck H]

...but you can safely use paraffin wax which melts around 68C so a tin floating in hot water is fine, just leave pickup inside long enough (1 hour?) so its deepest core reaches melting temperature.

Use a pot of hot water on the stove top. Put a couple of 1/4" wooden shims in the pan. Then put another pot on the wooden shims big enough to hold the pickup, the wax and maybe something heavy enough to keep it from wanting to float. That should work fine.

My wax potting rig is a repurposed coffee maker with a wide mouth carafe and a burner modification. The wife still has leftover blocks of paraffin from a candle making phase I stick a standard kitchen thermometer into the wax to monitor temperature.

[Helmholtz]

How do you retain all these details about vintage specifics!?! Between this and your tech chops you've been a real asset to the forum

Thanks. I have always been a researcher with a special interest in vintage stuff.

While my own '68 strat PUs are obviously wax potted (I can scratch off some wax with my fingernail), lacquer potting in later strat PUs was confirmed by people who do a lot of rewinds, see e.g. https://music-electronics-forum.com/...l=1#post449209 . There are also other interesting threads on this topic on the MEF.

Here is an original Fender factory spec. for Tele lead PUs dated '65/'67 (from A.R. Duchossoir's book "The Fender Telecaster"):

Seems they used lacquer to stabilize the magnets-flatwork assy before winding.

BTW, I would think that plain enamel is more susceptible to melting by lacquer thinners than formvar because its formula is oil-based. Testing by submerging a piece of wire in a jar with thinner would be easy and essential.

[Chuck H]

Testing by submerging a piece of wire in a jar with thinner would be easy and essential.

This^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

[Helmholtz]

In one of the older MEF threads/discussions on PU potting I found this interesting patent of Leo Fender:
Fender - Wax Potting pat4885970.pdf

His rationale is/was preventing moisture from penetrating the winding(s) and changing the tone. Strangely it was filed as late as 1987, even though Fender PUs were wax potted at least since the 50s.

[David King]

Fender dipped armatures prior to winding them in lacquer in an attempt to insulate the magnets and prevent the wire from shorting to them. It was not very reliable. When I rewound a 63 Jazz bass neck PU there was a layer of masking tape around the magnets. They were lightly wax potted, definitely not full penetration.

[Chuck H]

In one of the older MEF threads/discussions on PU potting I found this interesting patent of Leo Fender:
[ATTACH]53724[/ATTACH]

His rationale is/was preventing moisture from penetrating the winding(s) and changing the tone. Strangely it was filed as late as 1987, even though Fender PUs were wax potted at least since the 50s.

I don't think Leo ever assumed, even as it was happening with the gear he designed, that anyone would have problems with microphony due to acoustic feedback. The concept was outside of his understanding. Why the hell would anyone turn up an amp until it distorted everything???

[fieldwrangler]

Folks -

Sorry to have been absent through all this activity. It seems that I get email notifications of activity for a day or two and then they stop, even though the activity continues.

This particular coil is formed from American Formvar wire from maybe 1982. I did some experiments with coils of similar constitution, squirting ultra-thin Flu-Boost CA into the coils while watching my faithful Fluke ohmmeter. Small rise in resistance with heat, but when it all hardened (hours later without accelerator) the resistance was just where it started before the gluesquirt. Coil seems quite solid and resistant to translating physical shock into signal. Another experiment using accelerator was similarly benign to the insulation but seemed to stop penetration too early. An appointment with a vacuum oven next week could prove to have some significant results; if so I'll be back.

Bob Palmieri

[Bruce Johnson]

Bob;

I've been potting all of my pickups with a special super-thin epoxy called CPES (Clear Penetrating Epoxy Sealant), made by Smith & Co. It's a two-part epoxy, with a viscosity about like alcohol. It's made for sealing up old rotted timbers in wood reconstruction. It works beautifully for potting pickup coils. It soaks thoroughly down through the coil and cures. But it always stays a little rubbery; not brittle hard. It doesn't heat up at all, that I can tell, while curing. Doesn't harm the insulation on the wire. I lightly brush it onto the outside of the coil, and keep adding more until I can see that it's fully saturated. It cures in 4-6 hours.

I like it better than wax for potting coils. I've cut some coils apart, and it fully saturates and glues all the wire together. Forever; you're not going to unwind it. But you can cut the potted wire off a bobbin, if necessary. The biggest downside is that it stinks; powerful fumes while mixing and curing. You need ventilation while working with it.

I've been potting all my pickups with it since about 2005. Never had a pickup failure out in the field; never had any lab failures due to potting.

[Chuck H]

Bob;

I've been potting all of my pickups with a special super-thin epoxy called CPES (Clear Penetrating Epoxy Sealant), made by Smith & Co. It's a two-part epoxy, with a viscosity about like alcohol. It's made for sealing up old rotted timbers in wood reconstruction. It works beautifully for potting pickup coils. It soaks thoroughly down through the coil and cures. But it always stays a little rubbery; not brittle hard. It doesn't heat up at all, that I can tell, while curing. Doesn't harm the insulation on the wire. I lightly brush it onto the outside of the coil, and keep adding more until I can see that it's fully saturated. It cures in 4-6 hours.

I like it better than wax for potting coils. I've cut some coils apart, and it fully saturates and glues all the wire together. Forever; you're not going to unwind it. But you can cut the potted wire off a bobbin, if necessary. The biggest downside is that it stinks; powerful fumes while mixing and curing. You need ventilation while working with it.

I've been potting all my pickups with it since about 2005. Never had a pickup failure out in the field; never had any lab failures due to potting.

Brilliant product suggestion Of course, it can't be as thin as alcohol because it MUST have some solids in it to work and alcohol is LESS than water thin. But I get your point. It's thin enough that it doesn't impede the process of saturation. As a sealer I might have been suspicious of it effectively catalyzing in a penetrative situation (based on my own experience). But having a first hand account gives me A LOT more confidence. Thank you.

[Bruce Johnson]

I haven't seen an actual specification of the viscosity of CPES, but it's somewhere between alcohol and water, about like the super-thin CA glue. It definitely fully saturates tightly wound wire coils without any voids. I've cut coils apart and looked at them. As I said above, it stays a bit rubbery after curing. If you cut the coil with a knife, you can peel the bundle of wire back. It's still bendable, not rock hard. I like that as a characteristic for potting material. That tells me it's cushioning the wire a bit against vibration and thermal expansion.

As compared to wax, the advantages are:
It's a permanent cure, tougher material, never going to be affected by moisture or temperature. Should never dry out or crack over time.
Very simple neat application by daubing. No need to dunk it (although you could if you want to); no real cleanup needed afterwards.
No need for vacuum or temperature control. It soaks in completely and cures by itself.
No heat rise during the potting; no temperature damage to parts or connections or wire insulation.

Disadvantages:
It's a slower process than wax. Only a few minutes to daub it on, but then it needs 4-6 hours to cure.
It is permanent; you're not going to melt it off and unwind the wire.
It stinks. A strong odor from its solvent. Application and drying needs to be done in good ventilation. After it's fully cured, the smell is gone. That's how you tell it's cured.
CPES isn't cheap: $48 for 2-pint kit on Amazon. But 2 pints will probably do hundreds of pickups.

[Helmholtz]

Would be interesting to know the material's dielectric constant and electrical conductivity, in other words its potential influence on PU characteristics. Paraffin has excellent properties but does somewhat increase self-capacitance.

[Bruce Johnson]

I haven't been able to measure any change in resistance, capacitance, or overall sound between unpotted and potted. It's plastic, filling the space between the wires.

[Helmholtz]

It's plastic, filling the space between the wires.

And it's the material in the space between the wires which increases capacitance (compared to air). The dielectric constant determines how much the potting material increases the capacitance of air gaps. Plastics typically have a dielectric constant between 3 and 8 (some epoxies). Paraffin is around 2.

But as long as the total resulting self-capacitance is low compared to the capacitance of the guitar cable, there won't be much effect on PU sound.

[fieldwrangler]

Bob;

I've been potting all of my pickups with a special super-thin epoxy called CPES (Clear Penetrating Epoxy Sealant), made by Smith & Co. It's a two-part epoxy, with a viscosity about like alcohol. It's made for sealing up old rotted timbers in wood reconstruction. It works beautifully for potting pickup coils. It soaks thoroughly down through the coil and cures. But it always stays a little rubbery; not brittle hard. It doesn't heat up at all, that I can tell, while curing. Doesn't harm the insulation on the wire. I lightly brush it onto the outside of the coil, and keep adding more until I can see that it's fully saturated. It cures in 4-6 hours.

Bruce!

Wow... thanks for this - didn't know from this product at all.

By the way, the Glu-Boost Ultra Thin CA ended up working surprisingly well in the hour of my urgent need. All measurements remained in spec, no heat issues (without the dreaded accelerator) and my super-high-end guitar builder client is happy.

However, I'm definitely gonna try the CPES.

Bob Palmieri