Soldering - Precison Electronic Corporation (PEC) Pots

[RJ Guitars]

Hey solder gurus,

The spec sheet on the high quality PEC pots in 2 watt format (RV4) says they have a stainless steel housing on them. http://www.precisionelectronics.com/0-40a-010a.html In some amplifier builds there are ground bars soldered to the back of the pots.

In my experience soldering to stainless doesn't work with tin/lead solder and a soldering iron. Has anyone had any success soldering to the back of one of these RV4 pots?

or

Is there a high quality pot to use an alternative to these? I looked at the PEC RV2 with a plated brass housing. They looked excellent until the price quote came in at ~$50 each. http://www.precisionelectronics.com/0-40a-030a.html

Any thoughts?

rj

[RJ Guitars]

I found my own answer to some degree.... https://www.techni-tool.com/content/res ... ester.html

So how are the guys using these in Trainwreck traditional builds that have a ground buss wire soldered to the back of the pots?

rj

[Cliff Schecht]

Solder gun? That'll push out enough heat to solder to anything.. Just make sure not to melt the insides of the pot (i.e. be quick about your work).

[hitchcaster]

hey rj, hows it going man.. yeah, ive done it.. i used a dremel to rough them up a little.. then used high quality flux and solder and a digital iron as hot as it would go... worked no problem.. but some solders don't work very well... ive got a bunch of different kinds but wonder solder is the one that worked best for me...that doesn't mean whatever you have won't work... good luck with it!

oh yeah, i first got solder to attach to the pot... then later soldered the bar to the pot.. alot easier that way then trying to do it all at once

[Structo]

Yes it is advantageous to tin the area you are soldering to first to help with the heat transfer to the buss wire.

Stainless steel has always been hard to solder to.

I wonder if soldering containing silver would work any better?

[Cliff Schecht]

Probably not, the problem with soldering to stainless is it is harder than usual and requires more heat. Aren't there special fluxes for soldering to stainless?

[Zippy]

Probably not, the problem with soldering to stainless is it is harder than usual and requires more heat. Aren't there special fluxes for soldering to stainless?

What does hardness have to do with wetability?

More likely that the bad actor is the native layer of chromium oxide that migrates to the surface of stainless steel and provides the most excellent corrosion resistance. That is a good reason to sand/grind back the surface before you try to solder to it. Surface roughness will also give a more reactive surface as well as mechanical bonding.

Yes, there exist special fluxes for soldering to stainless steel. Another way to achieve success is to plate an interlayer on the surface.

63-37 solder is preferred.

[Blindog]

RJ,

You can use these ground lugs: http://ampgarage.com/forum/viewtopic.php?t=10514

I used the PEC's in my Rocket, but I went with a Dumble style ground bus because I could't get hold of those lugs at the time.

Mark

[FYL]

In my experience soldering to stainless doesn't work with tin/lead solder and a soldering iron. Has anyone had any success soldering to the back of one of these RV4 pots?

Easy: don't solder, use a dedicated ring/lug fitted to the bushing and solder your bus to it.

Silly: use a small electroplating pen/brush and deposit a copper layer on the back of the pot, then use it as a soldering base.

Old-style: use a brass bus between chassis and posts, Fender-style.

Gonzo: plenty of flux and a high-power iron on a scratched surface. Works most of the time, kills the conductive layer in others (plastic pots don't like the gonzo approach).

[Structo]

I never really cared for soldering the buss wire to the back of the pots.

Seems like unnecessary work when you can just run the wire above the pots.
I like to make a right angle bend in the buss wire over by the input jack the secure it firmly with a screw, star washer and nut to the bare chassis.
Then you have a nice ground for the cathodes and pots.

I don't want to heat up the pots anymore than necessary and if you ever have to replace the pot it makes it more difficult if that wire is there on the back.

But I know everybody has their favorite method so......carry on.

[RJ Guitars]

I never really cared for soldering the buss wire to the back of the pots...

Tom, personally I am with you on that. Where I am coming from on this is that PEC pots seem to be the ultimate choice for a high end build. Recently I've been working on putting together a complete Trainwreck amp kit so it doesn't take a visit to 20 different vendors to get all "the right stuff". However, in the Trainwreck world a guy can run into a lot of criticism if he deviates from the original and they have that buss bar soldered directly to the back of the pots. For myself I can live without that but to furnish the build parts for somebody else it is usually better to try and stick with the original idea. So we have a fundamental collision of technology and tradition. Excellent quality pots that won't let you easily solder to the housing... now you know there is some engineer back at PEC place saying "why would anybody do that"?

I am relatively savvy to the problems of soldering on stainless but was curious if there was something I didn't know (always possible). None of the presented ideas sound like a simple harmless solution except for the use of the added solder lugs... the winning solution in my book. This maybe askew of tradition but an improvement on that theme in my way of looking at it.

As a related note I think it was Geetar Picker Glen that mentioned the problems of getting the pot nuts tight enough for a good mechanical contact against the chassis when squeezing them up against a wooden faceplate. These solder lugs would also give a good option for solving that issue.

I suppose the additional shielding behind the signals inside the pot might eliminate some noise so the solder lug idea again works well to accomplish that.

Thanks for the good discussion,

rj

[Cliff Schecht]

Probably not, the problem with soldering to stainless is it is harder than usual and requires more heat. Aren't there special fluxes for soldering to stainless?

What does hardness have to do with wetability?

More likely that the bad actor is the native layer of chromium oxide that migrates to the surface of stainless steel and provides the most excellent corrosion resistance. That is a good reason to sand/grind back the surface before you try to solder to it. Surface roughness will also give a more reactive surface as well as mechanical bonding.

Yes, there exist special fluxes for soldering to stainless steel. Another way to achieve success is to plate an interlayer on the surface.

63-37 solder is preferred.

That was a guess having to do with, like you mentioned, the chromium content of the stainless. Stainless, being a much harder steel, would take more heat to properly flow the solder. Me thinks there is probably more than just chromium added to the mix too which would affect how well solder can actually flow. The fluxes I mentioned are acidic in nature which will slightly etch the surface allowing for the wetting to occur without as much heat.

[Tinkerer]

I have soldered stainless using 'stainless solder', but it's not recommended for electronics work...may have high resistance, but being that I was using it to solder motors into slot cars, it worked fine.
The last amp I built was an Express based amp I built for a good friend, who, just to be a PIA wanted PEC pots, so I was faced with same dilemma as you are facing now. I used a separate ground buss running over the top of the pots, but didn't bother trying to solder a dedicated earth to the pots. My logic was that the pots are clamped to the chassis which is already a huge buss in itself. I thought about the Fender brass plate, but opted out as combining so many different metals in one area...aluminium chassis, brass plate and stainless pots...was asking for electrolysis and or corrosion problems.
Not running the ground on the back of the pots hasn't hurt the amp in any way. I have no noise to speak of, and everything works the way it should.
The green wire running across the top of the pots is 18g Teflon wire that I used as the ground buss between the pots and the input jack. At the presence pot it becomes 16g going to a ground point. (White wire). As you can see, the board grounds also attach to this buss, and when checked with my trusty Fluke, all grounds read 0 Ohms resistance.

Hope this helps
Tinkerer

[Structo]

Do they have to be PEC pots?

How about the Bournes pots Apex sells?

Aren't those high quality?

[KellyBass]

Just say the word, RJ, and I will happily send you some of those solder lugs to try. The ones that I have don't have the holes...they've got a small "plate" area that can be bent upwards slightly to better accept a buss bar.