Marshall Bluesbreaker problem

[Karl]

Hello guys,

I got on my bench today a reissue Marshall Bluesbreaker. When it came to me it was losing all power after it had been running for 10-20 minutes. First thing I checked was all electrolytic caps. All elytic caps tested OK.

Next I replaced 3 potentiometers volume 1 and 2 and bass. The old ones were very stiff and almost impossible to rotate. After this I tested the amp and played it for 2 hours without having any problems.

My question to you guys is since I am only 21 years old and have limited experience.
Has anyone had similar problems with pots failing when they get hot?
Is it a common problem for pots to get extremely stiff in amps with the controls facing up?

Thanks

[xk49w]

Is it a common problem for pots to get extremely stiff in amps with the controls facing up?

It depends on how much beer and soda mixed with cig ashes and other unmentionable crud got spilled in there.

[tubeswell]

Yeah - and maybe they were squirting lubricant into the pots as well in the false hope that it would make things better/prevent problems. Sounds like the owner needs a talking to about the birds and the bees of amp maintenance.

[ampdoc1]

If any pots are getting hot, you've got a worse problem than you think.

Typical pots in a guitar amp are handling very little power. Dirt, and old age are completely another problem!

[Karl]

If any pots are getting hot, you've got a worse problem than you think.

Typical pots in a guitar amp are handling very little power. Dirt, and old age are completely another problem!

The pots are getting hot from the all the heat rising up from the tubes.

[6G6]

Someone may have tried a cleaner with no lubricant in it.
Yeah, it gets 'em clean...right before they seize up!

[Phil_S]

Worse yet, some genius may have squirted some WD-40 in there. I've seen flame wars over whether WD-40 is appropriate for pots and my amateur conclusion is "absolutely not."

[Jana]

One thing I have found with most pots made in the last decade or so is that they put a gob of greasy lubricant in them. It is usually put on the plastic part that rubs on the cover. But, whether from amp heat or heat from trying to solder the ground buss on the back of the pot, this grease will flow. One of the things I do when building an amp is take all the new pots apart and clean the excess grease out of them before installing them. I just use a Q-tip.

This grease is, I believe, one of the reasons pots become so hard to turn, the grease gets on the resistance trace and gets sticky. It also gets into the shaft/body area and tends to gel up.

[solderstain]

... a gob of greasy lubricant in them. It is usually put on the plastic part that rubs on the cover...

The purpose of that grease is to be an anti-static agent (it's conductive), precisely because the wiper in most pots these days is plastic, versus other non-conductive materials that got used in days past. The grease itself doesn't actually 'hurt' anything, and is more resistant to heat than you might think - the manufacturers know that the rear casing gets soldered to. It gets less viscous during the actual soldering time, but for most 'normal' operating conditions, including the temperatures inside the average tube amp, the grease doesn't go anywhere.

When I ordered my custom pots from CTS, I addressed the issue of that grease with my U.S. rep, and asked if I could get them without the grease (reduce the rotational resistance a little more); they talked me into leaving it there. I can't address the grease in other companies' pots, but I'd guess it has a similar purpose.

[Structo]

I work on quite a few guitars and had noticed over the last few years that the CTS pots I have been using seemed to be getting stiffer in rotation.

That isn't a good thing with some players who like to do pinky swells with their little finger.

Recently I completed a Telecaster build and used a couple CTS pots I bought through Allparts since I had to order a few other things there I bought the pots as well.
These pots rotate super easy and don't have that stiff greasy feel as I had been noticing before.
They are the solid shaft vintage style with the depression in the back.

[img:350:232]http://www.allparts.com/v/vspfiles/phot ... -000-2.jpg[/img]

I didn't take any apart so I don't know if they have the grease inside but I have a feeling they don't.
Most guitarists don't like a stiff pot and prefer an easy turning one for sure.
I'm not sure what all model pots they make anymore but it seems the 24mm ones I used to use were the series 295 ones. Or the series 270.
But I have also seen the smooth back ones as well.

[solderstain]

I work on quite a few guitars and had noticed over the last few years that the CTS pots I have been using seemed to be getting stiffer in rotation.

These pots rotate super easy and don't have that stiff greasy feel as I had been noticing before.

The anti-static grease in CTS pots is only part of the equation for rotational resistance. It's actually one of the items a buyer like me has to spec. They have a number of different rotational torque specs available. A lot of it is friction between the shaft and the bushing it rotates in. The degree of 'fit' between those two things is the big factor. There's also the tension of the wiper against the resistive element. And the anti-static grease.

Naturally, the lower the tolerance a buyer selects for each item on the list, the less each pot costs. That's one of the reasons some CTS pots are super-tight - the buyer spec'd them as cheap as possible.

[Structo]

Interesting.

On an amp I don't mind stiff pots, in fact I almost prefer them because that way there is no accidental slip on the master or volume taking it to 11 when you don't want to.

On a guitar it's different. These pots I bought almost seem too loose.
Really strange. I think you are right about the clearance between the shaft and bushing on these, because I have some heavy solid brass and chrome tele knobs on it, and you can rock the knob sideways to see how much play there is there. But the turn super easy and like I said, most guitarists like that.

[solderstain]

Really strange. I think you are right about the clearance between the shaft and bushing on these, because I have some heavy solid brass and chrome tele knobs on it, and you can rock the knob sideways to see how much play there is there. But the turn super easy and like I said, most guitarists like that.

The clearance is everything. The pots I buy for my business are directly for guitar installations. CTS doesn't have a generic 'spec' 500k 'guitar pot', contrary to what a whole lot of people think. Anyone who buys pots directly from CTS has their own order and spec sheet - there are a whole buncha things that have to be decided for each order. Rotational torque is one of them. When I was talking to my CTS rep (now retired... he was a great guy to talk to...) about deleting the anti-static grease, we also talked about the other factors that effect the rotational torque spec. Shaft-to-bushing clearance is the biggie. Interestingly, shaft and bushing material also have an effect - two more things you have to spec. The shafts on my pots wiggle a little bit too, so I'm not surprised yours do. Comes with the territory.

[Structo]

Yes, I was not aware of CTS making the custom pots for so many different companies.

Stands to reason why I have seen so many variations on them.
Like the ones Allparts sells, they are probably made to their specs.

I have another tele that has smooth backed CTS pots and it has about twice the rotational stiffness as this tele.

Good to know that it isn't just poor quality control but rather the way somebody had them made.

[Dai H.]

I did some googling on that stuff and I found it's called "damping grease" and it comes in different levels of viscosity. I don't think it's typically conductive but perhaps some are. Also used for switches (also moving parts such as on cameras) and sometimes (as commented on above) there can be a bit of excess(say a little lump or somewhereabouts). If you use something like alcohol that doesn't leave any lubricant behind, the pot can freeze up since all the damping grease gets washed away. I've wondered if it'd be a good idea for vendors to sell small portions and maybe a syringe in different levels of viscosity so one could change the rotational "feel" as they desired on a pot.