Replacing those unsalvageable rheostats

[Carlspeak]

Attached is a word document put together to show a severe example of corrosion that can occur in original AR rheostats. As many of you know, this is a common problem in vintage AR speakers over a range of models. Many original rheostats are indeed salvageable as long as the surfaces are not too pitted. This example is intended to illustrate a way to replace those that are not. Included are closeup pictures of the corrosion.

Additionl photos show a new, 15 ohm ceramic rheostat I have been using to replace the Aetna-Pollak version when it is no longer salvageable. The new rheostat is one that has been recommended via posts to this site a number of times. It is very well made with a carbon wiper chip. One slight drawback is the exposed wiper surface. To solve the problem I fabricated a wooden cover to help keep fiberglass stuffing away from it. It is worth noting that although the original rheostats look fully inclosed, they are not. They have 3 radial slots about 1/8th inch wide by 3/4 inch long.

Remember, it's all about the music

Carl

Carl's Custom Loudspeakers

[dynaco_dan]

Hi Carl;

Nice photos, Carl.

What are our costs on these new ceramic pots?

[Carlspeak]

>Hi Carl;

>Nice photos, Carl.

>What are our costs on these new ceramic pots?

Vern,

I don't sell them because they are easily obtainable at lower cost by those who are able to install them themselves. I charge $20 ea. to customers who are having a general upgrade of AR speakers done by me.

Remember, it's all about the music

Carl

Carl's Custom Loudspeakers

[Guest baumgrenze]

Carl,

Here's a thought for an alternative rheostat cover. I'd suggest cutting a polyester water bottle to an appropriate length. Retain the bottom end of the bottle. Cut it long enough so that you can make 3 or 4 flaps to fold outwards and then screw it to the back of the cabinet.

IMO it would be less massive and less likely to be reflective than a plywood cover.

In the next few weeks I'll be doing more work on a pair of AR-2AX speakers. They are packed with a coarse textured packing paper, not fiberglas. Does anyone else know how long this material was used? I am concerned that it is probably made from sulfite pulp, the fiber that has been blamed for acid degradation of books. I have some simple tests I'll try. I'll also post a photo or two to document what I'm talking about.

Baumgrenze

[dynaco_dan]

>>Hi Carl;

>>Nice photos, Carl.

>>What are our costs on these new ceramic pots?

>

>Vern,

>I don't sell them because they are easily obtainable at lower

>cost by those who are able to install them themselves. I

>charge $20 ea. to customers who are having a general upgrade

>of AR speakers done by me.

>

>Remember, it's all about the music

>

>Carl

>Carl's Custom Loudspeakers

Hi there;

To follow up my question to Carl, regarding our cost per pot.

I have read a lot of resourceful write-ups about the problems and solutions members have used to reduce or eliminate their pot troubles.

I live in Vancouver BC Canada, and for some reason our ocean proximity doesn't seem to have any effect on our pots here.

In my museum, I have kept a few NOS AR pots for my future needs, and here it is 35 years later, I have only needed to turn my pots a few times, in all these years.

There is that familiar screeching, as you turn the pots a few times, sure raises the hair on your back.

I now have a physical sample in my hand, of what appears to be an identical unit, to the one Carl took a photo of, along side an OEM AR unit.

I wrote up about one that was available from a friend of mine, last year, I believe.

I have never seen a sample of this particular replacement pot, on our website, or anywhere on the internet, prior to Carl's great photo.

If there was or is another photo, I guess it is buried pretty deep into the AR topics section.

I made 5 recommendations to my friend, who is in touch with the manufacturer.

If I had a cup of coffee, I might have thought of more, maybe.

My suggestions may involve patent rights or copyrights, so I cannot discus them.

What I did ask for, was, a price per unit in quantities of 1,000 and 10,000.

My thought is, that a group purchase for members may be possible down the road.

How many members own more than one pair, more than one dozen, yes.

At the present time, 0.5, 1, 2, 5, 10, 20, 50, 100 ohms are available in the 25 watt rating at $15.00 each, quantity one.

Presently 15 ohms is non-stock locally, but apparently is being manufactured.

They would be available in the present 15 ohm 25 watt wire wound pot form, but very much improved and totally compatible with our specific AR speaker needs.

Safety is also my priority with my suggestions.

If you are not aware of how hot an OEM AR pot can get, I will now give you a very brief description.

Can I ever be brief? lol

The earliest pots, that I am familiar with, had red or black plastic shafts, for us to turn.

When I worked at the local AR warantee depot, I saw more a few of the shafts had melted off from the heat.

I don't have any information, as to when they went to the aluminum shafts.

I am concerned with a fire inside an enclosure, if a flammable shield was to be used, in error.

If you look at your toaster coils, when toasting bread, that, is well over 1,000 watts and you can see that it is hot.

I know that I read recently, the kind of celcius temperature that comes from the speaker voice coils, it is amazing and goes un-noticed by listeners.

Also, just to mention, but too large in diameter for our use, maybe, 50 watt rating pots in 1, 2, 5, 10, 20, 30, 50, 100 ohms, these are 2 1/2" in diameter and also protrude 1 1/2" into the enclosure as well and sell for $20.00 each, quantity one.

Presently 15 ohms is non-stock.

The 50 watt units and the 25 watt units, presently have exposed contacts and coil.

The 25 watt version measures 1 5/8" +/- in diameter, the main body protrudes approximately 1 1/2" behind the speaker mounting board and they have a locating stop pin, which is adjustible.

It also includes one lock nut, no lockwasher.

The threaded portion allows for approximately 5/16" thick wall thickness.

The 1/4" shaft, at present, has a flat spot for a knob setscrew.

These are, as I wrote earlier, built like a tank and look to be of substantial quality, with a brush/coil contact rather than the present OEM stator/coil contact.

We shall see what the end cost is, and I will let you know more then.

Maybe an affordable solution for us all?

If this should come together, I would be happy to co-ordinate a bulk buy.

I do not want any money for myself in the process.

[Carlspeak]

In quotes below is a passage from Vern's e-mail.

"Also, just to mention, but too large in diameter for our use, maybe, 50 watt rating pots in 1, 2, 5, 10, 20, 30, 50, 100 ohms, these are 2 1/2" in diameter and also protrude 1 1/2" into the enclosure as well and sell for $20.00 each, quantity one.

Presently 15 ohms is non-stock.

The 50 watt units and the 25 watt units, presently have exposed contacts and coil.

The 25 watt version measures 1 5/8" +/- in diameter, the main body protrudes approximately 1 1/2" behind the speaker mounting board and they have a locating stop pin, which is adjustible.

It also includes one lock nut, no lockwasher.

The threaded portion allows for approximately 5/16" thick wall thickness.

The 1/4" shaft, at present, has a flat spot for a knob setscrew.

These are, as I wrote earlier, built like a tank and look to be of substantial quality, with a brush/coil contact rather than the present OEM stator/coil contact.

We shall see what the end cost is, and I will let you know more then."

The center distance between the pots on the classic AR speakers is in the range of 1 11/16 to 1 3/4 inches. Thus, you cannot install a pot any larger in body size than that. The L-pads available from Parts Express and MCM are all too large. Don't bother ordering them. I'll save you the trouble. I have, and ended up throwing them away.

The Ohmite pots I use and wrote about earlier have a body diameter (less solder tabs) of about 1 9/16 inches. 25 watt units are in stock and available for about $9 each including shipping. Check out Skycraft Surplus. I don't have a problem with readers of these posts who are capable of changing out the pots themselves buying them.

Remember, it's all about the music

Carl

Carl's Custom Loudspeakers

[dynaco_dan]

Hi Carl;

Thank you for being so helpful.

[dynaco_dan]

Thanks again, Carl.

Just thought of why I mentioned the larger 50 watt rated pots.

I have not looked inside a speaker enclosure for a long, long time now.

I was thinking that they might possibly fit single pot installations, for example, AR-4, AR-4X, AR-2X, AR-1, AR-1X, etc, and other brands as well.

[Guest rbferrell]

I'm working with a couple of the Ohmite pots* for one of my AR-3a's, and have devised and discounted several means for shielding the wiper from the fiberglass. First, plastic must not be used due to the heat, which leaves wood or metal (any other ideas?). Wood is bulky in the tight confines of these pots (only 3/16" between the pots and the speaker terminal machine screws). Metal must be used with caution to avoid sharp edges, shorts, and surfaces prone to vibration and resonance. I've considered separate round hoods fastened to the rear axis of each wiper (essentially a 1.5" diameter cap about 1/2" tall), and a single hood covering both pots and fastened to the masonite. No clear resolution yet on which method may be most practical and effective, but I'll post my experience when done.

Meanwhile, if others are further down this road than me, I'd appreciate the benefit of your experience.

* RHS15R, 15 ohm, 25W, $8 each from www.skycraftsurplus.com

[dynaco_dan]

Hi there;

Thank you for that link.

This is a product I have not seen before, at a very attractive price too.

[dynaco_dan]

>Hi there;

>

>Thank you for that link.

>

>This is a product I have not seen before, at a very attractive

>price too.

Hi there;

Thank you again for that source information.

I am following up on this Ohmite Rheostat.

The product appears very similar to the model I have on hand, and the sample Carl took a photo of.

It looks to be of good heavy duty quality, too bad about the open rearend as well.

If I have a choice, I will always choose an North American made product over an off-shore product, with very rare exception.

When I started in my own business, I went to one of our largest local wholesalers.

I told them, that I wanted my materials made in Canada first, USA second and Britain or Australia third, they laughed at me, and said, you take what we've got.

In consideration of those members that are using the Metric System, I am going to attempt to convert the measurements to Metric as well.

The Chinese rheostat model that I have on hand is 1 5/8" 1.625"/41.275 mm +/-in diameter, which is ever so slightly larger in diameter than Ohmites 1.560"/39.62 mm +/- diameter (1 9/16" =1.5625"/39.68 mm).

I checked out that particular source link you gave us, against 2 other outlets.

Surplus parts, and that is a surplus stock price, are always available at sub-retail pricing and sub-wholesale as well.

There is an end to their low prices when stock runs out.

Standard retail ongoing price appears to be $27.13 each quantity one.

Ohmite is a quality brand name from my personnal experience.

Thanks again for the tip.

It is a help to every member.

[Carlspeak]

Bob. I think you're trying to do too much to protect these rheostats. See what I did in my original post. There is a clear photo showing the wood cover I made and installed. Remember, the old Aetna-Pollak AR pots had nothing covering them except it's own construction which included three 1/8 inch wide slots about 3/4 inch long to let in god knows what. The batts of box stuffing tend to remain relatively motionless once the speaker is put in it's listening position and thus there is little opportunity for fiberglass to break off and start moving around. What I built protects the pots from above and in front of them. It's eash to construct,has a removable cover and is glued in place.

Remember, it's all about the music

Carl

Carl's Custom Loudspeakers