Ohmite Pot Cover Ideas?

[Shacky]

Does anyone have a good idea for covering these Ohmite pots? I tried landscape cloth glued around them - second picture - but the fabric hung up in the sweeper. I was thinking of cutting pieces of dowl and glueing/screwing them around outer edge to hold cloth up off the sweeper.

But wanted to see if some has a simpler solution first. A cup the right size might be the perfect solution.

[genek]

Does anyone have a good idea for covering these Ohmite pots? I tried landscape cloth glued around them - second picture - but the fabric hung up in the sweeper. I was thinking of cutting pieces of dowl and glueing/screwing them around outer edge to hold cloth up off the sweeper.

But wanted to see if some has a simpler solution first. A cup the right size might be the perfect solution.

Electric outlet box?

[JKent]

Hey Shack

I know Carl uses these pots. If you search a bit you should find a pic of his plywood shield. I’ve never been inside the AR3a but from your pics it looks like you could put a piece of Masonite or plywood across to bridge the upright pieces of plywood. If needed you could cap off the ends as well.

Question: How much of an issue is heat dissipation (how much air circulation is needed?). And a related question: How impermeable does the cover have to be (is the FG dust a problem, or just the fibers?). The outlet box sounds like it may be a good idea—the metal “1900” box would dissipate heat some, but they do have a few holes in them and fastening could be a challenge.

I am interested in the outcome because I have some of these Ohmites and am wondering if they are worth the hassle (compared to L-pads, which are cheap and simple).

Good luck and keep us posted!

Kent

[genek]

Question: How much of an issue is heat dissipation (how much air circulation is needed?). And a related question: How impermeable does the cover have to be (is the FG dust a problem, or just the fibers?). The outlet box sounds like it may be a good idea—the metal “1900” box would dissipate heat some, but they do have a few holes in them and fastening could be a challenge.

The original A-K units have slots in their cases, and most modern pots and L-pads are not hermetically sealed, and AR crossovers come from the factory with FG stuffed into all the spaces. I think the issue is mainly keeping the fibers from fouling the wipers and windiings.

[Shacky]

The original A-K units have slots in their cases, and most modern pots and L-pads are not hermetically sealed, and AR crossovers come from the factory with FG stuffed into all the spaces. I think the issue is mainly keeping the fibers from fouling the wipers and windiings.

Yes I think that's the case. I also wonder if it's an issue of preventing a "burn" of the wool with potential for worse if the damping material got into the wiper.

Someone on AK mentioned tupperware.

I do think it's worth the effort as my 3a's sound amazing. While the L-Pad with resistor is close, these Ohmites are closer and across entire range. I measured resistance on these and the original AP's and it's spot on.

I agrree though - the L-Pads are much easier - and cheaper.

[soundminded]

The original A-K units have slots in their cases, and most modern pots and L-pads are not hermetically sealed, and AR crossovers come from the factory with FG stuffed into all the spaces. I think the issue is mainly keeping the fibers from fouling the wipers and windiings.

I think the problem was that the stuffing was mineral wool which contained some sulfur bearing compounds. Over time, humidity will disolve some of the sulfides, sulfates or whatever and turn it into weak sulfuric or sulfurous acids or H2S all of which are corrosive. I think that is how the pots are damaged. You could try the tupperware idea and use a lot of clear GE silicone caulking to adhere it to the wood. It should form an excellent hermetic seal, it's a good glue, and if you ever have to, you can remove it and replace it. It takes about 24 hours to set so put a weight on the tuppeware while it cures. It's also non toxic and you can work it with your fingers but it is gooey. Also check the use by date on the outside of the tube. After that date, the volitile chemicals inside it have evaporated and it won't ever cure. I found that out the hard way after I called GE's answering center to see what went wrong with one of my projects. Who would have guessed if they didn't know?

Don't know about the heat. It could be a problem I suppose. The alternative is to take the whole crossover network outside the box. Might sound strange to say it but I think that is how it should have been done originally. After the experience with my early KLH Model 6s, while I regarded Henry Kloss as an excellent speaker designer, IMO he was a horrible manufacturing engineer. But that's how he did it with the first AS speakers and it set the pattern for the entire industry.

[genek]

I think the problem was that the stuffing was mineral wool which contained some sulfur bearing compounds.

Mineral wool is also more hygroscopic than FG (retains more moisture). It's the reason why you occaisionally see rusted drivers come out of boxes that used the stuff.

I think I would have boxed the crossovers the way the AR-1 enclosed the fullrange speaker, so that they could have been removed from the back. Of course, that would have increased the cost.

[Zilch]

The power dissipation is calculated in the switched attenuator kit thread.

The metal electrical box seems the best idea to me so far, because it would be thermally conductive, and have thermal mass. Run the wires through grommets in the holes and seal them with RTV. Put screws in the remaining empty ones, if any....

[Shacky]

Thanks for all the great ideas. This pair of 3';s came with pink FG inside - not the usual rock wool. My other project pair of 3's has the typical stuffing.

[Shacky]

Thanks for all the great ideas. This pair of 3';s came with pink FG inside - not the usual rock wool. My other project pair of 3's has the typical stuffing.

[JKent]

(posted incorrect info here--sorry)

Kent

[Carlspeak]

For those who want to try these, Skycraft has them back in stock:

http://skycraftsurplus.com/index.asp?PageA...amp;ProdID=2007

The units Skycraft is selling are 25 ohm, not 15 ohm although they look the same.

You can fabricate a 3 sided box with cover to cover the rheostats. Just snake the wires around and out thru the one open side. In your case, the open side should face down toward the base of the cabinet since your connections to the Ohmites are facing down.

[JKent]

The units Skycraft is selling are 25 ohm, not 15 ohm although they look the same.

OOPS! Sorry

Thanks for setting the record straight

I confused watts and ohms. The ones I bought last year are 15 ohm, 25 watt. Those the right ones?

Kent

[Carlspeak]

OOPS! Sorry

Thanks for setting the record straight

I confused watts and ohms. The ones I bought last year are 15 ohm, 25 watt. Those the right ones?

Kent

YUP!

[ADVENTAGIOUS]

I'm thinking about trying these for my Ohmite covers. They can be mounted using 2 sided tape, easy for wire pentration, and only cost abt. $.75. The product name is "Vapor Barrier Single Gang" for light switch and outlet boxes.

[Carlspeak]

You should also consider some porous fabric stapled around the Ohmites. Whatever you get should allow heat dissipation from the Ohmites. Don't seal them completely. The main concern is larger clumps of FG or PET getting between the carbon wiper and the coils.

[JKent]

Adventagious: Those look promising. There are many ways to skin a cat . I was considering some small Tupperware-like containers made by Glad or Ziplok. Here are 2 other possibilities:

First photo shows the existing braces in an AR3 built up with 3/4" wood. The idea was to then put a Masonite cover across the whole area, making a box about 7" square and 1-1/2" deep. This is what I was suggesting in post #3, but there was some concern this would decrease the box volume and alter the sound.

The 2nd photo shows a small project box. $3 each from PE. A little cramped but seems to work fine. They have larger boxes too, but then there's the cabinet volume issue.

3rd photo shows the box with aluminum cover on.

Kent

[ADVENTAGIOUS]

Thanks for the ideas! Kent did you try to mount the open side of the utility box to the back of the speaker (just cover the pots instead of putting them in the box)? Maybe even make a little frame 1/4" high same size as the box, run wire management through the frame, seal the openings, then attach the box to the frame. Heat dissipation would probably be better using your box instead of the V/Barrier. Compression in the box might even push the V/Barrier around. If they shouldn't be perfectly sealed a small air hole could be drilled through the pot mounting board on the back. Just some thoughts...

[JKent]

Thanks for the ideas! Kent did you try to mount the open side of the utility box to the back of the speaker (just cover the pots instead of putting them in the box)? Maybe even make a little frame 1/4" high same size as the box, run wire management through the frame, seal the openings, then attach the box to the frame. Heat dissipation would probably be better using your box instead of the V/Barrier. Compression in the box might even push the V/Barrier around. If they shouldn't be perfectly sealed a small air hole could be drilled through the pot mounting board on the back. Just some thoughts...

No, I did not try it that way but that could work too. I thought the metal lid "could" serve as a sort of heat sink, but who knows.

I used the equivalent of Parts Express part #320-420. 4-1/8" x 2-11/16".

Part # 320-430, at 5-1/4" x 3-1/4" would probably be easier to work with if mounting the parts inside the box.

As I mentioned, altering the volume of the speaker cabinet was a consideration. Besides, my local parts store didn't stock the larger size

FWIW, I'm not convinced the Ohmites are all that great. I "had" a bunch of them I'd bought for $8 each. Sold all but 6. Maybe should have held on to them because they seem to be selling for over $30 now. In the future I'm going to use L-pads with resistors, as recommended by Roy. Cheap and easy.

Good luck

Kent

[dynaco_dan]

No, I did not try it that way but that could work too. I thought the metal lid "could" serve as a sort of heat sink, but who knows.

I used the equivalent of Parts Express part #320-420. 4-1/8" x 2-11/16".

Part # 320-430, at 5-1/4" x 3-1/4" would probably be easier to work with if mounting the parts inside the box.

As I mentioned, altering the volume of the speaker cabinet was a consideration. Besides, my local parts store didn't stock the larger size

FWIW, I'm not convinced the Ohmites are all that great. I "had" a bunch of them I'd bought for $8 each. Sold all but 6. Maybe should have held on to them because they seem to be selling for over $30 now. In the future I'm going to use L-pads with resistors, as recommended by Roy. Cheap and easy.

Good luck

Kent

Hi Kent

I don't have my notes handy right now, for the part number, I did check an Ohmite data sheet and that 25 watt 15 ohm pot

is or was available with a rear enclosing shield.

In fact there is several levels of enclosure sealing quality,

No mention of price and we are not going to find many at $8.00 US.

I don't know if Ohmitie is still an active US manufacturer of these pots but I see old surplus only on the net.

Extremely heavy duty and well made to say the least, built like the proverbial tank.

I think there is or was a rebuild kit including a new wiper available as well.

Previously I did mention that I scouted out a few stores here in Vancouver for a pot cover.

I ended up in our Chinatown, looking for chinese tea cups.

There is a small collection of them downstairs here along with sterifoam coffee cups, urine sample cup, McDonald's ketchup container and small clay potting pots.

There is more than one manufacturer of tea cups so I bought a few of each.

When I went cruising I carried them with me in a pack, one of each, as well as the Ohmite and Chinese pot samples.

My concern with my experience of having seen melted plastic pot shafts and burnt fiberglass insulation, was three fold, they needed

to be both fire resistant and electrically non-conductive.

They also needed to fit over the rear of the pot without touching the rotor.

The earlier A-P pots had black or red plastic shafts and later they switched over to aluminum shafts.

If I had found a satisfactory unit that fit, both single and dual pot usuable, then I was thinking that they

could maybe be spot siliconed to the rear of the cabinet.

[ADVENTAGIOUS]

Kent, that is nice looking work...results speak for your effort in spite of the difficulty. Did you just replace the mid and high caps as shown? I can see something else there but not sure if it is new or original low cap. Thanks. I have 4 of those Ohmites that you bought. Do the L-pads work better or just easier to install??

[JKent]

Kent, that is nice looking work...results speak for your effort in spite of the difficulty. Did you just replace the mid and high caps as shown? I can see something else there but not sure if it is new or original low cap. Thanks. I have 4 of those Ohmites that you bought. Do the L-pads work better or just easier to install??

Thanks. The xo shown is AR3, so there are only 2 caps and 2 inductors. However, if you read my AR3/3a saga you'll see that these will be changed to 3a's so then there will be 3 caps and 3 inductors:

http://www.classicspeakerpages.net/IP.Board/index.php?showtopic=6284&pid=87031&st=0entry87031

Not sure if the L-pads work "better." Some members here have raved about Ohmites. Roy, my source of information for all of these mods (L-pads, HiVi tweeters) puts a 25 ohm 10 watt resistor in parallel with the L-pad. He said there are 2 benefits: The sound is more like the original, and the power-handling is improved. He said that on a pair of AR4x's he preferred the sound with the L-pads and resistor (over original cleaned A-P pots) YMMV.

Vern--I know what you mean about heat. Attached is a photo of a pot I pulled from an abused AR4x. As I recall, they had cheap replacement tweeters and there was a 4.7uF cap stuck in the circuit. Who knows how hard they were driven but you can see the result. Not a pretty sight.

Kent

[RoyC]

Not sure if the L-pads work "better."

Assuming the back cover issue is resolved, the Ohmite is technically the more authentic replacement. It is well constructed, and electrically identical to the original pots.

The l-pad/resistor arrangement is more "authentic" than an ordinary 8 ohm l-pad. It has an electrically similar effect to the original pots (and Ohmites) within the first half of the control's rotation off maximum. A 25 ohm resistor is placed in parallel with the driver, across the parallel leg of the l-pad (between tabs #2 and #3, as typically connected to an AR speaker). This works for most AR models except the midrange of the AR-5. The white dot ("normal") midrange pot setting of the AR-5 is in the 2nd half of the control's rotation, which calls for a different resistor arrangement. 4 ohms in series, and 12 ohms in parallel with the AR-5 mid/l-pad combo works well.

Of course, there are many people happily using l-pads with no additional resistors. L-pads are inexpensive and easy to install...and a better approach than simply "by-passing" the controls, or struggling with badly corroded original pots. The most noticeable effect of a straight midrange l-pad replacement is a more pronounced, unbalanced, midrange response. Due to the reticent nature of the original AR tweeters, the stronger output resulting from a tweeter l-pad replacement may be desirable to some listeners.

Roy

[ADVENTAGIOUS]

I`m going with the Ohmites. After poking in the box today I appreciate now what Kent was talking about...real estate space is an issue and whatever cover method is used must be carefully installed to not make contact with the speaker wire terminals. My PP caps arrived today and I didn`t order any series resistors for them. Is that an issue.... Thanks

[RoyC]

I didn`t order any series resistors for them. Is that an issue.... Thanks

No issue at all, imo...

Roy