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W5M transformer repair - to pot, or not to pot.

maxhifi

maxhifi

Addicted Member
Recently had a power transformer fail in one of my W5Ms. I have the "guts" of a spare W5M power transformer, which was obtained from someone who had a single transformer fail, and then replaced both with Hammonds, and wanted to use the cans.

Right now I have the open transformer sitting on a piece of wood, on top of the chassis, and everything works great. Looks pretty bad though, and I would be afraid to move or touch the amplifier.

Regarding the failed transformer, I heated it up, and removed the tar, and the failed core. So now I've got an empty can.

So the question is - can I just fasten the replacement transformer "guts" to the chassis, and use the can as a cover, or do I need to pot the transformer in order to ensure proper cooling? Is covering the transformer with the can, without potting material, going to create issues for me? I can't reuse the tar, because it got contaminated during removal, and I threw it out.
 
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Placing the transformer (xfmr) into the can without the potting would insulate the transformer and it could possibly overheat. The potting conducts the heat away from the transformer to the can, which then radiates the heat away. Best way to see if this is an issue would be to place the can over the transformer (no potting), operate the amplifier, and watch the transformer temperature to see if it stays at a reasonable level. The potting could also damp out for hum as well.
 
I didn't think that the potting had anything to do with helping cool a transformer. I was always under the impression that it was mainly used to seal it to keep out moisture, to reduce corrosion, and to also reduce physical noises, and a side benefit to protect other companies from copying their designs. Honestly, I would think an insulator would be more likely to keep heat in rather than allow the heat to dissipate. I know my Cary's use silicone as a potting material as silicone and handle heat better.
 
How does microcrystalline wax work for potting regarding heat transfer. I've seen or heard of some folks using it for transformers. And, it would seem to have the advantage that it would be melted out easily if ever the need arose.

I had a similar experience a couple of years ago with a W4 power transformer which failed. I temporarily installed a Stancor 8412. It was little high on B+ but using a 5U4 brought it into spec. I mounted it so that an original can that I had would fit over it. Didn't seem to get very hot. Eventually got another original. At the time I thought that adding some vent holes low and high on the back would help.
 
Ken Boyd said:
I didn't think that the potting had anything to do with helping cool a transformer. I was always under the impression that it was mainly used to seal it to keep out moisture, to reduce corrosion, and to also reduce physical noises, and a side benefit to protect other companies from copying their designs. Honestly, I would think an insulator would be more likely to keep heat in rather than allow the heat to dissipate. I know my Cary's use silicone as a potting material as silicone and handle heat better.
Click to expand...
Potting does those things, too.
 
Ideally, I would like something which can be easily removed, in case something fails again. The tar was not exactly pleasant to remove, but it also was not chipping away at a solid block of epoxy or silicone. Those sound like a removal nightmare to me. I'm thinking, maybe if i were to make aluminum spacers, so the transformer could conduct heat directly to the can, it might be a way to avoid messy potting material.

My worry about wax, is simply that if it's operating near the melting point, it may off gas and stink in operation.
 
I suppose it depends on what it used for the compound. Something with poor thermal transfer would be like putting a blanket over it.

McMaster has silicone potting compounds with an 0.2w/(m-C) rating but I suspect it would end up like a big tub of bathroom silicone, not a lot of fun to remove. Might honestly be tempted to try lining the can with wax paper so it can't actually bond to the metal and emptying it would be just a matter of sliding the whole lump out. Not sure what that might to the thermal transfer though.

or just not, and the can is just a cover.
 
Melting points for microcrystalline was seem to be fairly high. This web page says 145-195 degrees. I assume that is dependent on which was is used as there are different versions.
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From what I've found with some searches microcrystalline was has slightly better thermal conductivity than tar for what that's worth.
 
Thermally conductive epoxy. Forget about trying to get the can off after you pour it in. Even with wax paper lining. At least that was my experience.

By the way it works extremely well as a thermally conductive agent.
 

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That has 2-3 times the thermal conductivity of the wax and closer to 4 times more than tar. But as you say, it's once and for all time.
 
Did you identify what failed in your original PT, and what may have caused it? Are you aiming to add any protection to mitigate known likely causes of PT damage in the W5M ?

Given temperature rise may exacerbate the risk of a failure, then lowering the thermal resistance from the winding and the core to the can is worthwhile. There are a large variety of modern thermal mats used for thermal control of parts on pcbs etc that have one face with adhesive, and a variety of compresible thicknesses. Adding mats of suitable thickness to winding faces, and core faces, could allow the transformer to just slip inside the can, but still provide a sort of compression fit. Imho, any material interface has a lower thermal resistance than an air interface between transformer surfaces and the can. To that extent, painting the can exterior surface black, and providing forced air cooling may well be much better at restraining winding temperature, as well as enhanced protection to avoid the risk of overheating.
 
trobbins said:
Did you identify what failed in your original PT, and what may have caused it?
Click to expand...

Shorted turn in HT winding. Operator error combined with a known weak transformer. I had a KT66 fail in the other monoblock and have them both on a common power switch. I turned the amplifiers on and off while diagnosing the bad tube, without allowing adequate time to let the CL90 cool down. So at the end of the night, a bad KT66 in one amplifier became a bad KT66 in one amplifier and a bad power transformer in the other amp.

trobbins said:
Are you aiming to add any protection to mitigate known likely causes of PT damage in the W5M ?
Click to expand...

Already installed a CL90 when I rebuilt the amplifiers, but it's hardly effective if I don't wait for things to cool down before applying power.

Right now I've set them aside and I am using my Mullard 5-20 amplifiers instead. I'm a little disappointed by the mistake I made with the Heathkits.

I like your thermal pad idea. It also leaves the transformers serviceable. Going to look I to it with the next parts order.
 
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