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Old 6th October 2002, 11:49 PM   #1
ofb is offline ofb
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Default neophyte - transformer potting

i could use some basics and i'm afraid i'm not having much success with google.

can _any_ transformer be potted? should it?

at the moment i'm looking at a cheap beginner's amp and its rather ugly transformers.

naturally i'm wondering if they can be covered, but haven't prior experience to guess how oversize covers should be. nor do i understand if i should consider potting them in resin.

perhaps someone here could point me to a webpage or prior thread?

thanks
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Old 7th October 2002, 05:13 AM   #2
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<b>can _any_ transformer be potted? should it?</b>

Maybe and no respectively.
I had a post in my system before my HDDs untimely death from Mike of Magnequest transformers, explaining his experience with potting output transformers, and how it changed the measured and sonic performance. Different types of potting compound did different things, but all degraded performance. Here I bow to the experience of someone with vastly greater knowledge of the subject. If you search audioasylum under his moniker of MQracing you might find it.

Power transformers I wouldn't pot either. If a manufacturer has made and rated it unpotted, I think you would have to derate it once potted, and use the correct potting compound for the application, which would be..........stuffed if I know, but not standard resin.

Get a metalshop to make some covers for your transformers, and for the mains one, make sure it has some ventilation just to be safe. There's also no law requiring you to make your amp in the traditional style of a rectangular box with everything exposed on top. Put the tubes up in the breeze, and build the transformers into the case, making sure you have sufficient ventilation for them.

Cheers
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Old 7th October 2002, 07:04 AM   #3
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for toroidals, try an empty can of beans, soup, ravioli or whatever, just make sure centre bold don't touch both sides.
Simple experimenting solution. Every restaurant or household have tons of it each week.

Hi, I'm having a campbell tomato mains transformer, 1000va with hot-dog output.

You can also try sandwiched, bitumen or sand-filled, 2 or 3 concentric can solution.
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Old 7th October 2002, 07:51 PM   #4
dice45 is offline dice45  Germany
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Brett,
i tend to agree with you and MQracing.
OTOH, Tango and Tamura have gorgeous iron and they pot.
Famous vintage UTC and Chicago/AcroSound are potted.

But: Allen Wrigth reported that his new Lundahl LL1663 OPTs (5k->8R) sonically outperformed the OEM Tamura's he had before -- the Tamura is potted, the LUndahl not. There certainly are more differences, but it certainly gives a direction.

I plan to order a pair of LL1663 to compare it to my Tango XE-45-5. I expect the Lundahl to be atleast equivalent sonically.
I already know the iron will be under the top plate and have no benefit of the beautifully potted Tango. If the Lundahl is as good or better, i will probably sell my Tango's. Lundahl costs 1/3. And it's winding topology is perfectly suited for PP output stages.
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Bernhard
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Old 7th October 2002, 08:19 PM   #5
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Quote:
Originally posted by dice45
Brett,
i tend to agree with you and MQracing.
OTOH, Tango and Tamura have gorgeous iron and they pot.
Famous vintage UTC and Chicago/AcroSound are potted.
True, true. But the <i>designer</i> made the choice to pot and had the option of optimising any neccessary parameters. Plus we have no idea what they'd sound like <i>un</i>potted.

Quote:
But: Allen Wrigth reported that his new Lundahl LL1663 OPTs (5k->8R) sonically outperformed the OEM Tamura's he had before -- the Tamura is potted, the LUndahl not. There certainly are more differences, but it certainly gives a direction.

I plan to order a pair of LL1663 to compare it to my Tango XE-45-5. I expect the Lundahl to be atleast equivalent sonically.
I already know the iron will be under the top plate and have no benefit of the beautifully potted Tango. If the Lundahl is as good or better, i will probably sell my Tango's. Lundahl costs 1/3. And it's winding topology is perfectly suited for PP output stages.
I'm using LL1620PPs in my PP1C's for a 6k p-p load, mounted under the top-plate. Or at least they will be when the boxes are finished. Currently they're built on a peice of plywood scavenged from a building site.

I'll be interested in hearing what differences you find in the Lundahl and Tango sonically. I bet you sell the Tangos.

Ciao
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Old 7th October 2002, 09:11 PM   #6
ofb is offline ofb
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Quote:
Originally posted by Brett
<b>can _any_ transformer be potted? should it?</b>

Power transformers I wouldn't pot either. If a manufacturer has made and rated it unpotted, I think you would have to derate it once potted, and use the correct potting compound for the application, which would be..........stuffed if I know, but not standard resin.
fair point. thanks for all this, folks.

what i'm gathering is that potting does change the sonic signature, which is something the manufacturer can design for. hence it's not a good idea to pot one yourself except to experiment.

just curious: why are any transformers potted? is this just for heat and perhaps vibration, or is there more to it?


Quote:
Get a metalshop to make some covers for your transformers, and for the mains one, make sure it has some ventilation just to be safe. There's also no law requiring you to make your amp in the traditional style of a rectangular box with everything exposed on top. Put the tubes up in the breeze, and build the transformers into the case, making sure you have sufficient ventilation for them.
true enough.

the amp is still comfortably at the napkin sketch stage. at the moment i prefer to have either the outputs or the mains below, but not both. there's just four small tubes. starts to look a little odd alone on a big box.

aesthetically, i'd rather have the mains below because it's a lot bigger than the tubes. but i may also compromise and make a large hole to place it in the same center plane as the top plate, and use a half-height cover.

either way i'm thinking there should be a few holes in the top of a cover and beneath it for heat, but i'm not seeing that on other amps. is there anything wrong with the idea, or do people generally not like drilling more holes?
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Old 7th October 2002, 11:39 PM   #7
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Default Potting

The shrinkage of the potting compound is what degrades the transformer by putting mechanical stress on the core. This increrases the exciting VA (losses) required by the core. The silicon steel core material is very sensitive to stress (mu metal is worse) and the cores are stress relief annealed at 1500 F. to reduce the stress. Adding alot of filler (60%-70%)will minimize shrink and maximize thermal conductivity. Sand works well for this. Polyester resin shrinks more than urethane or epoxy. A low temperature cure will minimize the shrink also.
A closed can with no air vents is much worse than a can full of heavily filled resin.
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Old 13th December 2004, 12:51 AM   #8
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Just thought I would reactivate this post to ask if anyone has differing or further opinions regarding transformer potting. Brett, I'll certainly see if I can find Mike's AA post. I'm wondering if potting helps reduce unmusical vibrations? Wax of some lowish dielectric constant would seem a possible candidate as a potting compound if potting were perhaps thought beneficial by some. Is it right to assume that potting interferes with the magnetic flux in some undesirable way? Appreciate any opinions.

Cheers.
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Old 13th December 2004, 02:18 AM   #9
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BTW, Mike Lefevre (sp?) says he wrote an article for Sound Practices on Why Not Pot, or something like that, around year 2000 (1999?). Has anyone a copy of this article?

Also, what of potting in wood using low dielectric wax? Some waxes have a dielectric constant of about 2, less than teflon.

How deleterious are vibrations anyway?
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Old 13th December 2004, 03:33 AM   #10
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If you pot using wax, use beeswax, as it has a higher melting point than paraffin. I know some guitar folks who claim that it makes for better sounding pickups, although I haven't tried it--even though I keep bees and have bricks of wax sitting around. Your enemy is going to be heat, no matter how you approach this.
Most potting is done with epoxy these days, of course, but I have no opinion as to whether that's better or worse than anything else.

Grey
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