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neophyte - transformer potting

ofb

Member
2002-10-06 11:25 pm
canada
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
 

Brett

Member
2002-01-07 6:02 pm
<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
 

lieven

Member
2002-08-11 4:42 pm
Belgium
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.:D

You can also try sandwiched, bitumen or sand-filled, 2 or 3 concentric can solution.
 
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.
 

Brett

Member
2002-01-07 6:02 pm
dice45 said:
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.

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
 

ofb

Member
2002-10-06 11:25 pm
canada
Brett said:
<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?


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?
 
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.
 
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.
 
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?
 
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
 
Potting can be a real positive (reduction of vibration, environmental protection), but only if you select and use the correct materials. And, unfortunately, most people don't.

There are a few proper ways to do it, none of which are cheap. The best is to use a sealed can and fill it with a proper dielectric fluid (e.g., Fluorinert). You have to pay attention to thermal issues, and the can may have to be customized to achieve that. You can also use a low ionic content elastomeric potting compound. Good thermally conductive compounds with high fill levels are essential- think platinum-catalyzed silicone with beryllium oxide filler.
 
SY said:
Potting can be a real positive (reduction of vibration, environmental protection), but only if you select and use the correct materials. And, unfortunately, most people don't.

There are a few proper ways to do it, none of which are cheap. The best is to use a sealed can and fill it with a proper dielectric fluid (e.g., Fluorinert). You have to pay attention to thermal issues, and the can may have to be customized to achieve that. You can also use a low ionic content elastomeric potting compound. Good thermally conductive compounds with high fill levels are essential- think platinum-catalyzed silicone with beryllium oxide filler.

Sy, you again impress me with what you know. I can probably find my way on the Thomas Register, but do you know offhand where I might locate some flourinert?

Any suggestions re can material? Copper? T1?

Thanks.
 
It's cheating- I'm a materials guy by profession.

Fluorinert is made by DuPont. A quick scan of their website should shake loose the name of a distributor in your area. (PCBs work well, too, but there are some, ahem, other issues with those!) The trick is the can design- you want to be able to fill it, then seal it off. There has to be a controlled amount of headspace to accomodate thermal expansion.

BTW, the stresses from thermal expansion are the number one cause of potting problems when people try this using rigid compounds.