Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Midtex on November 04, 2010, 03:17:40 pm

Title: Transformer potting
Post by: Midtex on November 04, 2010, 03:17:40 pm

I searched and couldn't find an answer to my question - I hope this isn't a "thread-worn" topic. When potting a power transformer, I have heard recommendations to remove the end-bells, but what about the copper tape that goes completely around the frame? The old transformer that I removed from the enclosure didn't have copper tape, so I'm not sure if that is added only in conjunction with end-bells. Any suggestions for heat conductive potting epoxy? If too expensive, I am thinking of remelting the tar and reusing it for all but the last 1/2" or so, and then seal with epoxy to prevent tar from dripping out like the old days. Thanks for your responses!

Title: Re: Transformer potting
Post by: FYL on November 04, 2010, 03:39:54 pm

The copper band is an electrostatic shield between primaries and secondaries. It shouldn't be removed.

If you really want to pot a PT, MGC 832TC epoxy is one of the best.

http://www.mgchemicals.com/downloads/pdf/specsheets/832TC.pdf

Available from Mouser and other indus distribs. Doesn't come cheap : expect to pay $75 per pint.

http://www2.mouser.com/Tools-Supplies/Chemicals/_/N-wp63?Keyword=832TC

Title: Re: Transformer potting
Post by: triode on November 04, 2010, 07:52:42 pm

I could be off my rocker, but I have a friend that winds great trannies, and we talked about this once....

If a transformer is not potted, don't pot it. A good transformer winder will wind a transformer for potting
slightly differently than one that is not potted (due to increase in temp rise). If the transformer hums
(mechanical), your winder was not very good or he messed up slightly. Get a new tranny or isolate it with
rubber mounts.

If it is an output tranny and you are just doing it to be cool, or whatever, the temp argument still remains,
but most output trannys do not get hot unless you are really running them into the ground.

Read this on how to pot a tranny, at the bottom:

http://www.somis.org/D-amplifiers3.html

If you really, really want to do it, I can dig up my Sound Practices archives and get Mike Lafevres article
on potting trannies. He is one of the masters.

Title: Re: Transformer potting
Post by: FYL on November 04, 2010, 09:01:52 pm

Quote

I can dig up my Sound Practices archives and get Mike Lafevres article
on potting trannies.

"To pot... Or not. The myths of potted transformers", Mike LaFevre, Magnequest
SP V1#4, summer '93

Title: Re: Transformer potting
Post by: Midtex on November 04, 2010, 10:21:02 pm

Thanks everyone!