[Voodoochile]

Potting the transformers is almost done. I'm using some thermally conductive potting epoxy from MG Chemicals; stainless bolts and pots.

Step one was to cast about 1/8" of resin in the covers:

Attached leads and fiberglass sleeving to transformers:

Set leaded trafo on cured 1/8" epoxy "floor", to keep the
trafo away from the pot. Plenty of room:

Disc to hold studs and leads in position for curing:

The pot is about 3/4 full right now. When it's cured, the nuts and disc
will be removed, and the pot will be topped off to within 1/16th of an
inch of the rim, leaving just enough clearance for a 3/32" silicone pad.

The holes in the disc are an exact match to the top plate of the amplifier.
Details on the top plate will be in a seperate thread.

 

[Voodoochile]

Finished the epoxy backfill:

The nuts are flush with the rim of the can, so as to not allow torque to extract the bolts from the epoxy. There is a head and another nut at the head of the bolt, about 1.75" deep into the epoxy. They are 2" bolts.

A silicone pad was applied between the epoxy and the chassis plate, which will compress slightly upon installation.

 

[Voodoochile]

Quote:

Originally Posted by Voodoochile (In reference to my stock Ear+HD) The two power trafos are rather busy, and although you can't hear the filament trafo buzzing, you can feel a slight vibration. It is currently bolted right to the chassis panel. When I pot it, I'm going to bed a pair of bolts alongside, between the trafo and the pot, rather than through the trafo's frame. Epoxy is hard, but not as hard as iron bolted to steel. Plus, a thin asphalt pad between the potted unit and the top plate.

After testing the finished amp, I can say that there is no vibration when potted. In the end, the filament trafo was potted in thermally conductive epoxy, with no direct metallic contact to the chassis. There is a silicone rubber pad compressed within the ~1/16" gap between the epoxy and the stainless plate, an the potted mounting bolts (suspended within the epoxy) both have nuts mounted at the plate height, so as not to have the bolts break free from the epoxy upon tightening. Like that would ever happen!

Anyway, the potted trafos are very tight to the plate, and the filament trafo is conducting it's heat nicely to the outer surface. At first glance, it appears that the potted transformer is running hotter than the unpotted version. Well, it's heater is draining an additional 0.3A. But despite this, it is in direct contact with the highly conductive epoxy.

The former trafo is mounted in free air, with a box mounted over it. The surface of the box feels a bit cooler, but the transformer itself is warmer than the outer surface by a good margin. When I run this free-air transformer with the recitfier in-circuit, it runs far hotter, though still within it's safe operating region. Air, especially static air, is a good insulator. Although stainless is a poor heat conductor, the can offers more surface area, and is exposed to free-air, as opposed to being mounted within an enclosed box. I think I'm in good shape, but if something turns sour, I will let you know.

The practically unloaded output transformers are not potted, as they neither tend to vibrate, nor are they heavily loaded at any time. They are still, however, mounted on silicone rubber pads, with rubber washers and stainless washers above the ears.

 

[aerius]

Quote:

Originally Posted by erix Myself, I dropped dime on a pair of custom output transformers yesterday - I'll have them next week. Stay tuned for another FrankenEar... ok, erix

Speaking of which, there's a cheap alternative available at Digikey for about $20 a pop. Check out the Amveco 62080 series of mini toroidal power transformers which can be used as output transformers, just pick the one with the right turns ratio and pop it in. I decided to try it out after reading this page and they work surprisingly well. Better than the stock ones which came with my K-12M kit as well as the Hammonds I found in the local surplus store. Nice tone and a huge soundstage, might be a tiny bit rolled-off at the top compared to the others though, but overall I'm very happy with them. The latest issue of AudioXpress also has a project featuring these donuts as output transformers so it's not a complete crackpot idea.

 

[erix]

Yeah, PPL had a cheap-n-easy tube amp on the boards here for a while that used a pair of Talema toroids but I never got around to messing with it. Trouble is, they look wimpy so you've got to pot them in shells like Vood, or hide 'em inside the chassis.

Myself, I like iron, the bigger the better! My custom transformers weigh almost 5lbs a peice and have end bells so I can't paint them up real purty....

ok,
erix

 

[Voodoochile]

Quote:

Originally Posted by erix My custom transformers weigh almost 5lbs a peice and have end bells so I can't paint them up real purty....

Sounds like your transformers can kick my transformers' collective ass.
Can't wait to see it!