Potted transformers

[semitone]

Hi all. My first question (I am a new member) arises from the demise of the PT of my first amp that I finished last week. I basically rebuilt a Heathkit W3-AM using new parts, other than the original transformers. Other than aesthetics is there any other purpose for potting transformers? It seems to me that there is a fairly significant decrease in cooling properties over unpotted. I'm pretty sure the reason my Heath PT failed was due to too much idle current being drawn by the amp. I'm pretty sure it will be difficult finding another (Heath part # 54-13) in the near future. Hammond has a close replacement that would fit in the old "pot" but Hammond recommends against potting their transformers. Any thoughts on this? Thanks. -Paul

[BudP]

Semitone

As you noted the problem with potting a transformer is heat. A 1/4" of epoxy raises the hot spot temperature by 5 C, a layer of metal, end bell or can, raises it another 5 C. Hammond makes "commercial" transformers and so they are already running their devices at about a 65 C rise, in an ambient of 20 C.

A total of 85 C is not going to hurt the insulation system in a modern transformer. Modern class B materials will tolerate 130 C total temperature for 300k hours, mtbf. However, the 90 to 95 C temperature in your amp is going to degrade a lot of other components. Also, external box covers, over the entire device, add another 5 C.

Back in the day, the transformers were not able to operate at these temperatures, for the extended mtbf and so were designed more conservatively.

If you want to pot the Hammond in a new box, then a thermally conductive filled epoxy must be used. I do not know of any that are self curing so you will need to have a transformer company, with dedicated ovens bake it out for you. You cannot do this in your kitchen oven, period, don't even think about it.

Probably a better path is to look through the Plitron catalog for seperate B+ and filament toroids. They will almost certainly both fit into the box, have lower emitted fields and run cooler than one single E/I transformer. For power, it really is hard to beat a toroid. For audio signal, the two formats are much more evenly matched.

Anyway, welcome to the forum. Scout around a bit, there is an amazing amount of information available and a ton of polite folks willing to help, or argue with you, if you so choose.

Bud

[Boris_The_Blade]

I have successfully repotted a heathkit power transformer recently. I baked the old transformer out with a blow torch (goo!) and repotted a stancor PC8411 using medium casting resin from ebay. The transformer was from the infamous (for bad power transformers) W4 series. Works like a charm so far, and looks stock...well almost..I have to repaint it, but I'm too busy listening to it. I have recently devised a new plan for new metal potting cans, and will report back when I get the parts and try it out.

[tubewade]

My thoughts are... If you don't care whether it looks original or not, just use a non-potted transformer, such as the Hammond you mentioned. The amplifier won't care. I've had to do the same thing and it works fine.

Wade

[Wavebourn]

1. Magnetic and electrical field shield.
2. Protection against humidity and micro-organisms.
3. Protection against mechanical damages.
4. Aesthetics (questionable: depends on a taste; for my personal taste what is more functional that looke better. I don't like a metal painted and looked like a wood, neither a stove in a gear rack; when a material reflect it's properties I like it better).

However, thermal resistance of a compound matters increasing an overheating of a wire and it's insulation that may be damaged by high temperature, but an air gap between a transformer and a pot is usually less thermal conductive.

I would rather put grills on tubes to protect them mechanically and kids from their heat.

[Boris_The_Blade]

Originally posted by semitone
Hi all. My first question (I am a new member) arises from the demise of the PT of my first amp that I finished last week. I basically rebuilt a Heathkit W3-AM using new parts, other than the original transformers. Other than aesthetics is there any other purpose for potting transformers? It seems to me that there is a fairly significant decrease in cooling properties over unpotted. I'm pretty sure the reason my Heath PT failed was due to too much idle current being drawn by the amp. I'm pretty sure it will be difficult finding another (Heath part # 54-13) in the near future. Hammond has a close replacement that would fit in the old "pot" but Hammond recommends against potting their transformers. Any thoughts on this? Thanks. -Paul

You could call edcor or heyboer and tell them your situation, they could recommend an overrated transformer that would still fit in the can (you might have to specify dimensions to them) and then do what I did to pot a new one in there.

[semitone]

Such great feedback. Thank you. Based on your responses, I'm torn between trying to keep the look of my original design with the big potted OT in between the potted OT and choke, or just going for the simplicity of the naked PT. SO, Boris-the-Blade, you say the PT on the W4 was notoriously bad. Was it the same as the 54-13 on my W3? 'Cause that would make me feel a little better about having blown mine up!

Hmmm, so what do you guys think about me removing my dead Heath PT from the pot, mounting a new PT in the chassis (I am going to look into the sources you suggested), adding vent holes around the PT on the chassis, adding vent holes to the top of the old pot and then mounting the pot over the new PT. So, it's kind of an unpotted-potted transformer? I'd think with adequate ventilation this would work. Is this rookie thinking?

[Alastair E]

Originally posted by BudP
Semitone


If you want to pot the Hammond in a new box, then a thermally conductive filled epoxy must be used. I do not know of any that are self curing so you will need to have a transformer company, with dedicated ovens bake it out for you. You cannot do this in your kitchen oven, period, don't even think about it.


Bud

Guess who found out this the hard way while a teenager, and stunk out the whole house for a month!

[BudP]

semitone,

If you can remove the old transformer, without deforming the can or having to use a blow torch to remove old potting material. then there is no no other reason why you should not perform this swap.

I would not bother with cooling holes anywhere. A single metal cover is only going to raise the hot spot temp by 5 C. This should not be enough to cause failure in Hammond's product. If it does, they need to be notified quickly, because a serious mistake has been made in design.

Again, modern materials allow a much higher operating temperature than did materials as recently as 1980. If you were planning on using another NOS transformer for this, I would be much more uncomfortable. Hammond is a reputable manufacturer, but they are also a competitive manufacturer and the truth is, their customers are more than willing to trade heat, for money taken from the BOM.

My suggestions about toroids was to get you to seperate the high current low voltage, from the low current high voltage windings, by putting them on seperate core. This always sounds better in an audio amplifier, especially one based on minimal money power supply design. This being where large amounts of capacitance is traded for a power transformer with less than 3% no load to full load regulation and just enough capacitance to remove ripple.

Other than all of this verbiage I see no other problem with your scheme. Just make sure you are not exceeding the current draw limitations of the transformer. Psud2, with the no load sec voltage and DCR of primary and secondary entered in the edit box, will provide a very good estimation of true RMS AC current draw, for the transformer. The other acceptable method, is to calculate the primary AC current draw, plus DCR losses, on both sides of the transformer windings, calculate the resultant heat rise. Then measure the primary AC draw, under max duty, with an iron vane meter to make certain there is not excess current being drawn by the load. Psud2 is easier and faster and can be found here, if you don't already have it.http://www.duncanamps.com/psud2/download.html


Bud.

[BudP]

Alastair E,

Did you also learn not to let the smoke out of them, the hard way? An even more egregious smell, to avoid allowing into a volume, you intend to continue living in. Certainly caused me to want to move to another location, entirely.

Bud