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Power Transformer Temperature - what is too hot?

Jock Strap

Jock Strap

jacksonsinliss
Does anyone have any guidelines on how hot a power transformer should be?

I have a Leak Stereo 20 that I recently refurbished. It sounds excellent but I want to know what is normal for the transformer temperature. Currently, the power transformer is hot to touch but I can leave my hand on it without getting uncomfortable so I guess this is normal. But, how hot is too hot?

OPTs are just warm, not hot.
 
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After another two hours, the power transformer is hotter. I had to remove my hand after 5 seconds so now I am a bit concerned.
 
Judging by your description , it runs at about 130°F or close to , it's normal, like mine.

Some people says under 160°F is ok.

Good quality transformers have good isolation that can handle these temperatures.
 
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Some amps have power transformers which happen to run hot. The power transformer in my Dynaco ST-70 (circa 1974) has been running hot since I acquired the amp 17 years ago, and seems to be doing just fine in spite of it. As long as there aren't any odd smells from the transformer when ithe amp is running, and as long as its power draw is in line with the nameplate, you'll probably be OK.
-Adam
 
Agree with AdamAnt316, sounds like about 130 degrees F. I have several amps that run about this temperature on the PT and this is quite normal in my experience. I've had a couple that run hotter with no ill effect and have read other experienced hands also say that up to 160 degrees F is no problem.
 
If you can hold your hand on it for 5 secs, its not that hot. Once my ST70 gets up to full temp, I can only hold my fingers on it for maybe 2 seconds (about 60 deg.C., 140 deg.F.), yet its fine like that. (Note that my AC power runs about 122v on Ave.so the trans. is hotter than nominal.) It all depends on how much steel there is in the tranny. Some mfgs. use a transformer that's runs just under its maximum output. Probably to make the unit less expensive. Don't forget that modern enameled coil wire can stand 130-200 deg.C. (266-392 deg.F.) The highest being 250 deg.c. (482 deg.F.) Now that's the internal temperature of the coil but even with the lowest temp. rating of 266 deg., I doubt the core would reach above say, 166 deg.F. Don't worry about it. Tube amp power transformers get hot - nature of the beast.
 
When I was younger, the old electricians would use what they called the "3 second test". If you could leave your hand on an electrical device (transformer, motor, etc), without having to pull away, for 3 seconds or more, it was "safe".

No, it's not scientific- but it was a reasonable ballpark for the insulation technology of their time...

Regards,
Gordon.
 
Thank you all for your reassuring input. My other tube amps, Audio Innovations, have their transformers contained within the chassis/box so this is the first one I have that has an exposed transformer that I can touch.
 
BinaryMike said:
In your research, did you happen to find insulation temperature limits for transformer wire of vintage similar to a Leak Stereo 20?
Click to expand...
No, but I think it may have been the same. Only difference was that the enamel was much thicker then, so fewer turns could be used for a given coil.
 
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