Power transformer--how hot is too hot?

[mnexsen]

1970 Pro Reverb: the PT gets pretty hot, on the edge of too hot to hold. Should I be concerned?

[Gibsonman63]

How is your bias? I have a Bassman 100 that was getting really hot. It was converted from bias balance to bias adjustable bias voltage before I got it, but it was set too hot and I had to change the series resistance to get the adjustable range where it needed to be. Once I fixed the bias problem, it started running considerably cooler.

[JoeM]

1970 Pro Reverb...

40 years isnt too bad. My '65 Princeton's PT has always run very hot, but it still seems to be holding up.

[R.G.]

Edge of too hot to hold is OK.

The real scoop is that unless it was wound with special high temp wire and materials, the transformer is OK running with its hottest spot on the inside at 105C or more. That's 5C over the boiling point of water. Skipping over a lot of heat transfer math, it works out that a hot spot of 105C on an average sized amp transformer is about 130F on the outside of the transformer, and that happens to be about the temperature where an average person cannot hold their index finger tip on it because of the pain. Imagine that - a human fingertip is the useful temperature standard.

Hot is OK, good even, because you didn't pay too much money for the materials and transformer. Too hot to touch **may** be OK for special transformers, but these are not usually used in amps. Almost too hot to hold is probably OK. Smells bad and runs out wax, varnish, or smoke is too hot. Cooler is always better for long life, but transformers can be designed to run too hot to touch and still be OK. What matters is what's inside and 105C (Class A or Class 105 insulation, no relation to amp bias) is the lowest of the specified temp ranges. The iron and copper will work at temperatures which will melt any insulation. It's the melted insulation and shorts that kill the transformer, not a failure of the iron and copper at working temps.

If it were mine, I'd just check it from time to time. If it always gets to the same subjective temperature, stop worrying. By the way, it may take 2-3 hours or more for the transformer to come up to fully stable temperature. There is a lot of iron and copper in there to heat up.