Power Transformer Heat

[chunkitup]

How hot is too hot for a PT? The amp is a PV Butcher with an added 12ax7. The amp got a good workout last night being on for 4 hours and being played at no louder than 2 (loud enough for a hard rock band live) for about 2 hours of that time. I could touch the PT and hold my hand on it but it was pretty hot. I just want to know if the additional tube heater is pushing the PT past its limits.

[R.G.]

How hot is too hot for a PT? ... I could touch the PT and hold my hand on it but it was pretty hot. I just want to know if the additional tube heater is pushing the PT past its limits.

By accident, if you can voluntarily hold the fingerprint area of your index finger on the transformer, it's not too hot.

This only happens if the transformer has been run at load for some hours, as the thermal time constant (how long it takes it to heat up to steady temperature) is hours for a many-pound transformer.

Fortunately, you're testing at a reasonable interval.

The tech basis for this is that no commercial transformers use insulation classes which degrade at less than 105C/221F, also know as "Class A", and not the same as class A operation of a power transformer. For the materials used in class A insulation, the hottest spot inside the transformer being 221F or less corresponds to a surface temp of 130F/54.4C, and that's the hottest temperature an average human will willingly keep their index finger on.

Unusual humans exist who don't feel pain as much, can override the reflex, etc. exist (Gordon Liddy comes to mind), and some transformers can operate happily and well at much higher temps - Class H/180C insulation exists! - but if you're average, and you can keep your index finger on it, it's not overheating, however subjectively hot that feels.

Unfortunately, the hours-long time constant also means that a transformer can run past the insulation overtemp faster than the heat can get out to the surface, so a damaged one can burn out before the outside gets too hot. But hours in use is the right test case.

It's most likely fine.

[Amp Kat]

Dont know if you have ever accidently shorted the primaries on an EI power supply tranny but after it burns up and dies it is so hot that you could never keep you're index or any finger for that matter on it as it's smoking hot. I've heard that the Butchers trannys run pretty hot in nature.

[guitician]

I've got an old Class A P-P EL84 amp were the PT is so hot you couldn't keep your finger on the lamination's for a second. It's been happy that way for years. And it doesn't hum at all. Schumacher really made good transformers.

Still do I think.....