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View Full Version : Is a hot power transformer a bad sign?


Crow
09-15-2009, 07:39 AM
On one of my vintage Fender amps the power transformer gets hot. Is this a sign of it failing? The output tranny is pretty cool, however.

thanks,
Crow

hasserl
09-15-2009, 07:42 AM
Could be, depends on how hot. Power transformers can get pretty hot, but how hot is too hot? Can you hold you fingers on it for 10 seconds? What is the amp, are you running the correct tubes, correct impedance?

phsyconoodler
09-15-2009, 09:25 AM
Many power transformers get hot.An example of too hot is a recent Twin Reverb reissue I worked on.The PT got so hot you could burn your fingers on the front panel.A new transformer cured that and it just gets warm to the touch now.
Hasserl is correct;if you can hold your hand on it for at least 10 seconds without discomfort then it's probably fine.If you pull your hand away in pain in a few seconds you need to do some diagnosis.

Prairie Dawg
09-15-2009, 09:57 AM
Many power transformers get hot.An example of too hot is a recent Twin Reverb reissue I worked on.The PT got so hot you could burn your fingers on the front panel.A new transformer cured that and it just gets warm to the touch now.
Hasserl is correct;if you can hold your hand on it for at least 10 seconds without discomfort then it's probably fine.If you pull your hand away in pain in a few seconds you need to do some diagnosis.

If it's hot enough to cause pain in a few seconds it probably is drooling wax or asphalt and emitting transformer incense and is on its way to the great junkyard in the sky.

itkindaworks
09-15-2009, 10:36 AM
If it's hot enough to cause pain in a few seconds it probably is drooling wax or asphalt and emitting transformer incense and is on its way to the great junkyard in the sky.
That should be in the manual:D.

MKB
09-15-2009, 11:42 AM
It really does depend on the transformer, some get REALLY hot by design. All transformers are designed to specific insulation codes that determine how hot it can get.

The 10 second rule is a good one for guitar amp transformers, it's about all you can do unless you want to get into testing and math (which involves measuring the primary DC resistance when the transformer is cold then again when it is hot, then doing some math. You can get a good idea of the core temperature rise that way). But even then you may not know at what temperature it was designed to operate.

An old tranny guru I worked with once said "There is no limit to how small you can make a transformer, as long as you don't care how hot they get". The guru's dad designed some transformers for the Apollo moon missions, they were very small and light, and got extremely hot (they were designed with magnet wire with glass insulation). We designed a transformer with Class H insulation; I can't recall what temperature that works out to but it was extremely hot when operating correctly. It was designed to dissipate power as heat under certain operating conditions.

On thing a tech can check on a hot power transformer is to look at the secondary voltages; if they are significantly high but the supply sags way too much under load, you may have a partial primary short. I've seen a few transformers like this, and the only solution is to replace it.

Prairie Dawg
09-15-2009, 12:34 PM
It really does depend on the transformer, some get REALLY hot by design. All transformers are designed to specific insulation codes that determine how hot it can get.

The 10 second rule is a good one for guitar amp transformers, it's about all you can do unless you want to get into testing and math (which involves measuring the primary DC resistance when the transformer is cold then again when it is hot, then doing some math. You can get a good idea of the core temperature rise that way). But even then you may not know at what temperature it was designed to operate.

An old tranny guru I worked with once said "There is no limit to how small you can make a transformer, as long as you don't care how hot they get". The guru's dad designed some transformers for the Apollo moon missions, they were very small and light, and got extremely hot (they were designed with magnet wire with glass insulation). We designed a transformer with Class H insulation; I can't recall what temperature that works out to but it was extremely hot when operating correctly. It was designed to dissipate power as heat under certain operating conditions.

On thing a tech can check on a hot power transformer is to look at the secondary voltages; if they are significantly high but the supply sags way too much under load, you may have a partial primary short. I've seen a few transformers like this, and the only solution is to replace it.

This is a little off topic but it made me wonder what kind of iron was in some of the submarine cable repeaters.

http://atlantic-cable.com/Equipment/STC-1954-Repeaters/index.htm

davemccarthy707
09-15-2009, 01:58 PM
The tube rectified amps get a little hotter than then solid state rectified amps. In my experience. Also see how close the tubes are to the transformer.

donnyjaguar
09-16-2009, 01:30 PM
I'd run the transformer unloaded and see if it gets hot by itself. If so, you likely have a shorted windings. Good idea to check voltage of secondaries to see if they are all where they should be.

terminal
09-16-2009, 01:40 PM
pull the tubes and turn it on, see if the PT gets hot. If not replace them one at a time, until the condition returns.