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PostPosted: 15 Nov 2017, 21:06 
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There has been a lot of discussion on the forums concerning the fact that, in vacuum tube amps, the Hammond power transformers seem to run fairly warm. After listening to much speculation and anecdotal evidence, I finally decided to put the question directly to Hammond. And today I got a very polite and informative response to my inquiry.

First things first. From a thermal perspective, transformers are rated by their insulation temperature rating. These ratings are:
Class A: 105°C (221°F)
Class B: 150°C (302°F)
Class F: 180°C (356°F)
Class N: 200°C (392°F)
Class H: 220°C (428°F)
This is the maximum temperature that is allowed in any part of the transformer.

Using this insulation rating, assumptions and allowances are made to determine how much the temperature of the transformer may increase over ambient before the insulation is compromised (i.e thermal failure occurs). This is done according to the following relation:

T-rating = T-ambient + T-rise + T-hotspot

Now the assumptions are generally for an ambient environmental temperature of 40°C (104°F) and a hot spot allowance of 10°C (18°F). The Hammond "Classic" series (i.e. tube amp) transformers and chokes are all Class A, 105°C (221°F), rated. This means that with the assumptions above, the allowable temperate rise over ambient in the design is 55°C (99°F) (i.e. 105 - 10 - 40 = 55°C (99°F)).

So what this means is that if you are operating the transformer in a 25°C (77°F) environment, the maximum internal temperature of the transformer can go as high as 80°C (176°F) (25°C + 55°C = 80°C). Now the external temperature of the transformer will be lower than the internal temperature. How much is dependent on the thermal resistivity of the assembly. But seeing as how they are made of steel (i.e. a good thermal conductor), the thermal resistivity between the maximum internal temperature and the external temperature should not be that large. If one were to assume that the delta between maximum internal temperature and case temperature was 20°C (this is a very conservative estimate), then this would mean that it would be reasonable to see a case temperature of 60°C (140°F) at full load in a 25°C (77°F) environment and a case temperature of 85°C (185°F) in a 40°C (104°F) ambient environment. And none of these conditions would be out of specification for the transformer.

Just for the record, 60°C (140°F) is very warm to the touch. This is hot enough to scald or produce mild burns. And 85°C (185°F) will cause severe burns. For reference, 65°C (150°F) water can cause 3rd degree burns in only 2 seconds.

In my designs using Hammond transformers, I have not measured a case temperature higher than about 55°C (131°F) in a 25°C (77°F) environment. So while the case may have felt "hot" to the touch, in reality this was well within the specifications of the transformer.

So what does all this mean? I would say first, stop worrying about the temperature of the Hammond transformers. They are operating within specifications and should give good performance across their lifetimes. Second, even though here may be some other transformers that run "cooler" to the touch, that does not necessarily make them "better" than the Hammond units.

I hope that this information helps to put people's minds at ease. Your transformers are fine; enjoy your amps.

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PostPosted: 15 Nov 2017, 23:59 
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I'll try once more to measure the temp at the top of the tranni but last time I tried I got a reading of 40C about 100F well within range. I can just hold a few fingers on it for about one or two seconds.
Hard to believe I have stopped worrying but will try for another measurement.

Thanks for the info Matt.

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PostPosted: 16 Nov 2017, 00:38 
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Thank you Matt for your efforts and the information :D

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PostPosted: 16 Nov 2017, 15:58 
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Joined: 04 Jun 2008, 20:59
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Hi, Good info. In some of my various designs I have measured (spot) temperatures as much as 175F (about 80C). Still comfortably below the least of the specs. This was only after long periods of use. I find if you use good brands and keep the loading below about 75% of the ratings you will usually be fine. 50% is even better.

Good listening
Bruce

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PostPosted: 16 Nov 2017, 16:54 
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My best efforts to read the temp of the little HP amp at least agree - 40C about 100F. Therefore not worried.

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Website: retro-thermionic


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