Hammond Power Transformer temperature problem

The last EL84 amplifier I built (with the phono stage, tone control and headphone stage) has a Hammond 370FX power transformer. It was noticed this transformer gets uncomfortably hot to touch after about one hour’s use of the amplifier.

Being unfamiliar with how-hot-is-too-hot, I’ve adopted a cautious approach and ordered a higher spec transformer to replace the current unit. However this is a few weeks away (coming from Canada) so in the meantime I decided to measure the temperature rise to gain a deeper insight into the problem.

First thing I tested, before doing any measurements, was to pull the tubes from the headphone stage, thereby relieving the power supply of 44mA of B+ and 1600mA of 6.3volt. As expected, this resulted in a much slower heating up of the transformer.

With all tubes plugged in, the quiescent current on the B+ is 140mAThis is a centre-tapped transformer, so conventional wisdom is that in this mode of usage, the secondary should be rated at 1.2 times the desired DC current, which in this case would be 168mA
In the case of the 370FX the secondary is rated at 173mA

Assessment: OK

On the Low voltage side. the 6.3volt is rated at 5A and the total draw on it is 5.2A so a little over (by 5%)

Assessment: Not ideal
(The replacement unit ordered has an extra 1000mA there).

So. Down to the measurements. I ordered a digital pyrometer (infrared surface temperature measurement gun) and when that arrived, I ran the amplifier for 3 hours, measuring the surface temperature on the top of the transformer, every 5 minutes.

(Posed photo. Te measurement target shown was not the actual measurement location due to radiant heat from the output tubes).

Over three hours, this was the result:

The measurements were made each time at the same spot on the top of the transformer, from the same distance.

After around 45-50 mins the transformer became uncomfortably hot to touch if resting the hand on it. This corresponded to a temp of mid-to-high 40s. Once the temperature was in the low to mid 50s it became uncomfortable even to a fingertip.


1) This is an unscientific test with a cheap uncalibrated instrument from AliExpress. I have some confidence in it because the baseline temperature reported (22ºC) at zero minutes was exactly the ambient temperature in the room reported by multiple other thermometers.

2) Electronic components are rated at 105ºC. I do not know what the temperature difference between the windings and and the outside of the transformer would be, so I am going to make a totally wild and uninformed guess of 20°C. Therefore the windings are at around 80-85°

3) From reviewing others’ experiences with Hammond power transformers, it seems a commonly reported phenomenon that they run hot. Therefore this transformer is behaving as expected, although it is causing considerable unease in doing so.

4) Because of my wild assumption in (2) above, I have no confidence that this transformer will be safe or indeed what detrimental impact sustained running at high temperature will have on it.

5 thoughts on “Hammond Power Transformer temperature problem

  1. The problem with your mains transformer is two fold: You are using too much capacitance and secondly Hammond quality has deteriorated.

    No need to use a thermometer – just measure the resistance of your HT winding when cold and after about 6 hours. There is a formula than then calculates the winding temperature increase. Modern winding insulation will withstand up to 110 C. This is what I use (found it on the web):

    The way the pros do it is to use the copper windings as a thermometer. Copper has a temperature coefficient of resistance of nominally 0.003862/K. What that means is that for every degree it gets hotter, the resistance rises by a factor of 0.003862. So if you’re looking at 105C as being the maximum temperature for long insulation life (that’s what Class A/105 means) then you have 105C-35C = 70C rise that you can allow. (I used 35C as a guess at the internal air temp.) A 70C rise means that if the copper got that hot, its resistance would increase by a factor of 70*0.003862 = 0.2703.

    The red-red winding resistance is specified at 81.35 ohms +/-2%. So if you measure it cold, you’ll find out what your ohmmeter thinks it is, which varies with calibration. Then run it to heat it up. When it’s fully hot, working at maximum design temperature, it will rise to 81.35 * 1.2703 = 103.3 ohms. Again, your ohmmeter will vary a bit, but what you’re looking for is the percent increase. You have to disconnect the PT from power and from the circuit to measure this, but if you use a rectifier tube, you can just pull the rectifier tube and measure at the input pins to the rectifier.

    There are sources of error in this process that the pros would work to try to minimize, but it’s more accurate than measuring the surface temp and relying on the variation of the surface temp. Surface temp is good, resistance change is better.

    Regarding the too much capacitance:
    Use PSUD II (free software) to calculate the exact RMS that your transformer needs to deliver, this is lots more than the simple DC current depending if you use a bridge rectifier or a full-wave rectifier (2 diodes) setup.

    Don’t forget that your mains transformer will through magnetic coupling introduce likely 10mV – 25mV in the primary winding of your OPT so no sense in going overboard with the smoothing of B+.

    1. Very useful technique for measuring the resistance and calculating the temperature from there. I’d not read that anywhere else (my main literature sources are Merlin Blencowe and Morgan Jones and they didn’t mention it). I did run simulations in PSUD2 reducing filter capacitance to as low as 10µF but it made little difference. I think Hammond are specified at 60Hz and then “Seemed to run OK” at 50Hz so they added that to the spec. Though they should be de-rated for 50Hz use. But, not being a transformer expert, I’ll use an off-the-shelf part following the manufacturer’s guidelines, the guidance pages on Hammond’s site and on Sowter’s site seem to agree with each other. Both say that in a centre-tapped full-wave rectifier scenario, Isec should be ~1.2x Idc.
      However as stated in the article, it’s not reasonable to expect things not to get warm – so the effect is most likely just psychological discomfort 🙂

      As for mag coupling to output trafos – always a risk but I’m more concerned about stray magnetic flux from the transformer affecting the tubes in the preamp, particularly the phono stage. You can touch the body of the transformer with one hand and just bring your thumb from the same hand close to the tubes and hear the hum through the speakers as you move it around.
      The output transformers are rotated 90° to the mains transformer for that exact reason. Plus the negative feedback from teh speaker output to the first stage would help to correct anything induced. On the scope at the speaker output there’s a couple of mV of hum into 8Ohm dummyload, which I can live with. In usage it is inaudible.

  2. If you download from Hammond their document 5c-08.pdf
    ( http://www.hammondmfg.com/5ccat.htm ) and look at that documentation then the 370FX is rated at 150mA. Short term / intermittend it may be run at a slightly higher current so be wary. I also look at the drawings for the transformer and check when it was designed. Similarly you may find that e.g. the 370KX is not mentioned in the above document – I bought one of those and it runs rather hot – about 55C ~ 60C at about 180mA (rating is 322mA) but that is well within spec. The 370FX that I put in a SE EL84 stayed cool. (only running the two EL84 and an ECC83).

  3. After many years of buying Hammond power transformers, I’m finished with them. You bet Hammond quality has deteriorated. By all rights, Hammond Mfg. should be building world class transformers. The company has one hundred years plus experience in the transformer industry. I purchased new power transformers that hummed loudly (without a load!), with overheating issues and transformers with globs of ugly varnish solidified all over the core and end bells. These should never have passed quality control. Disgusting!

    As superior replacements, I’m now using Antek toroidal transformers.
    Finally, a company that offers a (very) wide range of quiet, well performing, decently priced toroidal power transformers for the vacuum tube enthusiast! They’re hobbyist friendly too. We’ve been waiting a long time for this. In my projects, they run cool and look great. Antek makes toroidal output transformers too, but I haven’t purchased any yet. Maybe later.

    1. I’ve not had the quality problems you describe with them, but I have shifted to a different supplier now. Although the mains transformers run quite warm, I’ve not had any warranty claims from them.

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