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
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.
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.