How hot is too hot for a transformer ?

[haenisch]

I finished my Zen V2 today, sounds pretty good !!

But I have a problem: My transformer gets hot, I think about 45 to 50 degree Centigrade (I can put my fingers on it, doesn't really hurt, but it's hot).

Since I have no information about the transformer I'am wondering if it is too small.

Can anybody give me a hint ?

Thanks in advance Till

[Shoog]

I would say that if your transformer is getting that warm you will start to encounter two problems. Regulation will increase with temp as the resistance in the wire increases.I would guess that beyond a certain point this will run away with itself and produce a meltdown. The effect of increased regulation is reduced power output as the line voltage drops.
Significant amounts of heat will stress the transformer wires enamel and eventually the transformer will short circuit.
The only advantage I experienced with my Zen V3 and a hot running transformer is that it acts as a power supply choke, which gives a nice warm sound to the amp and reduces hum.
The heat is also likely to shorten the life of your valuable filtering capacitors, which is a real problem.

Unfortunately I would say that you need to replace your transformer with something much beefier, I would only really be happy if my transformer were running at ambient temps. To do otherwise might be an expensive mistake.

Shoog

[Richard C]

45-50 C isn't particularly hot for a transformer, I've known them to run much hotter with no problems. Even though it feels hot to touch it would need to be much hotter before anything breaks down.

[peranders]

Normally the rating comes from when the transformer is 105 deg C hot inside, a toroid at least. = this is really hot outside too!

The limit is where the insulation gets damaged.

Measure the primary current so you can get an estimation of the VA into the transformer at least.

[Shoog]

Thanks for the correction.
I still personnally wouldn't be to happy, but thats just me.

Shoog

[Circlotron]

Fans work great on transformers. Have a look inside your microwave oven. WARNING! DONT TOUCH ANYTHING!! It is amazing how you can get ~1kW through such a small thing.

[sreten]

Just to say I agree with the sensible replies here.

A class A amplifier transformer is going to run hot, by definition.

50C is not hot for an operating temperature, by 65C which
is too hot to touch then things are becoming marginal.

Thermal cutouts fitted in transformers operate at at about
100C to 110C internal temperature of the transformer.

Offload and onload power supply voltages will tell you if the
transformer is too small.

sreten.

[Circlotron]

First thing in the morning, with the transformer cold and at a *known* temperature, measure the resistance of the primary. Then set the amp going for several hours, then unplug it and straight away measure the resistance again. There are formulas around for calculating the temperature rise of copper based on it's known increase of resistance with temperature. If you can't find one, post the cold and hot resistance values and the cold temp of the tranny and someone here will work it out.

[haenisch]

Thank you for the answers.

@ciclotron:
This method is very appealing from an asthetic point of view, using the device to test as a sensor is really nice ;-)

I will try this.

Thank you Till

[haenisch]

Now I'm able to answer my own question ;-)

From my measurement the primary reaches (after about 3 hours) a steady state temperature of approximately 70C (soft) to 80C (loud). So there should be no problem, at least in winter.

I have to think a bit, how it will behave with higher room temperature.

Thanks for your help Till