Depends on how hot you like your tea.. 
There are lots of guidelines one is supposed to follow in the selection and application of power transformers, but without knowing a whole lot more information it is hard to make a judgement.
Scorching burning smells or the smell of hot varnish after several hours of operation are bad signs.
Should the core be hot enough that you cannot even touch it after some period of operation is a clear indication of excessive temps.
Modern filament transformers usually have class A insulation which is rated for a maximum of 105 degrees C, class B is less common and has a rating of about 130 degrees C. Worst case design scenario at high line, full load, and max ambient should provide at least a 10 degree C margin.
With vintage transformers it is impossible to know the insulation class, but assume class A to be safe.
You can use the rise of resistance method with a good ohm meter to approximately determine the winding temperature, it won't be that precise but it will give you an approximate value.
If you can measure ambient temp so much the better, but usually assume either 20 or 25 degrees C.
Measure the cold resistance of the secondary with a good ohm meter, make sure you measure the probe resistance and subtract that from the resulting measurement.
Do the same for the hot measurement, note that speed is of the essence here. Remove power, pull the tube and measure quickly.
Here is the equation:
Degrees C Rise = (Rh – Rc)/ Rc x (234.5 + T)
Rc = Cold Winding Resistance in Ohms
Rh = Hot Winding Resistance in Ohms
T = Cold (ambient) Temperature in degrees C
As long as you get results below about 70 - 80 degrees C everything is probably fine.
Hope this helps..
There are lots of guidelines one is supposed to follow in the selection and application of power transformers, but without knowing a whole lot more information it is hard to make a judgement.
Scorching burning smells or the smell of hot varnish after several hours of operation are bad signs.
Should the core be hot enough that you cannot even touch it after some period of operation is a clear indication of excessive temps.
Modern filament transformers usually have class A insulation which is rated for a maximum of 105 degrees C, class B is less common and has a rating of about 130 degrees C. Worst case design scenario at high line, full load, and max ambient should provide at least a 10 degree C margin.
With vintage transformers it is impossible to know the insulation class, but assume class A to be safe.
You can use the rise of resistance method with a good ohm meter to approximately determine the winding temperature, it won't be that precise but it will give you an approximate value.
If you can measure ambient temp so much the better, but usually assume either 20 or 25 degrees C.
Measure the cold resistance of the secondary with a good ohm meter, make sure you measure the probe resistance and subtract that from the resulting measurement.
Do the same for the hot measurement, note that speed is of the essence here. Remove power, pull the tube and measure quickly.
Here is the equation:
Degrees C Rise = (Rh – Rc)/ Rc x (234.5 + T)
Rc = Cold Winding Resistance in Ohms
Rh = Hot Winding Resistance in Ohms
T = Cold (ambient) Temperature in degrees C
As long as you get results below about 70 - 80 degrees C everything is probably fine.
Hope this helps..
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