Matchless transformers info- must read

[Diablo]

I didn't let the rectifiers cook for too long before I checked the PT temperature. Only a few minutes for each rectifier, as I wanted to get the rectifier tube cool enough to swap easily for the next tube. Today, I'll run a longer test under load with the 5V4s to see if I can feel any temperature rise in the PT.

[J M Fahey]

In case it's important, *there is* a way to measure winding temperature using copper itself as a sensor.
It requires some way to measure DC resistance with precision ... which might be as simple as making a precision constant current supply and measuring voltage drop across winding .... power off and disconnected from tube filaments of course (easy to make adding some switch).
First measure cold, and repeat after, say, 30 minutes ON.
Power the filaments only .

Here's one full procedure : How to Calculate the Temperature Rise in Copper Windings From Resistance Measurements

I'd say that up to 20 to 30 deg C rise would be acceptable.