How to figure a transformer's output?

[MadMax]

If this has been asked before, I apologize. I just couldn't seem to find it on the board. I understand checking resistance on the input and output windings, and I understand that the ratio can be used to determine output voltage with a given input voltage. I've even measured (unloaded) output voltage on multi-tap transformers using a 10V input voltage. But I am not clear on how to determine the maximum safe amps I can use an unmarked transformer for. Surely it's more scientific than "just see if it gets too hot", right?
Along the same lines, If I want to calculate the total current needs of a tube amp, I need the total plate dissipation and the heater current, and what else?

If this has been asked and answered, please direct me to the board link. Thanks.

-Mad Max-

[mresistor]

Without some idea of the general specification provided for a power transformer I don't know how you would calculate it's maximum current capability other then judging it by its size and weight and relative dc resistance of the windings, size of wire, etc.  A transformer getting blistering hot would surely be telling you that you were approaching the current limit. 

To calculate the total current draw on a PT you can use handy tools like this one;

http://www.dreamtone.org/Calculate_Current_Form.htm

This thread might also be useful.

http://forum.allaboutcircuits.com/threads/assessing-an-unknown-transformer.38273/

[PRR]

> judging it by its size and weight

Right. Find something similar at Hammond Mfg.

For a multi-winding job, there is the question of how the power is proportioned among the windings. Some clue from the original app may help. Mostly tubes is tubes and heater/plate power ratio will be similar. You could run into something funny with a bazillion small tubes (lots of heater) all working at low plate current.

[mresistor]

Right. Find something similar at Hammond Mfg.

Or the Stancor or Triad catalogs, if they are available.

[MadMax]

All good information. I appreciate the responses. I like that calculator you linked Mresistor. It didn't list a 6X5GT (EZ80?) rectifier that one of my amps uses though. With the calculator, I'll try to start with the current needed and then guesstimate the appropriate transformer by size and weight.

I did read that forum discussion. Do you guys agree with the 10% drop-off from unloaded voltage as a max current draw approximation?

-Mad Max-

[HotBluePlates]

Do you guys agree with the 10% drop-off from unloaded voltage as a max current draw approximation?

That's probably reasonable. But the drop might be higher when hitting the rated limit of a tiny PT, and may barely sag at all on a huge PT.

There's a couple of mechanisms which cause sag in a power transformer, both of which become more pronounced when a transformer is designed to minimize size, weight and cost.