Load testing filament current?

[Blair]

How long should it take to get a transformer up to maximum temp?

I've tried several times to go through the "dumpster transformer" calculations for filament current and I'm sure there is a conservative approach there, but using the calculations on known transformers sometimes yields about 1/2 the filament delivery.

Right now, I have a few transformers I want to use for preamps, and I'm planning to rectify a 10V winding to run filaments at 12VDC. Yesterday, I loaded the transformer with two EL34s in series and adjusted the mains voltage with my variac to get to 12.6V. I loaded it with the tube filament and it dropped by .2V. So far so good. After a full hour of operation, I'm still at 12.4V and the transformer is maybe a degree or two warmer than the room. There are no bells on it, so I can touch the paper around the windings as well as the laminates.

How long should I run it before I call it good?

Thanks!

[DeathRex]

How long should it take to get a transformer up to maximum temp?

I've tried several times to go through the "dumpster transformer" calculations for filament current and I'm sure there is a conservative approach there, but using the calculations on known transformers sometimes yields about 1/2 the filament delivery.

Right now, I have a few transformers I want to use for preamps, and I'm planning to rectify a 10V winding to run filaments at 12VDC.

Yesterday, I loaded the transformer with two EL34s in series and adjusted the mains voltage with my variac to get to 12.6VAC. I loaded it with the tube filament and it dropped by .2V. So far so good. After a full hour of operation, I'm still at 12.4VAC and the transformer is maybe a degree or two warmer than the room. There are no bells on it, so I can touch the paper around the windings as well as the laminates.

How long should I run it before I call it good?

Thanks!

It is AC right?
Filament transformers should be easy. You can measure voltage drop and wire size. If you have only a filament transformer, measure the wire size (I use a stripper) and you can find the approximate current. Only wire size I know is 18 gauge is 3 amps. I've seen some filament transformers drop 5% when loaded and some 20%.

As for temperature, I usually give a power transformer 5 hours, and it would be good if it was under 60deg C, preferably under 55.