find mA rating for unknown power transformer?

I've got an old power transformer that is meant for a tube amplifier. It
has 3 filament windings and one HT secondary. Running the primary direct
from the wall supply, 120v, the HT secondary with out a load is 655vac
across the outer legs. It has a center tap that I ignored for this purpose.
So, I can guess that 327-0-327 is probably 300-0-300 give or take 10-15v.
The hard part is figuring out how much current capacity exists without
killing the transformer.

I have tested with 10W sandblock resistors (what I have on-hand) across the
HT secondary and have these results. 14K7 = 643vac, 9K8=640vac, 5K8=634vac,
and 1K5 smoked & toasted at 590vac. Now, I realize that 1K5/590v is 390mA
and 230W. This seems well beyond what is appropriate for this transformer.
I am guessing it is capable of something around 150-180mA. But all this is
trial and error.

Is there a more definitive approach to uncovering the required information
and properly back-solving for an answer? To repeat, the question is how
many mA capacity is there?

Regards,
Phil



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Relevant Pages

  • Re: find mA rating for unknown power transformer?
    ... Running the primary direct ... from the wall supply, 120v, the HT secondary with out a load is 655vac ... The hard part is figuring out how much current capacity exists without ... This seems well beyond what is appropriate for this transformer. ...
    (sci.electronics)
  • Re: where does all the extra current go?
    ... Your transformer can deliver up to 1.5A but will do so only if its ... load demands it. ... If you are drawing 100mA from your 1A transformer it means that the ... transformer still has a further 900mA capacity which you aren't using ...
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  • Re: Cheaper output transformer
    ... You can try using a power transformer backwards, ... plates, and the filament windings, kinda match ... A single ended amp probably will sound really bad because of the DC ...
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