[sdiy] guessing HV power transformer's current capacity by its dimensions?

[Bob Weigel]

Usually dimensions are a rough way to tell but you can assess more by of 
course finding the guage of wire used in the windings if you can get it 
apart to peek.  Then use a milliohmeter or ohmeter as needed to measure 
the DC impedences of the coils.

Guages of the wires will tell you roughly how much current they can 
safely handle.  But of course if they are tightly wound you can also 
make estimates on how much heat in watts can be radiated at the top 
temperature you'd want to run the thing compounded by each layer's 
thermal conductivity limitation.  In other words it'll get hottest 
inside barring any really bad connections etc. :-)  -Bob

anthony wrote:

> I was wondering if you can somewhat accurately judge the current 
> handling capability of a high-voltage power transformer by its size. I 
> have a rather hefty transformer that was in a pice of test equipment 
> that had a lot of tubes in it. The secondary doesn't have a center-tap 
> and the rectifiers were solid state so I can't guess based on what the 
> rectifier can handle (and the original solid state diodes are long 
> lost). It has 2 filament windings, both without center-taps, one green 
> which is visibly much stouter than the other, yellow one. Most of the 
> tubes were 12AX7's , but there were 6BK7B's, 12AT7's, 12AU7's, some 
> acorns, a Nuvistor, various small pentodes, and 4 EL84's working as 
> current amps in the voltage regulated power supply.
>
> The fact that it is using 4 EL84's in the power supply might signify 
> that the xfrmr indeed handles a lot of current, but using an 0A2 and a 
> 5651, would require current amplification if there was even modest 
> demands on their current handling capability (which isn't great).
>
> I guess it seems like folly to try to guess at it this way. I mean 
> most of the space for the windings may be for the filament windings 
> and the primary winding to go with them. But I think it would be an 
> even greater folly if I was trying to judge something like a power 
> transformer from a 'scope (which do work very well for other uses if 
> you tuck the unneeded leads safely away...).
>
> Why I'm even fussed about it is I'm trying to determine if this would 
> be enough or too much transformer for powering 2 GA-5T Crestline 
> circuits (2x2x6AQ5A push-pull) biased to run nearly 100% Class A on 
> the same chassis.
>
> It's about the size of a large-ish grapefruit or more like a smallish 
> musk melon and weighs I'd say close to 10 lbs.
>
> And I'm actually not so much worried about it not hadling enough 
> current, but being wasted on a project that could make do with a 
> smaller transformer and using it for an amp that puts out much more 
> power, like something with 2 6L6GT's or 2 EL34's...
>
> AA
>
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