[AMRadio] Unusual transformer

Donald Chester k4kyv at charter.net
Sun Oct 22 11:52:03 EDT 2017

> I'm puzzling over a transformer I found in my junk, which is clearly identified
> as a Thoardson T-21M66, but even after finding it listed in the 1947 catalog
> online, I'm still not sure how it was used.     It is described as a "Matching Line
> to RF Load Modulation (M) Transformer" and has a single 500 ohm primary
> (that would be the "line" part), and five secondary taps that are only
> described as "5M-6M-7M-8M-9M-10M" which must be the "RF Matching"
> part, but darned if I know what these values are or how the transformer
> would be used.   The audio power rating is 50 watts and the secondary is
> good for 200MA.    A scan of the actual catalog section can be seen here:
> https://i.imgur.com/2eyXns2.png
> I've included the multi-match modulation transformer section above so you
> can see what the column headings refer to.   Can anyone shed light on what
> this transformer was used for and what the secondary values mean?
> Bob W9RAN

Looks like it's a modulation transformer designed to be fed with a 500-ohm line, instead of directly by the plates of the modulator tubes.  The modulator tubes need a plate(s) to 500-ohm output transformer to feed the line.  Could be useful when the modulator is in a separate rack some distance from the RF section, and one would prefer to feed the audio throug a 500-ohm line instead of running the high  voltage from the modulator or the final through long  leads running between the racks.

5M-6M-7M-8M-9M-10M means the same thing as 5K-6K-7K..., i.e. 5000, 6000, 7000 etc. "M" is sometimes used as an abbreviation for "mill" = thousand.


Don k4kyv

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