[AMRadio] RCA Transformer ID?

Donald Chester k4kyv at charter.net
Sat Jul 15 23:17:19 EDT 2017

It's a modulation transformer, but wasn't designed for voice. Thousands of
them must have been released as surplus after WWII; it may have been the
most popular modulation transformer for KW rigs in the early 1950s.  It was
designed by RCA for tone modulated CW (MCW) on Liberty Ships during the war.
Definitely not broadcast iron, the audio quality is on par with low-cost
"amateur radio quality" multi-match modulation transformers, such as the UTC
S-22 and maybe ever the VM- series. They display enough phase shift
distortion to be visible on a scope when running sine wave tone modulation
near 100%, but still work pretty well for ham stations.

One of the drawbacks is the 5500:5500 (one to one) turns ratio.  This causes
a mis-match with most of the popular modulator tubes, but the distortion
level is tolerable.  It has a tertiary winding to modulate the screen of a
tetrode or pentode.  The turns ratio gives good positive modulation peaks if
the proper modulator tubes are used, for example a quad of 811-As in

The  name plate figures, giving impedance, turns ratio, maximum final
amplifier plate current through the secondary, plate voltage and modulator
power rating, don't add up.

Although zillions of these transformers circulated after the war, very few,
if any of the transmitter they went into have appeared on  the surplus
market, or even any descriptions, technical or otherwise of the actual
transmitters.  It has always been somewhat of a mystery item.

Don k4kyv

> -----Original Message-----
> From: amradio-bounces at mailman.qth.net [mailto:amradio- 
> bounces at mailman.qth.net] On Behalf Of Mike Durff via AMRadio
> Sent: Friday, 14 July, 2017 19:39
> To: amradio at mailman.qth.net
> Subject: [AMRadio] RCA Transformer ID?
> RCA 901765-501 Tag says:Class = ModulationPri= 5500 ohmssec = 5500 ohms.
> I thought it was a modulation transformer, but others say no. Google 
> doesn't yield  conclusive results.
> Any guesses?
> Thanks, Durff
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