[CW] Tone Modulated CW

[Donald Chester]

One of the most commonly used AM modulation transfromers in ham rigs after 
WWII was an open frame job made by RCA.  It resembled the ones in their 
250-watt broadcast transmitters, but was slightly smaller and had a 1:1 
impedance ratio, with a tertiary winding for modulating the screen grid of 
the final.  There must have been a jillion of those things made, and they 
were once an abundant surplus item and were often seen at hamfests.  The 
data on the nameplate does not add up mathematically (impedance, DC current 
through the winding, power rating, etc), and the audio quality is not 
exactly high fidelity, but they work acceptably well in ham rigs up to 1 kw 
input.

I have never seen anything published about the rig these transformers came 
out of or were designed to go into.  Recently, a ham in England with whom I 
corresponded via e-mail told me he had the data on these transformers. He 
even mentioned the model number of the transmitter.  He said they were 
designed for a plate-modualted MCW (type A-2 emission) rig that was widely 
used on the Liberty Ships during WWII.  That explains the relatively poor 
audio quality for voice.

Does anyone have any idea why the Liberty ships would have used tone 
modulated CW (type A-2 emission)instead of regualr CW?  I am aware that 
early tube type CW rigs would modulate the carrier using a rotary mechanical 
chopper, but I always assumed this early MCW was used to make the signal 
readable on a diode or other detector in existing receivers that had been 
originally designed for spark.

One advantage MCW would have had over regular CW (A-1) is that frequency 
stability would not be so cricical in reciver or transmitter.  However, by 
WWII, xtal control was the order of the day, and stable receivers were in 
common use.  One good example was the pre-WWII National HRO, which once 
warmed up, would stay on frequency on 20m. for hours at a time without 
needing to retouch the tuning.

Does anyone have any ideas?

73, Don K4KYV

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