[Boatanchors] Audio output transformers in VC/NVA radio set

[William L Howard]

Someone asked what I was going to do with the audio output transformer.
For those who have not been to my web site on Technical Intelligence and
or did not look at the section on radios of the VC/NVA, I will make a
brief summary. I have been able to study in great detail, two VC
Homebrew radio sets. I have also built 6 copies of some other sets,
based on Technical Intelligence reports, CIA studies,and some detailed
photos. In addition, I have several of my own designs and have also made
"Ivo's Flea Power Transmitter", a 2-12MC set based on a 6LN8 tube.

I made one copy of the set at the Aberdeen Proving Ground Museum ad had
trouble getting it to work. I took it to Don Dean in Sarasota and went
over it and got it working. He made the comment, "Why so many tubes, all
you need is three" and the set had about 7.

One has to ask, "Where did they get their designs from?"  The only
radios that they had any contact with were Japanese WW II sets, US
military sets and most likely the OSS SSR5 receiver. The other options
were publications such as the ARRL handbooks from the 1950s. Most of the
sets made use of the miniature tubes such as the 1S5, 1R5 and 1T4s. This
brings to mind the late war Japanese 1568 set which made use of three
1T4 or the Japanese equivalent.  Most of the WW II Japanese radios made
use of 1930s circuits and tubes and in mid was the aircraft radios began
to use the octal base tubes. I have never run across a VC Homebrew that
did not make use of the smaller tubes so I conclude the circuits came
from 1950s manuals.

The only NVA factory made radio was the VTS 2 receiver that used tubes
for RF amp and detector circuits and then transistors for audio
amplification.

I am currently building another "VC Homebrew set". Most of the VC
communication was in the 2 - 12 MC range. In addition, in my opinion,
most of the sets were more complex than they needed to be. One
construction feature was that B - was grounded to the chassis. Now the
standard 1/4" jack socket has one of the contacts also grounded. Now if
you connect B+ to the headphones and then to the tube plate you have an
instant short circuit. To get around this they used an output
transformer with the B+ going to the primary winding and the other end
to the tube socket. The secondary winding was connected to ground and
one wire to the headphone socket. It uses less wire, makes for a simpler
wiring harness. Another construction feature was the use of bare wire
busses for the filament supply and for the B+ line. Copper seems to be
the A+ and a silver colored wire buss for A -.
Parts were then wired from tube socket to the appropriate buss wire. In
most of the sets, the stages were capacitor connected, eliminating the
need for costly, hard to find, bulky interstage transformers, ala the
1930s vintage sets. My most recent acquisition, a VC homebrew supposedly
made in Cambodia, made use of transistor sized interstage IF
transformers. I am still trying to figure out the circuit they used.

In the meantime, that's what the audio output transformer was to be used
in.

Bill Howard