[Milsurplus] SCR vs. TVI

[Ray Fantini]

With all this talk of TVI and how the old Mil sets don't generate it
makes me wonder why I have had such issues with the TBW that I have been
using on AM for the last six months. I am using an unmodified TBW with a
external power supply cabinet. The center power supply section that I
obtained from eBay was gutted when I received it, so had no problems
rebuilding it with 60 cycle stuff while trying to use as many original
components as possible, but being 60 cycles that meant all new filter
capacitors and chokes and an external solid state plate supply. I did
take efforts to use the original rectifier tube for the 500 volt supply
and spent months tracking down the 843 triode for the modulator. But the
transmitter section is completely original. For the last couple months I
have been working with different methods of tuning and loading to try to
get the spurious and harmonic emissions down, knowing that my second and
third harmonics are only about 43 to 45 dB below the carrier and
regularly see spurious emissions well out to 175 MHz at 55 to 60 dB
below the carrier. I have found that tuning by the book for maximum grid
current and using original plate voltage will overdrive the IA and PA
stages and produce spurious products. I am now running reduced plate
voltage on the oscillator and driver stage with a 3 to 6 dB improvement.
So on the TBW running it strictly by the book will not result in the
cleanest operation. Other modifications include reduced plate voltage to
around twelve to fourteen hundred volts from the recommended two
thousand and I use a Johnson Low Pass filter that attenuates everything
above 30 MHz to at least 80 dB below the carrier. Before this I was
using a TCS transmitter for AM and found that its second and third
harmonics were around 45 dB down, but with the low power output of the
TCS TVI was not an issue. In a world before cable television where many
viewers used outside antennas, preamplifiers and televisions that were
prone to front end overloading 40 dB down on a two hundred watt AM Ham
station was well capable of knocking out several neighbors' television
reception. My point is that first: Late thirties and early forties
designs are not as effective as modern designs in suppressing harmonics
and spurious emissions and second: That running them strictly by the
book will not always result in the best operation. I understand that
this will upset the "absolute truth" of the book crowd, and pleas
understand that I am in no way saying to discount the importance of the
original documentation but to say that this is the way is was designed
and this is the only way something can be used and by default must be
the best way is not always the case. Collecting, restoring and operating
vintage radio equipment incorporates a large and diverse group of
people, from those who feel that everything must be exactly as it was
when the item was produced to those who are willing to modify procedures
and sometimes equipment to facilitate better operation, I would like to
think that the members of this reflector have the capabilities to
respect a broad range of opinions that are expressed here. I do take
exception with those who gut or modify NOS material but that's just
me. As a footnote 97.307 says that all harmonics of a HF transmitter
running over 5 watts should be a minimum of 40 dB below the carrier and
not exceed 50 mW
Ray Fantini KA3EKH