[BC] FM Grounded Grid Triodes & FM Tetrodes

[Mark Humphrey]

Back in Syracuse in the early '70s, I remember visiting WDDS 93.1 (now
WNTQ) which was still running a big 10 kW RCA with grounded grid
amplifier into a 10 bay Andrew "V" antenna for 97 kW ERP.   As I
recall, the final didn't have much power gain and required almost 3 kW
of drive.  By that time, the original RCA Iron Fireman has been
retired in favor of a Serrasoid exciter.

As you might guess, WDDS played "dentist office music" on main
channel, with 41 and 67 kHz SCAs for Muzak and targeted spots, which
is how they made most of their income.    (I'm surprised that call
sign hasn't been picked up -- someone could build a lame AC format
around it, with Perry O'Dontal as the morning man, Flossy as his
female sidekick, etc. -- well, never mind.)

At WRTI, we had a 1 kW ITA backup with grounded-grid tetrode PA -- not
in the best of shape, but it kept us on the air when the Wilkinson 10
kW main (serial 001) would go down, but that's a story for another
day.

I quickly learned to be careful when tuning the ITA, otherwise it
would go into "spread spectrum" mode with spurs every 350 kHz across
most of the educational band.  I'll have to say the 3CX series used by
CCA was a much more stable approach to grounded grid.

Mark

On Thu, Nov 11, 2010 at 9:40 AM, Dave Hultsman <DHults1043 at aol.com> wrote:
>
> RCA may have pioneered tetrode FM power amplifiers,  but their early FM transmitters used ground grid for 3,5,10 kW.  and even the later few FM 50 kW. all using the 5762 finals.   The 50 kw. used the 7 triode tube ring type final amplifier used in TV aural amplifiers.   The original RCA 50 kW. FM transmitter of which, I believe, there were three made, I seem to recall used three triodes in parallel in the output stage.   WBRC-FM and a station in Madison Wisconson used that transmitter in the early FM days.