[BC] FM Exciters (Pic's)

[Phil Alexander]

On 26 Feb 2006 at 10:09, Mark Humphrey wrote:

> I have a manual for the RCA BTE-10B (with the cute CRT, successor to the
> "Iron Fireman") in front of me.  The FM oscillator was a 6AQ5 Hartley type,
> operating at 1/18 carrier frequency, with two 6AQ5 reactance tubes for main
> channel in a "push-pull" arrangement, plus a 6CL6 reactance modulator for
> the SCA.    The output of this master oscillator was divided by 240 and
> compared with the crystal reference oscillator (divided by 5) with a phase
> detector operating somewhere between 20 and 25 kc.  This phase detection
> circuit was just a couple of transformers with a pair of 1N34A diodes and
> filter network, to generate the DC error voltage which was fed back to the
> reactance tubes.

Ah, I see too much of the TE-1 spilled over into my memories of the RCA.
At least I got the crystal frequency in the correct range. <g>

> This brings back some memories.   At my Class D college station, we bought
> one of these used from WBVP-FM in Beaver Falls, PA to get on the air cheap.
> AFC stability wasn't a big problem, but I could never get it to make the
> full 10 watt rated power.  Let's just say its performance was not in
> proportion to its weight.

About 12 RU of 19" rack as I recall. Let's see if I came close on that one. <g>
Of course the RCA was not alone in anemic RF output. The Gates M-6095 which
was probably one of the most widely sold exciters of the era, could barely
make 10 W with a new 6360, and if the screen voltage was left at that level,
the tube would quickly die. This was only a problem in the earlier Gates
transmitters such as the FM-1C, because the IPA of the "G" series required
only about 3-4 W input.

Far and away the best (IMHO) Tx for a Class D NCE in those days was the GE.
It *was* truly crystal controlled and the exciter drove a separate panel 
that used an 815 as a push pull PA. Note: 815 = pr. 2E26's in a common
envelope - a little fat tube with two plate caps - R2D2 prototype only 
shorter. <g> Performance of that box was amazing, as was the sound of
the Phasitron, although making a proof at both 30 and 50 Hz at 100% mod.
was quite a challenge in the days before the 30 Hz requirement was dropped
from the FM audio proof.

> WGRQ 96.9 in Buffalo had one on the air through the late '70s. I wonder how
> many are still running.

I there are any, I do hope they are in museums.


Phil Alexander, CSRE, AMD
Broadcast Engineering Services and Technology 
(a Div. of Advanced Parts Corporation) 
Ph. (317) 335-2065   FAX (317) 335-9037





-- 
No virus found in this outgoing message.
Checked by AVG Free Edition.
Version: 7.1.375 / Virus Database: 268.1.0/269 - Release Date: 2/24/06