AM Transmitters

[rabruner at aol.com]

Actually, RCA sold quite a few Ampliphase transmitters, most in higher powered configurations as the increased complexity brought decreasing benefits as the power went down. But I know of at least half a dozen Ampliphase rigs still on the air in power levels ranging from 10 kW to 50 kW.  Plate modulated transmitters are capable of excellent fidelity in all power levels, but especially in 5kW and under range where audio response up to 15 kHz is achievable with distortion <= 1% not at all uncommon.  Certainly the transmitter is capable of more fidelity than the average AM receiver. When you hear poor HF response, it's mostly in the IF of the receiver, sometimes in the telephone line feeding the transmitter. Distortion can come from anywhere and often is the result of processing gear jazzed up too much, or simply sloppy production practices at the studio.  There just isn't the advantage to a sophisticated, tricky to tune transmitter below 5 kW that there is above that level. These transmitters do exist, but they are not common.
    Western Electric made Doherty amplifiers in 5kW.  I know where two of them were in use at one time, I have never seen a 5 kW Ampliphase, I don't know if RCA made them in that power level or not.  The Continental version of the Carrier/ Peak tube transmitter was made in a 10 kW version, but I don't know of any smaller than that.  The drawback to all those carrier peak tube transmitters is that they take some careful tweaking.  The two final tubes are tied together on the output by a 90 degree line, which means the drive also has to be phase shifted to get additive power into the load from each tube.  The Continental transmitters have a small oscilloscope mounted on the panel to display the 90 degree phase shift at various points in the transmitter for tuning purposes.  The W-E transmitters had cabinets full of adjustable coils that had to be set up with a static bridge for specific impedances, often achieved while needing to maintain the correct phase shift. Getting the transmitter out of phase could turn it iinto a gigantic oscillator.  However, when adjusted, they were marvelous sounding beasts, but as someone has said, not at all frequency agile
    The Continental transmitter was able to work because of the use of Tetrodes in the final.  (the W-E xmtrs were all-triode)  The two tubes were essentially in parallel through the 90 degree delay lines.  The screen of one, the carrier tube, was tied to the plate of the a 3CX3000 with close to normal voltage on it. The screen of the other final, the 'peak' tube, was biased several hundred volts below ground, negative enough that it did not conduct more than resting current during no modulation conditions.  This sreen was tied to the cathode of the other 4CX3000.  The two audio tubes were driven in parallel and would modulate the carrier tube downward and the and the peak tube upward as needed.  The output envelope was detected and the resulting signal used as negative feedback for the transmitter.  The original W-E transmitters did this as well.  Both techniques produced wide frequency response and low distortion on the dummy load.  Of course, the bandwidth of the antenna system and other factors in the load could affect performance, but over all these were excellent transmitters.  
    Once we were using a Barker and Williams oscillator during some performance measurements.  These oscillators had a tendency to 'bounce' and if they were not carefully tweaked could begin oscillating on subsonic frequencies.  The other engineer that night called out to come look and we could see the modulation envelope on the 317C going to 100% at some frequency below a couple of cycles per second without even a whimper.  That same night, again on the dummy load, we ran the frequency up and hit 30 kHz before it began to fall off.  Of course, we couldn't get that kind of HF response through the antenna networks, but it was interesting to know the transmitter was capable of it. Very good square wave performance for an AM transmitter.

Bob Bruner 
W9TAJ


Complexity is relative.  Ampliphase may have been more efficient than plate 
modulation, but I don't think that it caught on that much in the USA.  I assume 
that station owners didn't see enough of an advantage to offset their worries.  
They didn't really understand even the simple plate modulation technology, in my 
opinion, or I think that they would have gone with triode audio power tubes and 
tetrode or pentode RF power tubes in the conventional class-AB / class C 
plate-modulated transmitters pretty much from the 30s.  So convincing them to go 
with some remarkable vector combination  system like Ampli-Phase, as cool as it 
really was, was a tough sell.  To me, the reduction in overmod spatter, the good 
efficiency, and the improvement in audio quality, would have made the sale.  No 
big heavy modulation transformer to blow out, either.  But they were nervous 
investors.

   Bob, WA3WDR

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