At one of the National Association of Broadcasters conventions (1984) I
presented a paper/talk on FM transmitter design, titled Transmitter
Performance Requirements for Subcarrier Operation. While we concentrated on
bandwidth, group delay, and incidental AM coming from power amplifier
cavities using tetrodes (and the effects of the input and output tuning
circuits), we made a real measurement of the gain of a new Eimac tetrode in
grid driven and cathode driven operation, with the same output cavity. We
built the socket on a shelf that could be swapped out, and in one the
complete filament/cathode arrangment was floated for RF. Parts of this
research are included in the 7th Edition of the NAB Engineering Handbook,
1985. We did not even mess with 3000-5000 watt triodes, since the gain was
much lower and the feedback higher. We did build a good 8877 amplifier at
1500 Watts, which was able to work with about 75 Watts of drive.
In summary, the 4CX3500A tetrode was operated with 5 KW output, class C, at
two plate voltages.
The drive power and gain is shown here:
4.5 KVDC 5.2
KVDC
GG (cathode drive) 340 Watts/11.6 dB 280 Watts/12.5 dB
GC (grid drive) 190 Watts/14.2 dB 140 Watts/15.5 dB
Sorry if the chart fails to come over right, darn email formatting...
We settled on grid driven operastion, as we could get by with a single two
transistor IPA using a pair of SD Thomson SD1460 bipolars or TRW TP9383's.
It, in turn was driven by some Acrian part at that time.
We used the lower voltage single phase unit for up to 3800 watts, and a
three phase for higher plate voltage and up to 5500 watts. Many (hundreds)
of these rigs are on the air worldwide, I think.
It was a learning experience, as you can see that there was about a 3 dB
advantage at 100 MHz! This is sort of what one should expect. The phase and
amplitude bandwidth trade off was another major factor, and too much to
regurgitate here.
73
John
K5PRO
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