[Milsurplus] What's With 2,000 Volt, 50-Watt Xmtrs??

[Bruce Gentry]

David Stinson wrote:
> I've been looking at some transmitter diagrams 
> that use tubes like the RCA 803 and its kin.
> Take the RCA AVT-12:  This is a simple, late '30s
> xtal-osc-power-amp transmitter intended for 
> small aircraft which delivers about 50 watts out on phone.
> The high B+ for this thing, supplied by an external 
> dynamotor over a long run of cable, is like 1700 volts.
> Why?  RCA designed similar rigs about the same time
> that delivered that level of power with 500-600 volts.
> Get a GP and start thinking about 2000+ volts to 
> deliver less than 100 watts.  That kind of voltage
> in the dirty, greasy, gas-vapor-y environment 
> of a 1930s-40s aircraft seems...ummm....
> not well thought-out (read "stupid and suicidal").
> Other period equipment demonstrates doing so was unnecessary.
>
> Whenever someone otherwise brilliant pens a design
> that makes as little sense as creating the 803 to deliver
> modest power with Tesla-esk voltages, I smell money
> at the root of it.  Did RCA tube designers build the 803 and
> other tubes like it to get around someone's patents?
> That would certainly fit-in with all the radio patent fights
> in the 30s. How many different ways can you make a tube,
> anyhow?  They'd have to do something different to avoid
> royalties to someone, like design a 50-watt circuit 
> with 2200 volts of B+. 
>
> OK, People-Smarter-Than-Me:  What do you think was the 
> motivation to design low-power-out circuits with 
> dangerously high B+ voltages?
>
> 73 Dave S.
>
>
>
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>   
I  make a guess the early oxide and thoriated tungsten filaments  could 
not handle higher current densities.  Older AM broadcast transmitters 
used about 5000 V. for a 5 KW. and  18,000 for a 50 KW.  I have a couple 
Western Electric audio amps that use push-pull  211s  and 845s with 
about 1000 V. on the plates. The 211s give 15 watts and the 845s about 
65 watts.  I strongly prefer the sound of audio amps using high plate 
voltages at modest current over lower voltages at high current.