[ARC5] [Milsurplus] What's With 2,000 Volt, 50-Watt Xmtrs??

[howard holden]

At least a part of this could be due to the need to "meet spec". When I was back east, working aboard the USS Ling, we happened across the TBL transmitter final acceptance tests. The TBL, with 2 803's is rated at a pittance 200 watts out, with 2KV on the plates. Yep, seems stupid. BUT...on the acceptance tests, the transmitter put out over 500W at the lower HF frequencies, tapering off to about 225 at the upper end, 18 Mhz. Granted, the called-for plate current was not at the max for the tubes, but it was not too low either. As I recall, the power input with spec'd plate current was around the 1KW area at the lowser freq ranges, tapering off as the freq was raised. So, in this case, at least, and given that the 803 does not have a very high upper end freq rating, the seemingly stupid power ratings make a small amount of sense. Of course, they could have scaled the published transmitter ratings to match the tubes' capabilities over the freq range, but that would have been too logical for the military/industrial complex thinking!

Howie WB2AWQ

----- Original Message ----- 
  From: David Stinson<mailto:arc5 at ix.netcom.com> 
  To: arc5 at mailman.qth.net<mailto:arc5 at mailman.qth.net> ; milsurplus at mailman.qth.net<mailto:milsurplus at mailman.qth.net> 
  Sent: Wednesday, November 17, 2010 9:11 AM
  Subject: [Milsurplus] What's With 2,000 Volt, 50-Watt Xmtrs??


  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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