[Amps] tube output power

[David G4FTC]

>
> When I worked for BE, I designed the cavity amplifier that was put into production in the FM3.5A and FM5A FM
transmitters. The 'standard' tube used for 3 - 5 kW FMs that used tetrodes was the 4CX5000A, in RCA and other
makes of power amplifiers. George Badger of Eimac showed us their new shortened 4CX3500A, which broke the
traditional wisdom, and it ran beautifully in the 5 kW model (actually 5.5 kW with 110% power type accepted). 
While this isn't 2 x the dissipation rating, it does show that the tube chosen for a particular power level depends a
lot on other things like cathode emission, expected lifetime, class of operation, screen dissipation, as well as plate 
dissipation. We did built a triode transmitter as well, using the 3CX1500A7/8877, although it was only running 1750 
watts output. And the 8990/4CX20,000A was used in the FM30. In no case am I aware that we actually built PAs 
that produced output powers that were exceeding twice the plate dissipation. We were fairly
> conservative, but we had a lot of customers and didn't want to fail in the marketplace at the time.
>

>From my experience of professional amplifiers/transmitters, the yardstick which seems to be
used by most manufacturers is that the tube plate dissipation equals the RF Output.

I know that much more power can be obtained and still stay within the manufacturer's 
rated plate dissipation, but the above is just my observation.

Regards

David G4FTC
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