[Amps] About single 4CX250b output power

[Bill Fuqua]

I noticed a cathode driven amplifier in one of my old Radio Handbooks.  It 
used a pair of 4CX250 tubes. I suspect cathode drive would provide less 
distortion. Also, oddly enough in a very old QST there was a  single 
cathode driven 4X150A amplifier but it was operating zero bias.  This is 
the only one that I have seen like this. I suspect the tubes did not last 
long.   I knew a ham that tried all sorts of things to operate 4x150As 
cathode driven with zero bias. He only had two successes. One was with a 
large resistor in series with the control grid. And the other was with the 
control grid connected to the cathode. The second seemed to work but took 
lots of drive power. Naturally he did not make any IMD measurements. This 
was about 30 years ago.

73
Bill wa4lav

At 04:48 PM 11/3/2003 +0000, you wrote:
>Mike Tubby wrote:
>>
>>Page 65 of Eimac's "Care and feeding of power grid tubes" shows a 
>>selection guide for SSB service for which the 4CX250B is listed as being 
>>capable of 348W "useful power output" in AB1 with a 3rd order distortion 
>>product of -25dB (doesn't say whether its relative to carrier power or 
>>one tone of two tones).
>Relative to PEP, ie only -19dB relative to either tone.
>
>Eimac were being quite irresponsible by implying that level of IMD is fit 
>to be put on the air, and that data sheet has given the 4CX250B an 
>undeserved reputation for poor linearity.
>
>If you're less greedy than Eimac, you can get 250W PEP from a single tube 
>with excellent linearity.
>
>
>--
>73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
>                            Editor, 'The VHF/UHF DX Book'
>http://www.ifwtech.co.uk/g3sek
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