Hi Tom,
  In answer to a couple notes:

  20 feet of coax should pose no problem as long as both ends are 50 ohms.  

  There may be some modern devices that perform better than those ancient TRW things I suggested.  I'm not really current with those parts.  Texas Instruments has some great Op-Amps which, at around 100mW are not quite as powerful as the old hybrids, sport really fine distortion numbers at 4Mhz.  Take a look at the OPA695.

  I don't know much about the insides of the FT-1K.  A look with a spectrum analyzer would be better than my speculation.  :-)

  Running the tubes "lighter" improves the numbers at a faster rate than negative feedback.  NF nets about a dB distortion reduction for each dB of NF.  Reducing the stage power 3dB reduces the 3rd order products (theoretically) 9dB -- the cube!   The higher order products diminish at progressively higher rates.

  Since you running at a relatively low frequency, you might be able to implement a feedback loop around three stages.  The phase shift is the limiting factor.

  To answer the questions about the 10KW Hughes amp:
     That drawing is from more of a marketing than engineering document.  It does contain a few errors & some (obvious) omissions.  I added the ground symbol to the final autotuning sense ckt. myself before scanning.

  Some numbers:
     Grid bias:    
      4CX250R:   -62V
      4CX1000D:  -340V  (adj for 500mA resting plate current)
    A resistive feed to the bottom of the grid tuning coils, as Karl-Arne noted, is valid to feed the bias.

    Signal levels:
      Input: 100mW / 2.25V RMS
      4CX250R grid:  22V RMS      (this illustrates how light the 6146 is being run!)
      4CX1000D grid:  210V RMS  (this illustrates how light the 4CX250R is being run!)

    Power Out:
       10KW avg.  14 KW peak
       better than -40db IMD @ peak power

   Negative feedback: 12dB

   A resistive bias feed to the bottom of the grid tuning coils, as Karl-Arne noted, is valid to feed the bias.


73 & Good morning,
  Marv WC6W

P.S. -- Steve T. posited in another post that there was little intrinsic difference between a vanilla 4CX350A and the 4CX350FJ.  The data sheet claims 10dB lower distortion at equal power.  That's a lot.  I wonder if Eimac uses a striped cathode aligned grid scheme, as in the 4CX600J & 4CX1500B, in the '350FJ?   







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