[Amps] 4CX1500B project

[Radio WC6W]

Hi Vic,
   Recognizing that this isn't an optimum tube for CW, one might consider exploring Class C operation since IMD is of no concern in CW.

   Start with perhaps -85 or -90 volts grid bias so that the tube is essentially cutoff.

   Then apply something in the neighborhood of 100 to 110 volts peak RF drive.  This tube has a 1 watt grid dissipation rating that would need to be respected & monitored.

   Increase the screen voltage as necessary until the desired output is acheived.

   The drive requirements may be reduced by employing a 1:4, or 1:9, impedance ratio RF transmission line transformer with 200 ohm, or 450 ohm, swamping resistors respectively.   10 watts drive should suffice with the 1:9 selection.

   The increase in grid circuit impedance will substantially worsen the effect of the tube's input capacitance and will require adding an inductor to the grid circuit.  This will need to be variable or bandswitched (w/ relays?).   To maintain a reasonable L/C ratio, some additional capacitance may be desirable on the lower frequencies.  More parts.  :-)

   If this works, legal limit output may be acheivable with around 2KW input; at perhaps 75% efficiency, only 500 watts plate dissipation.  That's way "cooler" than the typical operation numbers for this tube. 

   Just a thought.

73 & Good morning,
   Marv WC6W


--- On Wed, 8/8/12, Vic K2VCO <k2vco.vic at gmail.com> wrote:

> From: Vic K2VCO <k2vco.vic at gmail.com>
> Subject: [Amps] 4CX1500B project
> To: "Amps reflector" <amps at contesting.com>
> Date: Wednesday, August 8, 2012, 8:06 PM
> I've been thinking about my next
> project for some time, and I can't seem to shake the idea 
> of an 160-10M amplifier using the 4CX1500B. The tubes and
> the beautiful SK-800B socket are 
> just burning a hole in my workbench, as it were.
> 
> So I am thinking of running it in AB1 according to the
> parameters in the 4CX1000 datasheet 
> -- 3kV on the plate, 325V on the screen and -60V bias. This
> will give me full legal output 
> from one tube.  It is possible to reduce the bias and
> screen voltage and run it closer to 
> class A for much better IMD, but as a CW operator I'd prefer
> a few more watts (actually, a 
> CW/SSB switch is a possibility).
> 
> I am considering a grid-driven circuit with a 50 ohm
> swamping resistor. It seems to me 
> that it would require 60v peak to just overcome the bias,
> which comes out to 72 watts 
> drive for AB1 operation. Is this a reasonable assumption?
> 
> This configuration would make it unnecessary to build and
> fiddle with a tuned input 
> circuit. I'm also thinking about using the G3SEK tetrode
> board, which would solve a lot of 
> little problems in one package.
> 
> Does anyone have comments or suggestions?
> 
> -- 
> Vic, K2VCO
> Fresno CA
> http://www.qsl.net/k2vco/   
>     
> 
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