2X 4CX250B Amplifier






for






160 Meter AM/CW









As conversions go this one was relatively easy, as the equipment
involved
was designed for operation in the .500 Khz 



to 3 Mhz frequency range. This amplifier was used in a Mass
Spectrometer system made by Varian.
It came with

 provisions for monitoring plate, screen, and grid currents and also power output from the
pair of 4CX250B
tubes.

 This unit came in a standard 19 inch wide, rack mount configuration.









  


  
 The front panel:




  

    

  

  



Close-up of the metering.








  
There is a top and bottom shield for the tubes, covering the
plate circuit
and the grid / screen circuits.Which keeps
the errant feedback from causing problems in the amplifier in
normal operation.
Parasitic suppressors in the plate
circuit handle that quite easily, there is a separate R. F. choke
for each
tube with its' own HV bypass cap.








  

  



  

  
The Plate compartment.


  

  



  

  
The Grid compartment.


  
The socket on the left mounts a printed circuit board that drives
the metering
circuits. While the socket on the
right mounts the integral frequency control and driver board. The
original
crystal on this board was at 1823 Khz.
The crystal was removed and a piece of RG-174/U coaxial cable
soldered
to the socket and run along the
wiring harness to a chassis mounted BNC connector. This connector
is driven
by the "Lab" PTS-040 signal source
which makes the rig agile with coverage of the entire 160 Meter
Band. The
plate circuit blocking capacitors, which
has one for each tube is a special separate plate affair which
isolates
the HV from the RF circuitry. The RF output
is driven independently from each 4CX250B, and thus there is a
coil and
tuning variable capacitor for the the two
tubes. This output is then mixed in an RF duplexing arrangement
external
to the R.F. Deck. Note the VERY large
angled pipe which brings cooling air flow into the amplifier
compartment
to protect the tubes from excessive heat
within the compartment. The two coaxial lines have been
restructured and
the plate circuits mixed internally to
facilitate driving a single 50 ohm line to the antenna coupling
circuits.

    

  

  



  
The HV power supply is the "rescued Savant" power supply
mentioned earlier
on the web site. This supply can
provide up to 5,000 VDC at 500 Ma. to this amplifier, however
with the
variac control of the A.C. input to
the HV transformer the output is set at 2000 VDC for the
4CX250Bs'. The
screen supply and bias supply are
provided by the Screen and bias transformer from a scrapped out
DX-100.
In keeping with the "Glow-Bug"
theme the circuit for this supplies is lifted completely from the
DX-100
schematic, using 6AL5 and 5V4 rectifiers
and the same choke and capacitor arrangement as well. In looking
at the
above picture, in the lower left corner
there is shown three potentiometers, which set screen volts for
both tubes,
and individual "pots" for setting the bias
voltage to the tubes. The lifted schematic drawing for the screen
and bias
supply is here: , and
the schematic for
the "Savant" HV supply is here: A CW keyline goes to the exciter
board
and keys a 2n2219A for CW operation
of this converted RF amplifier. Neat Huh? The Modulator is a
"Coming Attraction"!
Have to find a correct match
in a modulation transformer for a pair of 4CX250B modulated by a
pair of
4CX250Bs.


The schematic is in three parts, and just so you can see what the unit looks like electrically, if you like you can cut and

paste the three sections together for a better idea of what is going on. Schem1, Schem2, Schem3.

    

If you come across one of these and want to put it on the Ham-Bands, it works fine and should be good from 160 - 10 meters.

    



  



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