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PLASMA AMP TO 40 METER CONVERSION

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I found this 3CX3000F7 RF Deck from a 13.56 mHz Eratron Model 80030 Plasma Generator.  I was originally planning an easy conversion to 20 meters, but abandoned the idea since I am a night guy and 40 is a much better band at night.

ORIGINAL UNMODIFIED 13.56 MHZ RF DECK:

            
       Front view               Front-top view              Top view                   Bottom view 

 

COMPLETED 40 METER RF DECK:

                             
  Front view                Top view               Bottom view             Back View 

 

RF DECK  SCHEMATICS:             

                  
Original Cathode         Original Plate          Completed Cathode     Completed Plate
                                                                                                    (see  mods below)

 

CATHODE CIRCUIT CONVERSION:

           
Original 13 mHz pi-network cathode circuit:  The PI-network capacitors had very short shafts with slotted ends, that extended only through the RF compartment; however, there are two holes in the front panel to allow for screwdriver adjustments of these capacitors.

 

        
Completed 40 meter pi-network cathode circuit:  Removed the original 3 turn coil and replaced the original doorknob caps with 650 pfd units.  Wound a 9 turn coil of #8 bare copper around a flashlight.  Hooked up the MFJ-259 and the 51 ohm loading resistor and by trial and error, trimmed down the coil until I found 6 turns would be perfect.  ( 6 turns, 1.75" diameter, 2.25" long)  Low Q, flat from 6.8 to 7.5 mHz without retuning.

 

PLATE CIRCUIT CONVERSION:

    
Original 13 mHz pi-network plate circuit:

 

            
Completed 40 meter pi-network plate circuit:  The original pi-network output caps had fixed values to feed a fixed load of 50 ohms.  To accommodate loading into other than 50 ohms, I added a 150 pfd breadslicer. (*** see modifications below as this was changed***). The original 100 pfd units were replaced with 200 pfd units.  The resulting coil was 4.25" long, 3.75" in diameter wound with 1/2 in wide silver plated ribbon (aprox 4.6 uhy).  Also added was a 90 microhenry protection choke.

 

INSTALLED TURNS COUNTER FOR THE VACUUM VARIABLE:

   
There was no room inside the cabinet, so the final amp pi-network turns-counter was externally installed.  It's ugly, but it has a very solid connection to the vacuum variable.

 

INSTALLED TUBE COOLING BLOWER:

              

    
Installed and spray painted (Rust-oleum flat black) 3/4 inch plywood shelf inside a 6 foot relay rack using four 3" x 3" angle brackets.  Cut a rectangular hole in the plywood shelf and added the Dayton 4C870 blower motor:  1/10 HP, 1570 RPM.  Fan is 6" in diameter and 4" wide.  RF Deck will be slid over hole in plywood, as the hole was cut to line up with the tube.

CONSTRUCTED 2 AMP PLATE CURRENT METER SHUNT:

        
The process involved putting the test meter in series with a known "standard" 2 amp DC meter in series with a power source that could be varied.  By trial and error, a certain length of shunt wire will make the test meter read the same as the standard meter.  I found that a 3 foot length of #20 hookup wire did the trick.  I also added back to back diodes for meter protection.

 

 BUILT PUSH-TO-TALK CONTROL CIRCUITS:

         
Using a series resistor in the DC path, the power supply provides about 17 VDC to turn on the 12 VDC relay and then drops to 8.5VDC to hold it on.  I expect the push-to-talk wire to be quite long from the shack to the garage.

 

 3CX3000F7 FILAMENT TRANSFORMER:


Expecting a 120VAC transformer, I was surprised to find it was actually built for 220 VAC.

 

BUILT HIGH VOLTAGE POWER SUPPLY:

            
5000 volt supply       Filament and blower control

     
Installed the Plate transformer:  260 pounds !  Primary terminals: 230 to -5 = 220 in, 230 to 0 = 230 in, 230 to +5 = 240 in.  Secondary: 2378-1980-1694-0-1694-1980-2378.  Secondary Terminals 10 to 8 = 4700 VDC no load, 4200 volts under power, Terminals 11 to 7 = 5500 VDC no load, 5000 volts under power.  In a capacitive input, full wave bridge configuration, this transformer would yield between 4970 to 6724 VDC.  At 2 amps CCS, this transformer will loaf (be asleep) at legal limit power.  Serious overkill, but the transformer was free ! 

 

         
Installed transformer primary control module:  Built the modular control unit on a 7.5" X 17" painted 3/4 inch plywood board.  Has 30 second delay step-start via a 25 ohm resistor and a shorting contactor.  Connected the control module to the AC power subpanel via a 50 amp dryer plug and socket. 

 

        
Installed (8) K2AW rectifiers on a 5" x 8" x 1.25" aluminum heat sink and wired them in parallel.  Installed (16) 1900 MFD @450 VDC computer grade caps on a painted 3/4 inch plywood and wired them with 100K equalizing resistorsTotal value is 118 mfd @ 7200 VDC. (note:  1.6 meg makes a poor bleeder; however, the 450 ma amplifier idling current stabilizes the supply nicely and when I turn off the high voltage primary and key the amp without drive, the tube conducts and the high voltage decays in about 4 seconds).

 

                                     
Installed a high voltage fuse out of a 3 inch piece of #30 magnet wire on standoff insulators.  Installed a plate meter multiplier via 17 resistors on Plexiglas.

 


Completed power supply:

 

         
Completed amplifier.

 

BUILD ANTENNA SWITCHING RELAY:

    
Installed a 25 amp DPDT relay on a thick piece of aluminum to which was installed 4 coax connectors.

 

 TESTS AND OBSERVATIONS:

1.  The power supply came right up with no problems and the DC voltage came out as calculated.  The no-load high voltage was 4900 (note: 1.6 megohm bleeder).  The high voltage dropped to 4700 under a 450 ma load, when the amp was turned on with no drive.  At a 1 amp load, the high voltage was 4600.

2.  The first time I turned on the RF Deck, I applied 20 watts input and the pi-network tuned correctly and output was developed.  I increased power to 100 watts and got approximately 2000 watts out. I was very pleased that the power supply and amp came right up without smoke, fire or tripped circuit breakers !

3.  The amplifier puts out 20 times the input, thus a 18 watt carrier input yields 360 watts output and a 300 watt input yields 6000 watts.  (On the amps I have built without a tuned input, the amplification factor was only 14).

4.  The input pi-network worked fine and the driver could be tuned to 1:1 SWR.

MODIFICATIONS:

1.  While tuning up under AM modulation conditions, monitoring with a scope, I found that I couldn't quite load the amp to full output into loads different than 50  ohms because the 150 pfd pi-network cap was not enough capacitance.  I should have put in a large variable right from the start.  I now have a 27-1000 pfd variable and loading is perfect into dummy load or antenna.

    

 

I love this amplifier.  It's works flawlessly and has sufficient plate dissipation such that I can move around the band and not be concerned about less than ideal SWR.

 

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