KL7UW
144-MHz 1500W 8877 Power Amp



 
   This page describes the construction of a 1500-watt 2m amplifier using an 8877.  The design follows 
   the classic W6PO design which is described in the Eimac Notes.  I am using the Triode Board
   built by G3SEK.  I purchased the Plate lines and output tuning capacitors as a kit from Harry, WA4OFS(sk).

   Here are some photos of the amplifier under construction:  (click to see larger image)













    Block diagrams of the amplifier showing control, bias, and power wiring:  (click to see larger image)






















   QRO HV power supply acquired:
    

  • Schematic
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  • Front and top views of the surplus HV Power Supply
 























   Here are photos of the assembled PA Control Panel (before wiring):
    

  • Front view of Control Panel.
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  • Close up view of Meters.
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  • Backside view-1.





















    

  • Backside view-2.
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  • High Voltage Divider to provide HV detection & metering.
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  • High Voltage switch.




















    

  • High Voltage Connectors.

















    

  • Front Panel view of RF Assembly.
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  • Side view showing terminal strip and filament transformer.
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  • Close up view of Filament Transformer.


















    

  • Input cavity of RF Assembly.
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  • Detail showing input coils and tuning cap.
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  • Rear view.




















    

  • View of other side.
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  • Top view.
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  • Top view two.




















    

  • Top view showing RF circuits.
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  • Close up of HV RF Choke, bypass cap. and MHV connector.

















    

  • Workbench testing setup.
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  • Amplifier with new cover and fan.
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  • Another view.






















    

  • Finished control panel (open).
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  • Finished control panel (covered)





















    

  • Amplifier installed in cabinet.
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  • Amplifier showing operation at 600w.





























  Testing of 2m-8877:
    

  • Filament voltage has been applied for several hours over several days.
Filament in-rush circuit results in 2-second delay of full filament voltage (4.9vac).
    • 

  • 8877 temperature barely detectable with filaments powered indicating blower is working well
    • 

  • Repair of the Triode Board and re-calibration of metering/bias is done
    • 

  • Application of HV was done in stages up to 3.1 kV with no problems
    • 

  • Setting of bias is adjustable up to 150 mA anode standby current
    • 

  • Application of low power (5w) RF shows a high SWR preventing further testing
    • 

  • Input coils were modified: L1-6T, closed-spaced; L2-3T, wide-spaced
    • 

  • Testing the input impedance shows the grid circuit is resonant at 144 MHz with SWR=1.7
    • 

  • Use of a MFJ-921 antenna tuner matches the FT-847 so that it may drive the 8877

  • Reset bias for 8.29v, resting Ip< 10 mA
    • 

  • With 27w input, getting 600w output: Ep= 2400v, Ip= 410 mA, Ig= 16 mA,
984w dc input with 60.9% efficiency and 13.6-dB gain
    • 

  • With 30w drive, 650w output is possible, but with more plate current
    • 

  • I found that the 8877 will drive without the tuner by retuning the input, but takes 30w drive and Ip= 420 Ma
    • 

  • Reconditioning a surplus CAI HVPS will permit running the 8877 at full output


I discovered that using a galvanized steel screen for the cover of the amplifier enclosure
was not the best choice.  Apparently, a magnetic metal is subject to higher circulating RF 
currents.  First, I mounted the 1/4 wavelength copper lines too high such there was indadequate
clearance between the anode straps and the top cover (3/16-inch).  This resulted in a HV flashover
(arc) to occur resulting in a very small spot to appear on the screen directly above the anode 
straps (carrying high RF currents).

I deformed the straps to increase the air gap between the anode straps and the enclosure cover.
My solution resulted in RF heating in the area directly above the straps (see photo).  Two large
dark circles appeared in the mesh directly above the anode straps.

My solution was to use the original sheet aluminum cover and make a large vent by drilling a pattern
of 3/8-inch holes too form a vent.  To this I add a six-inch radial fan above the vent to aid air flow
out of the anode cavity.  The cover was raised by using 3/4 square channel as a spacer around 
the upper lip of the enclosure.  This resulted in roughly 1-inch separation of the anode straps to the 
cover.  This worked out well, with no HV arcing or heating evident.

I have now run about ten hours time on the amplifier (March 3, 2009), with it preforming very reliably.     




   Return to 2-meter EME page.