70 cm GI14b (GI39b) Power Amplifier
by
OK1VPZ

This PA has constructed between 1986 and 1990 years. With 600W PEP output it was at that time one of the most powerful 432MHz power amplifiers in OK and first of such construction with GI14b radar tube.

Soviet radar tube GI14b was produced since mid of 50-ties till mid of 70-ties and later replaced by GI39b tube, which has almost the same performance. Compare well known GS31b, resp, GS35b tube these radar tubes have lower plate dissipation, only about 500W and much average shorter life time. But for amateur radio use on the end of 80-ties we had nothing better.

In 2006, after 1,5 year of heavy use by OK1TEH the tube has changed by new one and PA performance was measured again.

Here it is:  

PA 432 MHz by OK1VPZ

Tube: GI14b (GI39b) Tube datasheet here.

Plate voltage: 2750V (idly)  HV transformer ca 1150W has 6 windings 300V AC each

Plate voltage [V] 2350 2175 2100 2300
Plate current [mA] 280 400 500 520 *)
Modulation FM FM FM SSB
Plate input [W] 658 870 1050 1196 *)
Grid voltage [V] 18.5 18.5 18.5 18.5
Heater voltage [V] 12,4 12,2 12.0 12,4
Heater current [A] 3,4 3,4 3,3 3,4
Drive power [W] 10 20 28 30 *)
Output power [W] 210 410 **) 550 640 *)
Plate dissipation [W] 440 460 500 556 *)
Efficiency [%] 32 47 52 53

*)  peak value  **) for WSJT max 400W output is recommended due to tube temperature limit

Gain aprox.:  13 dB

Q of unloaded plate circuit:  > 1000  (less than 400kHz width for -3dB)

Q of by antenna loaded plate circuit: aprox. 130  (cca 3,5 MHz width for -3dB)

Attenuation of reflection on the input: aprox. -18 dB

Reverse attenuation output to input:  > 23 dB

Note: for lower output to input transfer, and improved neutralization a teflon strip ca 35mm width was inserted between grid ring and tube

Construction: aluminium casting with 10mm wall. Grid ring from brass, silvered. Plate stripline copper, 4mm thick, used as well as heatsink for tube. Cathode ring made from bronze, stripline from brass plate. All dimensions below.

This tube can be pushed to higher output power, aprox. up to 800W+ RF, but due to lower dissipation of these radar tubes for SSB/CW operation only. If you want to get higher output, use higher plate voltage up to 3 kV and more powerful HV transformer. But in such operation shorter lifetime of tube shall be expected.

73! OK1VPZ

Construction drawings:


Pictures of this PA:

Broken GI 14b tube:

HV transformer:  

Original OK1VPZ web page.

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