QRO for 23cm
PAs using Russian GS-34 & GS-31/35 tubes, designed and built by
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Water Cooled GS-23b
Air cooled GS-34 & GS-31/35 PAs
23cm GS-23b PA
Water cooled GS-34 PA
23cm GS-23b PA
Construction details
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RW1AW biographical and station data

The amplifiers described here use Russian "GS" triode tubes. Most HAMs might think the Thompson TH-308-338 triodes would be better suited for 23cm operation. This is not necessarily true. Both the relatively inexpensive "GS" and the Thompson tubes have maximum requency ratings to 1000MHz. Both work acceptably well at 1296MHz, the Thompson with efficiencies up to about 40%. The GS-34 PA shown here achieves efficiencies approaching 50%, and the GS-31/35 operates at 36/38%, respectively. Maximum dissipation rating for the Thompson triodes is from the TH-338 at 1200W. The GS-35 when water cooled is rated at 2000W, nearly twice as much. The output capacitance of the "GS" triodes is as good or better than the Thompson line: 4 - 4.3pF.

All is not perfect with the "GS" triodes, since the anode cylinder is much longer than with the TH-line, the grid rings on the "GS" tubes vary in diameter, and it is dificult to tame internal capacitance changes due to thermal effects. Even with these negatives, however, careful design and construction of the cavities used for the "GS" series, and employing low duty cycle modes such as SSB, can work together to bring their 23cm operation to par with the Thompson line!

Links to tube specifications:   GS-31B   GS-34   GS-35B  

Click on pictures for zoom

Large PA is GS-31/35 which requires >300 m3 / hour - blower is rated at 450 m3 / hour. Small PA is GS-34 "driver" - uses only 150 m3 / hour


Both PA cavities are 100% homebrew by RW1AW in his shack. No surplus military or commercial products were used!

Small PA (with GS-34) is the 23cm driver amplifier. Ua is lowered from 2500V to 1500V, decreasing power out from > 350W to 90-120W.


The large PA is driven at 95-100W with a GS-31 installed, and 120W with a GS-35 installed.

With 15W drive, this GS-34 puts 260-280W out. Max. (drift-free) output from this PA is approx. 400W with 25-28W drive, Ua = 2500V.


An air-cooled GS-31 in this PA does about 900W output. An air-cooled GS-35 does about 1200W out and, with water cooling, about 1500W.

RW1AW shack after final PA tests were completed.


GS-31b and GS-34 Cavity PAs...and raw materials for future experiments!

A water cooled version of the GS-34 PA design is shown HERE.


At left is the GS-34 driver PA. Experience shows that 23cm amplifiers using this tube are capable of approximately 200W output in hard use without drift caused by thermal problems. It will do 400W out if used for very low duty cycle modes such as SSB, but "key-down" for more tha a couple of seconds can send it into thermal "runaway". 150 cubic meters per hour (88 CFM) forced air provides sufficient cooling.

Below is the detail drawing of the plate bypass capacitor as used on the "new" GS-31/35 PA design for 23cm. At the bottom of the page is a drawing of the "old" and "new" GS-31/35 cavity PAs.

Please note the differences between the "old" PA at the left, and the "new" PA on the right; the plate bypass capacitor of new design, and it is now placed directly against the top of the GS-31/35 ceramic structure. The lengths of both the anode cylinder and the tube used for the cathode cavity are longer. It is this type of adjustment and care which make it possible for the "GS" series triodes to operate at a level of performance very comparable to that of the Thompson TH-line on the 23cm band!

Note:  It is not particularly good to allow the plate current of the GS-31b or GS-35b to exceed 1A. Anode voltage should be kept in the range of 2.6 to 3.8KV, with care taken to limit plate current as noted!

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