PAs using Russian GS-34 & GS-31/35 tubes, designed and built by
RW1AW
Click on callsign for e-mail
|
Air cooled GS-34 & GS-31/35 PAs |
|
|
|
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!
|
|
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!
|
go to the top