If this was already covered please excuse my redundancy. The input
capacitance of the GS31B is likely on the order of 22pf. Two of them 44pf.
These
values as well as any strays in the circuit have to be considered as part of
the
output C in your tuning network.
The nominal input impedance of the tube develops at full drive and is
related to peak cathode voltage divided by peak cathode current. Note that the
peak
current includes plate and grid current, both of which will be at least
three times what the d.c. meters are reading. If you plan to run the amplifier
in
loaf mode, the currents will about the same as a single tube and the input
impedance developed will also be about the same as a single tube. Your input
network may need to be tailored for more like 50 ohms to 50 ohms in this case.
You can cold measure your network tuning by placing a resistor across the
tube cathode to ground that approximates the impedance. Naturally the tubes
have
to be in place as they are providing part of the network capacitance.
Sounds like a fun project!
73,
Gerald K5GW
In a message dated 1/3/2006 12:13:08 P.M. Central Standard Time,
Geert_Hofman@Jabil.com writes:
I am working on a contest amplifier with 2 x GS 31B which should run
smoothly (cool) at 1.5 kW RTTY.
Prepared the tuned input circuit based on a Q of 2 for all contest bands
( so 50 to 25 Ohms impedance matching ) , and tested it out with my MFJ
analyser, but found out that I was way too high on nearly all bands
(except 80 meter where I was pretty close). Now I might have been
mistaking possibly by 1 winding on the cores, but certainly not more
than that. I am using hermetically sealed Siemens (HF) relays for the
switching of the bands. To give you an idea my tuning on 10 meters
turned out at 40 + Mhz with a resistive 25 Ohms loading!!. I had to
decrease my initial cap values both on input and output from about 120
pF to about 20 pF...
After some further reading I found an article which said tuning/setting
the filter with the low output analyser is not trustworthy. You should
do it with the tube(s) in place . I agree and will do so later , but I
am somehow surprised to see I am so far off on all bands ... and am now
doubting if I did something wrong after all. I cross checked my values
with what had been posted some months ago on the reflector here for 2x
GS35 ( KD7QAE : see below) and my calculations are in line with those.
Cin 0.01uF
Zin 50
Zout 25
160m 1600p 2.7u 1200p
80m 1000p 1.5u 1000p
40m 470p 0.72u 270p
20m 270p 0.42u 270p
17/15m 180p 0.27u 100p
10m 150p 0.19u 130p
1600p = 2x500p SM fixed + ARCO 4611
1200p = 500p SM fixed + ARCO 4611
1000p = 500p SM fixed + ARCO 4611
470p = ARCO 4611
270 & 180p = ARCO 465
150, 130 & 100p = ARCO 463
Any feedback on this mystery would be appreciated...
Geert Hofman
Amateur radio Station : ON4IG
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