[Amps] Tuned input for 2 x GS 31 B

[Geert Hofman]

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