[Amps] Tuned input for 2 x GS 31 B

TexasRF@aol.com TexasRF at aol.com
Wed Jan 4 10:32:59 EST 2006


 
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 at 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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