[Amps] Tuned input for GG 50 mhz amp

[Jim Thomson]

On Behalf Of TL_IARC
Sent: Sunday, March 21, 2010 3:16 PM
To: Carl; amps at contesting.com
Subject: Re: [Amps] Tuned Input for G-G 50Mhz amp

Hi Carl,
Thanks.
I thought a "Tee" would be easy to tune outside the amp in real time
(one
cap), and would basically achieve the same result as the "Pi".
Someone elsewhere mentioned better harmonic treatment with the Pi, but I

can't relate to that as I have limited knowledge.

73
Isaac, 4Z4TL

##   We just went through this on a buddy's GS35B 6m amp. The original
one tube amp used a T input. and no amount of  tweaking the cap, or both coils,
would result in a flat swr.  It would get down to 1.5:1 , and that's about it.  
The amp had layout problems as well.   Like gerald sez... the GG tube already
has stray C  from cathode to grid, which will make any T  network into a  CLCL 
 network.   2nd amp  was a 2 x GS35B 6 m amp. 

##  PI net was used, and 2 x small trimmer caps used, like  5-120 pf types.  The C2 cap
of the PI net has to be close to the cathode. [ the C2 cap is what kills the harmonics]
 You don't want the tuned input to be external to the amp at all.  Both caps could be 
adjusted from the top of the chassis, for adjustment. The stator + rotor plates  for each
trimmer cap are below the chassis.   With the Pi net,input SWR is dead flat. 

The stray C  between cathode and grid is absorbed by the C2 cap.  This Pi is a LP filter. 
The amp has to be driven for both caps to be tweaked.   Once input swr is dead flat, you
don't touch it again. Then you also get max grid current.  Power output of the  2 x GS35B
amp on 50.125 mhz, with 200 watts of drive was + 66.72 dbm.  

Later... Jim   VE7RF