[Amps] Grounded grid matching circuit

[Jim Thomson]

Date: Fri, 16 Apr 2010 18:56:01 -0500
From: Mike Lucas <W5CHR at Bellsouth.net>
Subject: [Amps] Grounded grid matching circuit


In the March 1978 HAM RADIO magazine, Ulrich Rohde wrote about an 
elliptic filter covering the amateur bands
from 1.8 to 30MHz, suitable for an input circuit to an amp needing a 50 
ohm to 100 ohm step-up. A corrected
schematic was published in the August 1978 SHORT CIRCUITS. I have run 
the numbers through Jim Tonne's
software, and the results (both the original circuit and the corrected 
one) don't look anywhere near flat enough to
work. VSWR is wild and off the charts! I'd  bet good money that Rohde  
was right and I blithered somewhere, but
I haven't found where yet. Has anyone in the audience seen the article, 
and what were your conclusions?
Anyone? Anyone?

Mike W5CHR
Memphis

##  It's on Page 48  of the  March 1979  Ham radio magazine.  The article is 
posted under the files section of the yahoo  ham_ amplifiers grp. 

## the circuit does not work.    If it did, we would all be using it ! 

## he also erroneously states that a PI network wont' suppress harmonics..
which is nonsense.  The PI  tuned input, inside a GG amp, will attenuate
harmonics.  In the ten tec manual for the Centurion  2 x 3-500z amp...
they refer to the individual pi nets.. as  ' LP filters'    which is actually
what they are... and is correct. 

## the idea behind the Pi-net is to kill the harmonics of the amp itself,
not the exciter driving it.  The Pi net will provide the required flywheel
effect with loaded Q of aprx 3.  It also does any Z matching. It works best,
then the C2 cap [facing the cathode].. is closest to the cathode.  If the tube's
input Z is aprx 50 ohms... 50 ohm coax can be run between output of tuned input
and the cathode, with no ill effects.... it's like a line extension.   If the tube is 
25/35 ohms... then parallel 50/70  ohm coax can be used. 

later... Jim   VE7RF