[AMRadio] Pi-Net vs Link Couple - WD5JKO


Jim candela jcandela at prodigy.net
Thu Sep 22 21:48:11 EDT 2005


 Hi all,

   I have a different twist on this wonderful topic. Years ago I was going
over the very issue, and I had a Viking I transmitter, and I wanted to run
the legal limit of about 375 watts carrier 100% modulated am. So that would
be 6 db to go from say 100 watts to 400 watts. I went through a lot of
charts, curves, and calculator work, and I worked out a design. Of course I
lost my notes, so this description is from memory.

   What I came up with was a parallel grounded grid linear using low mu
304TL's with a tuned filament choke input, and pi network output. Using
grounded grid should reduce, or eliminate the need to neutralize, well at
least from 160-40 meters that is. With no neutralizing circuitry, things get
a lot simpler. A 3 band pi network on the output is doable, as is resonating
the high current filament choke on multiple bands.

   So I was guessing a little on the gain, but 6 db was the target, and a
low mu 304 TL is going to have low gain in G-G mode. Since class B RF linear
amplifiers on AM get about 33% efficiency (no modulation), I figured 900
watts DC input would provide 300 watts RF output. A rough guess is the drive
would be 100 watts from the Viking I, 25 of which would be lost to drive the
304TL's, and the remaining 75 would feed through to the output (normal
characteristic for a G-G amplifier). So the output would be 300 + 75 = 375
watts.

So in summary:

exciter output 100 w

304TL drive power 25 w

final dc input 900 w

final amp % eff (un-mod) 33%

feed through power from exciter 75 w

rf output = (900 * .33) + 75 = 375 watts

304 TL Pd each tube = 300 watts

  So fellows, here is a big rig running AM legal power with big triodes,
pi-net output, NO neutralizing circuit, with NO modulator, band switched,
and driven by a Viking I or other 100 watt class AM rig. I never built the
beast.

Do you folks think this would work?

Regards,
Jim Candela
WD5JKO
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