Scott I built YU1AW's design a couple of years ago. Used 1/16 inch copper
tubing from O'Reilly's auto store. Think it was about 6 - 8 turns maybe 2
in. in dia. Worked like a champ no c nages with the output cap. Change
that coil using his dimensions and value for cap and you'll do fine. By the
way let that GI7B warm up a good couple of minutes b4 keying.
73 Mike K4ELV
----- Original Message -----
From: "KB0NLY" <kb0nly@mchsi.com>
To: <amps@contesting.com>
Sent: Wednesday, February 25, 2009 5:36 PM
Subject: [Amps] Output Matching Network, Need Advice
Gents,
As some here are aware, i am building a 6M amp using a GI-7B Triode. I
constructed the amp from a combination of info from YU1AW and a few that
have actually built his design.
Today i am trying to finish building and adjusting the output network. I
borrowed an MFJ analyer and put it on the output, calculated the value of
the resistor i should put from the anode to chassis using all the info and
references you guys provided and that should be a 2200 ohm, or 2.2k
resistor, tripped the t/r relay and started working on it. I could't get the
SWR meter to go below infinity, and the resistance would adjust over range
of 20-100 ohms with the load cap. So i removed some plates from the plate
cap, slowly only pair at a time until i started seeing a dip in SWR as i
tried it each time.
I got down to the last four plates, four on the stator four on the rotor,
and i wanted to stop and ask some questions before i take off any more as
its a destructive process with the air variable i have and i can't put them
back if i mess up! I found some references that said i should go up and
down in frequency from my target frequency and try adjusting it each time to
find out where its resonant. I found through trial and error that i could
get it totally flat, 1:1 and 50 ohms, at 40Mhz. So i need to go up in
frequency.
The questions..
Should i remove more plates from the tuning cap to bring up the resonant
frequency? I think i might still be too high in value, most designs online
show around 5-30pf for it and i probably don't have it that low yet.
Or should i make a change to my plate coil? I have that at 4 1/2 turns of
10awg wire, spaced less than the thickness of the wire apart at roughly 1
1/8" inside diameter. That was the tightest and closest i could get it to
the specs online. I tried changing the gap on the turns, more and less, and
it didn't seem to affect it any.
73,
Scott KBØNLY
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