[Amps] Output Matching Network, Need Advice

[K4elv@bellsouth,net]

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 at mchsi.com>
To: <amps at 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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