[Amps] Ameritron AL-1200 problem on 160 meters

[Rick Stealey]

There are a lot of questions on the reflector similar to this one.  Usually the advice
given is to check the bandswitch, relay, padding caps and measure the swr on the 
input circuit to be sure you are getting drive into the tube.  But it's tough to 
do anything with the amp live.  No, let me rephrase that.  It's impossible
to do any work inside the amp when it is live.
There is a technique that is well known to experienced amp guys that you might
be able to make use of, and since I have had great success with it myself 
I'll explain how it works.  It is really very simple.  You do the required safety 
things first (unplug, short out the B+ etc), then disconnect the plate cap from
the final and clip any small resistor from there to ground.  The R must be equal 
to the plate impedance of the tube.  Use R=plate volts/(plate currentX1.8) close
enough.  Then attach your antenna analyzer across the loading capacitor.  
The analyzer is then looking back toward the tube,looking at the pi network
which is transforming the resistor value to 50 ohms. No antenna connected.
In the case of my homebrew amp, I was trying to get it down to 160 meters 
and not getting a nice dip, the plate tuning was fully meshed, the load wasn't 
doing anything and I didn't know where it was tuning, or the value of padding
caps I needed.  The analyzer showed me the pi net was only tuning down 
to 2.2 MHz.  I added 267 pf across the tuning cap and voila!  When I fired
the amp back up it tuned perfectly.
Now in your case if you use this technique you will see where its resonant,
or if it isn't you can change out components, clean the bandswitch, 
put your fingers all over the place - real convienent, takes all the guesswork
out of the process.    GL, 73,
Rick  K2XT

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