I was told to also disconnect the plate choke and the ouput safety choke
when doing this measurement.
Steve KK7UV
----- Original Message -----
From: "KB0NLY" <kb0nly@mchsi.com>
To: <amps@contesting.com>
Sent: Saturday, February 21, 2009 12:44 AM
Subject: [Amps] MFJ Analyzer for adjusting matching network...
> After all the discussion on the subject of selecting the proper value of
> the plate and loading cap i came across a lot of articles in my research
> on using an MFJ analyzer to do this. I have access to one so i think i'm
> going to give that a shot. I found some pretty good explanations how to
> do this.
>
> ""Adjusting the output pi-network:
>
> ABSOLUTELY REMOVE ALL TUBE VOLTAGES. Leave the tube in circuit. The plate
> load impedance of your amplifier is expressed approximately as: [(plate
> voltage in volts) divided by (1.8 times the plate current in amps)].""
>
> For me that would be 2400 plate volts and 600 ma of plate current max,
> this works out to 2222 ohms. So a 2.2k resistor should be close enough.
>
> ""Make up a resistance value close to this number with low inductance
> resistors, and temporarily place this resistor string from the tube anode
> connection to ground. This simulates the plate load impedance of the
> amplifier. The purpose of the pi-network in the amplifier is to change
> this relatively high plate load impedance value to 50 ohms for your
> transmission line. Now, hook up a SWR analyzer to the amplifier's RF
> OUTPUT connector. Adjust the SWR analyzer for a frequency of 50.1 MHz
> Adjust the copper coil windings (slightly expand or squeeze together) in
> conjunction with adjusting the output loading capacitor. Adjust for a 1 to
> 1 SWR reading on the SWR analyzer. When the SWR reading is flat, your
> adjustment is finished. The top RF deck shielding cover on this amplifier
> had a small effect on this setting, so a tiny adjustment was necessary
> when the amplifier was running at full output. It's amazing how close this
> procedure gets your amp to the final settings. If you don't h
> ave a SWR analyzer, consider getting one. Remember to remove the temporary
> resistor you installed during this procedure. ""
>
> Sounds easy enough, or am i missing something here?
>
> 73,
>
> Scott
>
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