[AMPS] Re: Neutralization 572Bs

[Tom Rauch]

> Date:          Sat, 23 May 1998 14:20:09 +1200
> From:          Martin Ellis <jmellis@ihug.co.nz>
GM Martin,

> In the early 1980s I had a bad case of instability on 10M
> in a FL2100B which uses a pair of 572Bs.  My elmer was
> Ray ZL1GG (one grounded-grid), who gave me a copy of the FL2100z
> circuit. The FL2100z uses a 30pf variable neutralizing capacitor
> to feed some RF from the 50 ohm output of the tank
> (taken at the change-over relay) to the input at the cathode.
> Ray explained that there is a 180 degree phase difference
> between these points.
 
Phase shift is only 180 degrees when the pi- network has a very 
high Q. If the Q is the minimum to match the anode to the load, 
phase shift is 90 degrees. Normal phase shift is about the 120-140 
degree range (as I recall), for a Q in the ten or fifteen range.

The phase will also vary with control positions.

That's why a tab near the anode, and a broadband transformer is 
"better". Of course, any cancellation of feedthrough capacitance is 
better than none.

 (On ten meters it might indeed be about 180 deg with the pi, since 
the loaded Q is probably pretty high on that band.)


73, Tom W8JI
w8ji.tom@MCIONE.com

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