Jim wrote...

>I have an old tube RF amp in which the neutralization
>can be adjusted with the power off - by measurement
>of the S12 or reverse direction gain A/K/A isolation
>figure from output to input - adjustment calls
>for adjusting the tuning for a *minimum* or a null
>in the signal from output to input using a signal
>generator and spectrum analyzer/receiver.

My very first paying job, back when I was in high school, was on
a production line doing just that.  We were making these little
2-meter Nuvistor preamps for somebody, and my job was sitting
there tweaking the trimmers to get that null.  Over and over, all
day long...

DD

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