> > The best cure for that problem is to neutralize the amp. If you don't
> > want to do that, make sure you have a LONG input cable and always run
> > the loading control up towards the open position.
> >
>
> How would go about designing and testing a neutralization circuit. This
> seems like an interesting project...
Actually it is quite simple overall. The only difficult part is the
feedback capacitor.
You need a feedback capacitor that equals the feedthrough
capacitance of the tube. That can be a metal plate near the anode
of a tube(s), or a separate VERY high voltage adjustable cap
connected to the tuning capacitor.
It ties through a 1:1 ratio phase inverting transformer back to the
filament through two coupling capacitors that split it to each side of
the filament.
A 61 material core works well for the core of the transformer, and
you can use teflon #16-20 ga wire for the winding.
Adjust the cap, with power off and relay closed, for minimum
feedthrough power on 15 meters. That will compromise the
feedback for the higher three bands, where it is most important.
You should be able to virtually null out all RF feeding through the
amp when the amp is "cold" (and the relay activated).
73, Tom W8JI
w8ji@contesting.com
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