[AMRadio] Globe Champion

[Donald Chester]

>From: Brett Gazdzinski <brett.gazdzinski at wcom.com>

>On my home brew rig, there is no shielding between the neutralizing
>caps, the grid coil, the plates of the tubes, etc.
>In all the push pull rigs I have looked at, the layout was similar,
>no shielding between the grid coil and the output stages,
>but there was shielding between the power stage and the driver.
>Your typical push pull deck in the Bill Orr books has
>the tubes, coils, caps, grid coil, neutralizing caps, all
>on top of the deck with no shielding at all, and no problems.

Both my homebrew rig is like that too.  One precaution is to place the axis 
of the grid coil perpendicular to that of the plate tank coil.  I notice on 
some of the old Globe rigs they did not do that, placing the coils so that 
the axes would be parallel, but there was shielding between the plate tank 
and grid tank.  If I recall correctly, the old GC175 used the HV oil-filled 
filter caps as shielding.

I did an experiment once, and determined that shielding alone does little to 
isolate the coupling between two tuned circuits adjusted to the same 
frequency.  I suspect that the shielding is effective for reducing 
electrostatic coupling but does little to procect against magnetic coupling. 
  The only cure for the latter is proper placement of the positions of the 
coils, and separating them by distanc.  When I built my rig years ago, I 
excited each coil with rf from a low power transmitter, and experimentally 
"played chess" with the coils until I found the mounting positions with the 
least coupling.  In every case I was able to find a specific position with a 
strong null in the coupling.

Don K4KYV

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