[AMRadio] Globe Champion

[Brett Gazdzinski]

Don,
My grid coil is parallel with the output coil, but
on the opposite side of the rf deck, far away as I could get it.
Most of the Bill Orr rigs are similar, mostly to
maintain the balance of the push pull section.
I have exactly the same length between connections,
so its a totally balanced setup.
I don't know how you would maintain perfect balance
if you turn the grid coil, you would have to mount
it between the tubes, too close to the output coil
and the tube plates.

In laying out the RF deck, I was careful to mount the
output coil, the tubes, the neutralizing caps, and the grid
coil so that every wire/strap is of equal length, and the
parts are an equal distance apart.
I think this is more important than shielding in a push
pull triode output section.

Since the grid coil is a plug in coil, it had to be above deck,
but the tuning cap can be below deck, but still needs
to be centered so everything is balanced.

I think its all about balance here, and since you neutralize
the stage, the amount of rf picked up in the grid coil
is directly offset by the feedback.


Now its likely very different with the driver/multiplier
sections as they don't need much input on the grid to
cause trouble.

Brett
N2DTS


> >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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