> sooner or later I'll get it right. I've been watching the discuusions
> on
> "neutralizing GG amps". It's interesting but I've never seen anything
> in writing on the subject! Can anyone tell me where to get some info
> on it?? I'm always willing to learn something, if that's possible at
> this late date, HI, HI!! thanks. carl / kz5ca
Hi Carl,
Examples of GG tubes that often need neutralized include the
3CX1200A7 and D7, pairs of 572B's, and (4) 811A's. The reason
they need neutralized is the long grid leads and poor
cathode/anode shielding inside the tube allows a considerable
amount of feedback capacitance from anode to cathode.
Regenerative feedback shows up as a lack of having maximum
output at minimum plate current on higher HF bands, or as a
stability problem near the operating frequency under "light" or no
loading of the PA.
The earliest commercial amps to neutralize the tubes include the
Heath Warrior (4 811A's) and the Gonset 4 811A amplifier. In
those amps, they added a third winding to the filament choke and
used a conventional neutralizing-style capacitor between the
ground end of that filament transformer end of that winding and the
tuning cap end of the tank. The end of the winding nearest the
filaments was grounded.
The AL-1200, AL-811H, and AL-572 Ameritrons all use a separate
broadband transformer to invert phase and the first two have a
neutralizing probe while the 572 has a coaxial slide capacitor for
neutralizing.
This is actually a form of bridge-type neutralization, and you can
find it described in many books. It has many applications, including
grid driven systems.
This does not cure the "problems" wrongly attributed to
"parasitics", such as excessive tank voltage caused by incorrect
loading, exciter transients, or antenna system arcs or faults that
unload the tank and allow excessive voltages to develop. It does
cure instability near the operating frequency of the PA.
73, Tom W8JI
w8ji@contesting.com
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