Everyone seems to agree the idea of negative feedback is a bust.
That is was not technically sound in an amplifier with grid current,
or a triode. One valid reason for floating the grids is equalizing grid
currents in multiple parallel tubes with directly heated cathodes.
Other than that, anything else can be accomplished some other
way.
One problem overlooked with fusing or floating the grids in GG
amps is that during a fault the grid can rise to supply voltage. If the
grid rises to supply voltage or near supply voltage (that is the
ONLY way the grid resistor or choke would limit fault current) then
there is no protection back to the cathode to prevent the cathode
from rising to supply voltage.
The grid serves the very important function of isolating the anode
voltage from the cathode during a fault. Without it securely
grounded, you should always use some other protection to prevent
the exciter from taking a "hit" during a fault.
The best idea is to ground the grid, and install a glitch resistor in
the anode supply if necessary.
73, Tom W8JI
w8ji@contesting.com
--
FAQ on WWW: http://www.contesting.com/FAQ/amps
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
|