ORIGINAL MESSAGE:
On Tue, 21 Jul 2009 22:18:31 -0500, "Gary Schafer" <garyschafer@comcast.net>
wrote:
>
>It is well known that a grounded grid amplifier places a varying load on its
>driver stage.
REPLY:
That is true only if the tube is driven into the positive grid region. If the
drive is kept below that point, the load is constant. In fact, the load is
infinite - no drive power is required except to make up for losses in the input
circuit. .
Nearly all triodes have to be driven positive for full output, but that is a
function of the tube design, not whether it is operated in grounded grid or not.
Most modern tetrodes such as the 4CX1000A and similar do NOT have to have the
grids driven plosive for full output, regardless of whether they are operated GG
or GC. Spend a few minutes with the characteristic curve charts for both triodes
and tetrodes and you will see why. Especially note the plate current with zero
control grid volts. Tetrodes of the same approximate power level will have much
higher plate currents and that is why they do not have to have the control grid
driven positive. In fact, that characteristic is why tetrodes were invented in
the first place - higher gain.
Take that same triode and operate it grounded cathode and the grid will still
have to be driven positive for full output, and thus will present a varying
load.
73, Bill W6WRT
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