> From: Rich Measures <measures@vc.net>
>
> If this be the case, then why doesn't Eimac specify a maximum DC current
> rating for the 8877's grid?
Rich,
You claim the loss is I^2 R loss. Resistance is easy to define, even
if it has a slope with frequency due to skin effect.
If the loss causing heat was I^2 R loss, Eimac COULD publish a
current rating.
The loss is due to the time integrated value of grid current time
grid to cathode voltage. Because of that, the WAVESHAPE of the
voltage and current affects the dissipation. Since the waveshape
varies with tuning, bias, and other EXTERNAL circuit
parameters...Eimac can not give a grid current rating.
The AC RF heating component of the grid is very small at HF, just
like it is in the anode, cathode, and other elements. Only the time
averaged heating of the grid by electrons striking the grid is
important in nearly all cases.
The velocity of the electrons is controlled by the voltage difference
between the target element (grid) and the emitter of electrons
(cathode). The of each electron is fixed, so the total amount of
electrons also is a factor. We measure the acceleration by voltage,
and the number of electrons by current.
Hundreds of years ago they decided to call the various
measurement units VOLTS, AMPERES, and WATTS.
One volt of accelerating voltage times one ampere of current is one
watt of power. If the accelerating potential across a resistor is
1000 volts, and the current flowing is one ampere, the power is
1000 watts. The same is true in a vacuum tube, except the heat is
confined to the target electrode...since the vacuum is "empty".
The approximation Phil gave you, Ig times Vpk, is ONLY an
approximation. It is the typical power in the grid of a class B or
C amplifier, and it is from Eimac data. It does NOT include AC
heating effects, and it is not precise. It is an only a rough
approximation, but close enough for practical systems.
It is derived from typical values of heating caused by the time
varying dissipation of a power amplifier grid, and has nothing to do
with displacement currents caused by plate/grid capacitance. It only
considers the main parameter that causes heat, electrons smashing
into the surface of the grid.
> - Has anyone on this discussion group looked through a low power
> microscope at the grid of a gold-sputter damaged 8877?
More important, have you ever looked through a Physics book in the
areas discussing heat?
73 Tom
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