[AMPS] 8877 Grid Dissipation

Tue, 12 Aug 1997 14:32:56 +0000

> 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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