> From: Rich Measures <measures@vc.net>
To: <amps@contesting.com>
> Date: Fri, 8 Aug 97 07:24:35 +0000
> - There appears to be a lot of technoblather floating around about the
> extreme delicacy of the 8877's grid. However, at the maximum ratings of
> 250MHz, 1A, and 4000V, over 40 rms amperes of RF current flows through
> the grid to chassis ground/common.
Rich,
You really should not give false advice like this to people trying to
learn. Twisted statements like you made above give people the
totally wrong idea about grid dissipation ratings.
Grid heating is almost entirely due to the kinetic energy of
electrons hitting the grid, and is a direct function of the mass of
the electrons and their velocity. The velocity is determined by the
voltage that accelerates them, they all have the same mass, and the
current indicates how many strike the grid. If the accelerating
voltage is 100 volts, and the current is one ampere, the heating
caused by kinetic energy is 100 watts.
That is why the electrons leaving the cathode heat the anode and NOT
the cathode (the electrons actually cool the cathode, but an
insignificant amount, as they leave). That is why a tube's anode
dissipation is calculated by anode current times anode to cathode
voltage.
Stating grid heating is due to I^2 R losses may be a way to bolster
sales of nichrome, but it is technically false. It might be a good
idea to not mislead people in a way that could ruin their 600 dollar
tubes.
73 Tom
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