Hi Ian,
> It depends what you call "delicate". The 4CX800 has a rated maximum grid
> dissipation of 2W, which means that at the normal bias voltage of -60V
> you could run as much as 30mA grid current without damage.
Grid dissipation is not bias voltage times grid current. It is
the value of POSITIVE grid voltage (with respect to the
cathode) times grid current integrated over the conducting portion of
the grid drive cycle.
I have no idea what the exact value is, but from a rough look at the
4CX800A data it looks like 30 mA indicated on a grid meter would cook
the grid rather quickly. It would take considerably more than 60
volts positive voltage to drive the grid to 30mA current. So if
the time integrated (by the meter's response) grid current is 30 mA
we can be certain the time integrated positive grid voltage is
VERY much higher than 60 volts!
> It's also notable that "zero" is only seen in data sheets that are 30
> years old now. It was probably meant to educate people about those new
> tubes that were not intended to run grid current, but it was over-
> dramatic and created a "disaster" mentality among users. With later
> tetrodes intended to be operated in class AB1, Eimac and Svetlana have
> started to give more realistic numbers.
The grid can indeed be ruined by grid currents of more than a few mA
with tubes like the 4CX1000A Ian. This is a sad fact many owners of
30S1's discovered.
The problem relates to deflecting the electron stream from the
cathode into the area where the grid is "smacked" by electrons. Tubes
like the 4CX1000 have the grid's gold quickly damaged long before
heating becomes severe. Remember the kinetic energy of the electrons
will work away at the plating LONG before the temperature of the grid
becomes elevated. The 4CX1000 and other small MOX cathode tetrodes
have particularly poor tolerance for gold migration. Much worse than
triodes.
73, Tom W8JI
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