Hi Ian,
> >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!
>
> Oops - my mistake, thanks for the correction, Tom.
>
> I extrapolated too far from the simple fact that the 4CX800 and 400 have
> the same 2W maximum grid dissipation as the 4CX250 which can be operated
> in class C. Actually the V/I characteristics are quite different - at
> the same instantaneous positive grid voltage, the 400 and 800 will draw
> a lot more grid current than the 250.
I see why you said that, looking at the 4CX250. I just didn't want
people to assume grid dissipation is given by multiplying
grid current times dc grid bias. Clearly, that is NOT the case.
There are enough misconceptions about grids floating about.
Everyone should remember the primary source of heat (other than
radiant and conducted heat from the cathode) is simply due to
kinetic energy from electrons hitting the grid, and the simple
fact electrons strike the grid dislodges gold.
The only element in the tube dominated by resistive heating is the
filament or heater. All other elements are heated primarily by the
electrons striking their surfaces, and the heat produced is given by
the voltage between that element and the electron source WHILE THE
ELEMENT IS BIASED INTO CONDUCTION.
All this stuff about skin effect heating (which would be I squared
R loss) is nonsense.
> Actually the notes in the rear of the 4CX1000 data sheet say that "peak
> grid currents of less than 5mA, as read on a 5mA meter" are allowed for
> "peak-signal monitoring purposes". Reading between the lines, this means
> that you can run up to 5mA of grid current on modulation peaks, in order
> to generate an ALC signal.
That's correct. The last thing in the world you want to do is
accelerate electrons and let them smash into the grid. The electrons
will dislodge gold atoms (long before the grid melts), and
contaminate the tube.
> Under those operating conditions, including a *small* amount of grid
> current, Eimac says that the 3rd/5th order IMD of the 4CX1500B is about
> -40/-47dB.
Also agreed. This stuff about grid current causing distortion is
overblown, the primary concern is grid damage. The distortion is
mostly caused by non-linearity on peaks, and allowing the grid to be
driven into greatly increased negative bias, while it does protect
the grid, will NOT reduce distortion. Quite the opposite is true,
the increased negative voltage will simply make IMD much worse.
You are exactly correct on your point. Using a grid supply
with low impedance will reduce distortion if the tube is over driven.
Rich is exactly incorrect.
73, Tom W8JI
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