> From: Rich Measures <measures@vc.net>
> >The grid dissipation limit of a 3CX800 is reached at about 130 mA per
> >pair of tubes. .....
> >
> >73, Tom W8JI
>
> Hmmm. The grid dissipating rating is 5w per grid, or 10w for a pair. If
> E = P/I, then E = 10w/0.13a = 77v. According to Eimac's characteristic
> curves, the driving potential is roughly 30v. Apparently, Georg Simon
> Ohm's formulas are not 'EXACTLY correct'.
>
> Rich---.
The waveform is not a sine wave, so RMS voltage does not apply. You
actually would have to do a Fourier analysis to determine the
effective voltage G-K voltage.
Eimac, in their literature, gives a grid dissipation approximation as
Egk peak times Ig average. But that isn't always exact.
Here's a typical operating condition for a pair of 3CX800's at 2400
volts dc (this assumes a perfect match to the published curves) for
a plate load resistance (presented by the tank) of 1500 ohms.
Minimum plate voltage 300 volts
Peak plate current 2.9 amps
Average plate current .95 amperes
Average grid current .04 amperes
Plate dissipation 800 watts
Grid dissipation 1.8 watts
Changing the load to 3000 ohms we get:
Minimum plate voltage 100 volts
Peak plate current 1.7 amperes
Average anode current .5 amperes
Average grid current .16 amperes
Plate dissipation 190 watts
Grid dissipation 8.5 watts
Other operating conditions can produce 8 watts of grid dissipation
(the maximum for two tubes) at even less grid current.
That's why it's best to limit Ig to 100 mA maximum in a typical
3CX800 (x2) PA, unless you KNOW the dissipation.
UNDERLOADING a PA is destructive to both the grid and the tank
components. This example represents a load increase of ONLY 50 ohms
if the PA was tuned into a 50 ohm load initially (a change from 1:1
to 2:1 SWR, without tank retuning).
Now imagine what happens in the TL-922 and other PA's when the relay
doesn't switch, and the SWR is near infinity! ALL cathode driven
PA's using low grid dissipation tubes absolutely should have an
electronic overload system. A fuse will not do, and all the "magic
nichrome" in the world won't cure a load fault.
Rich recommends REMOVING fast electronic overload circuits as a
"helpful suggestion". Think about that advice!
73, Tom W8JI
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