> From: Rich Measures <measures@vc.net>
To: <amps@contesting.com>
> Date: Wed, 3 Sep 97 08:01:45 +0000
I wrote:
> >The grid dissipation is FIVE watts. Rated grid dissipation is reached
> >at about 60 mA of grid current in the SB-230.
Rich replied:
> According to Eimac's constant current curves for the 8873/4/5, the
> cathode-to-grid driving voltage is roughly 40V-peak, or 28 V rms.
> 28V*0.06A = 1.68W. . . It sorta looks like that in the Rauchian World,
> P=3(I*E).
It is incorrect to multiply RMS cathode voltage times indicated grid
current to find power, since the waveform in the tube is NOT a sine
wave.
It might be a good idea, before you pull out your data sheets, to
read the instructions on using them. ALL through the guidelines grid
power is mentioned as PEAK grid voltage times indicated grid
current. Of course that's just an approximation, the exact
value needs to be determined in the actual PA through a fourier
analysis.
> >Anyone in their
right mind would know a 100 watt exciter could easily
> >cook a five watt grid if the PA is underloaded. ...
>
> SB-230s reportedly destroy the 8873 some time after being loaded
> correctly.
My car got a flat tire after being reportedly driven down a road
with no nails.
> >Anyone doubting that should ask Eimac,...
> ...snip...
>
> Sounds like a good idea, Mr. Rauch. In 1986, I asked Eimac. . .
> However, when you call Eimac these days, you won't be talking to tube
> engineer.
I'm sure the world revolves around an alleged phone conversation you
had way back in 1986, and there are now NO engineers
anywhere in the world qualified to refute your alleged phone
conversation.
Rich, for the most part your nichrome is harmless (other than the
reduction in HF Q near ten meters), but your nonsense about tube
grids IS VERY harmful to the people you claim to be "looking out
for".
When Eimac rates a grid at five watts, the grid absolutely needs
protection in a grounded grid PA. Grid dissipation varies wildly with
loading, and even a few seconds of mis-tuning can damage the grid.
Since you can't or won't understand harmonic waveforms consider
this. An exciter puts 50 watts into the cathode, the tube grid
normally dissipates two watts to reach full output power. The other
48 watts contributes to the output power (less the amount dissipated
in the PA anode).
If the anode is mistuned, the feedthrough power from cathode to
anode is greatly reduced. The 48 watts has to go "somewhere", so a
large portion of it goes to the grid.
It's very easy to dissipate HALF of the drive power in the grid if
the PA is underloaded.
Eimac determines the grid dissipation by measuring emission
from the control grid, since emission is proportional to grid
temperature. When the emission starts to "ramp-up", the grid is
becoming hot enough to boil electrons off.
IMHO, running the soft gold plating on the grid near the
temperature where it becomes a cathode is bad advice to give, even if
it helps you sell your "kits".
73, Tom W8JI
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