>How does one estimate the grid dissipation in valves like the
>3-500Z, 3CX800 etc. DC Volts x DC amps is simple, RMS volts
>and RMS current also easy. But what about RF losses?
I go by avg. I x avg. E pretty much =s avg. P, however the RF matter is
>Say I have 50W input to a 5-300Z on 6m, cathode biased to 5V
>3kV EHT at 400mA and 130mA grid current. What is the grid dissipation?
The cathode-grid potential is c. 105v-peak (Eimac constant current
characteristics). Pgrid = c 70v-avg. x 0.13a avg. = c. 9w due to the
DC current. To calculate the AC loss: The grid to anode C is 5pF. At
50MHz, 5pF has 636 ohms of Xc. . The peak AC anode voltage 3000v
-250v(Eimac constant current characteristics) =2750v-pk , so the anode
potential is 0.707 x 2750v= 1940V-rms. Thus, the AC grid current is
1940v/636ohms= 3.05A-rms. The next step requires a Z-analyzer or Q-meter
to measure the R of the grid at 50MHz. My wild guess is that this is
less than 0.5 ohm. Using P=I^2 x R, the additional dissipation due to the
AC current is 3A^2 x 0.5 ohm = 4.5w, for a grand total guesstimate of 9w
+ 4.5w = 13.5w, although the second guess is that it is probably a bit
>I know it is not 50W, but it has to be more than the 7W I calculated.
>Say there is 200mA of grid current during tune up. How long can that
>situation continue before damage occurs?
Quite probably a long time. . . I have never seen a 3-500Z that failed
from melted grid wires, however, I have seen many that failed from sudden
grid to filament shorts due to a bent filament helix which manifested
itself immediately after an arc and "big bang". Last week, I tested an
Eimac 3-500Z that was virtually brand new. It had recently been
installed in an out of the box, new TL-922. Within several minutes,
there was a "big bang", and reverse grid current plus anode current was
indicated on standby. . . When I tested the tube I discovered a
grid-filament short. . . It is my opinion that the tube shorted due to
an intermittent oscillation at c. 120MHz. A similarly-shorted 3-500Z
appears in the photograph on page 15 in the Sept. 1990 *QST*.
R. L. Measures, 805-386-3734, AG6K
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