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[AMPS] 3-500Z amplifiers and grid current

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Subject: [AMPS] 3-500Z amplifiers and grid current
From: (Rich Measures)
Date: Mon, 16 Mar 98 07:29:35 -0800
>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 
more complex.  

>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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