Amps
[Top] [All Lists]

## [AMPS] 3-500Z amplifiers and grid current

 To: [AMPS] 3-500Z amplifiers and grid current measures@vc.net (Rich Measures) 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 less. >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*. cheers Rich... R. L. Measures, 805-386-3734, AG6K -- FAQ on WWW: http://www.contesting.com/ampfaq.html Submissions: amps@contesting.com Administrative requests: amps-REQUEST@contesting.com Problems: owner-amps@contesting.com Search: http://www.contesting.com/km9p/search.htm ```
 Current Thread [AMPS] 3-500Z amplifiers and grid current, Rich Measures <= [AMPS] 3-500Z amplifiers and grid current, km1h @ juno.com [AMPS] 3-500Z amplifiers and grid current, Rich Measures [AMPS] 3-500Z amplifiers and grid current, km1h @ juno.com [AMPS] 3-500Z amplifiers and grid current, Rich Measures [AMPS] 3-500Z amplifiers and grid current, km1h @ juno.com [AMPS] 3-500Z amplifiers and grid current, Rich Measures [AMPS] 3-500Z amplifiers and grid current, km1h @ juno.com Fwd: [AMPS] 3-500Z amplifiers and grid current, KA2YKC [AMPS] 3-500Z amplifiers and grid current, Rich Measures Fwd: [AMPS] 3-500Z amplifiers and grid current, KA2YKC