>At 08:36 AM 12/12/97 -0800, you wrote:
>
>>There are no smallish wires in the the grid of an 8877. There are 100+
>>rectangular bars. At Eimac's maximum ratings of 4000v and 250MHz, the
>>grid of an 8877 carries over 40 RMS amperes. // There are no small
>>wires in the 8877 cathode element.
>
>
>Great to hear that, Rich! Most of my amps are 8877's so I have a broad
>cushion with the 4 Joule limited anode supply. It was not the intent
>of the Eimac paper to cover each tube type individually. The 4 Joule
>limit was devised as a blanket value to cover ALL their small TX types.
This is starting to sound like guess work, Phil, not anything based on
scientific measurements. - - We have a tube that has a 25
watt-continuous rated grid which is designed to carry 40A of grid current
at 250MHz, yet the safe limit during an anode/grid arc is 4 watt-seconds.
However, I have never seen gas vanish in an 8877, so if one has an
anode/grid arc, one need not worry about damaging the "delicate grid
wires" - because the tube is KAPUT. However folks report hearing a
'big bang' from an alleged arc inside a near vacuum, and lots of current
seems to be flowing in the cathode because the cathode bias supply diode
often shorts, which hardly seems likely because the grid is directly
grounded. .
>I am sure some of the smaller glass tubes are closer to disaster at the
>4 Joule limit than an 8877, ... ... ...
Maybe not. Mr. Rauch and I have run a 3-500Z grid so hot it glowed, yet
no damage was done.
>but the blanket recommendation covers all
>of their smaller tubes in one whack. As I mentioned earlier, the 3CX3000A7
>and larger tubes are allowed a limit of 50 Joules. They seem to make
>the 4 Joule/50 Joule dividing line at 2000-3000 watt dissipation tubes.
>This gives us only 2 numbers to remember when designing the glitch
>resistor for a power supply.
>
How would you calculate the size of an 8877 glitch resistor using a 35uF
filter, operating at 4000v?
Rich...
R. L. Measures, 805-386-3734, AG6K
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