Phil Clements wrote:
>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.
>I am sure some of the smaller glass tubes are closer to disaster at the
>4 Joule limit than an 8877, 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.
EIMAC Application Bulletin #17 applies the 4J limit to "oxide cathode
tube types up to about 1500 watts plate dissipation".
That puts the 8877 right on the borderline. No doubt it is much more
capable of handling 4J than say an 8874 or even a 3CX100.
AB#17 goes on to say that:
"Most tubes employing thoriated tungesten filaments are capable of
withstanding higher energy arcs than are the oxide cathode types.
However, some of the smaller types such as the 4XC1500A, 3-500Z and
5CX1500A as well as some larger high-mu triodes (such as the 3CX3000A7,
3CX10000A7 or 3CX20000A7) while employing thoriated tungesten emitters,
have a very fine wire grid and cathode structures and should be
protected in much the same way as the smaller oxide cathode tubes.
However, larger tubes should be capable of withstanding arcs containing
up to 50 joules total energy."
Reads like it was written by a lawyer, but you get the picture.
One interesting feature is that, in EIMAC's experience, arcs from the
plate DO cause damage to oxide cathodes. This implies that there IS -
somehow - a current path through the cathode as well as the grid... and
therefore through the cathode bias zener as well.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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