>At 08:23 AM 2/16/98 -0800, you wrote:
>>>At 01:16 PM 2/16/98, you wrote:
>
>>€ Mr. Subich: On the 8873/4/5 and 3cx800A7 Eimac spec. sheet, no maximum
>>grid current rating appears to be given.
>
>So what advice would YOU give the poor guy about grid current limits before
>he fries his 8875s?
>
ok, Pete
- The cathode/grid driving potential is roughly 27v-rms. To stay within the 5w rating, continuous grid current probably should not be be allowed to exceed 0.18A. In 'Typical Operation', single-tone, Eimac indicates Igrid= 0.098A. From my experiences with the tube, Igrid was slightly under this value when the amplifier was tuned for max. RF output.
>
Based on damage I see in autopsied tubes, is my opinion that 8873/4/5s are most often damaged by:
1. gold sputtering off of the grid. {loose gold melt-balls tend to cause internal arcs and cathode poisoning}.
2. overdrive {strips the cathode coating, causing g-k shorts}.
In 1986, an Eimac rep., the 8877 design team concluded that gold sputtering is due to an 'oscillation condition' .
- Overdrive is anything over 350mA of keydown single-tone anode/plate current, or over 500 mA in SSB voice service. It is that my opinion that the MLA2500 does not exhibit good VHF stability. The likely reasons: to cut costs, no Eimac grid-grounding clips were used; the lead from the anodes to the tune cap is so long that it creates a Pandora's Box of VHF resonances; there is virtually no VHF loading on the cathode; the VHF suppressors could be improved to reduce VHF gain.
- Photographs of the gold-sputtering and cathode stripping maladies are in the Sept. 1990 *QST* article "Parasitics Revisited".
cheers
Rich...
R. L. Measures, 805-386-3734, AG6K
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