>
>> // Some Alphas apparently C-E-short their interrupt transistor.
>
>If I am not mistaken, the original Alpha 77 (model PA-77) uses such a
>grid-protection circuit. However, I don't recall how well the
>switching transistor is buffered and isolated from the grid. If I get a
>chance tonight, I'll look at the schematic.
>
>On the other hand, my Alpha PA-70V (with a single 3CV1500A7) uses a
>factory-designed 1/4-amp/3AG grid protection fuse. The fuse
>seems to work well and blows quite nicely when I accidentally apply full
>exciter power at 100-watts. What I don't know is the
>long-term detrimental effect on the tube caused by these brief and
>occasional periods of excessive grid current. Of course, the
>fuse blows well before the grid ammeter has an opportunity to display the
>excessive condition (due to the meter ballistics). But,
>how fast is fast enough?
>
// Fast enough is no observable damage to the electron tube with a
high-pot tester,
Over-current fuses open to limit current, but over-current transistors
often short, which does not limit current. The 8877 grid is capable of
dissipating 25w in AØ mode. Since typical grid-cathode potential is c.
57Vrms, 25w works out to c. 440 grid mA. I have never seen a kaput 8877
grid that had uniform gold evaporation - such as would be the case with
too much DC grid current. Mr, Murphy said that everything is more
complex than it first looks.
cheers, Paul
- R. L. Measures, 805.386.3734,AG6K, www.vcnet.com/measures.
end
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