[AMPS] AL1500

[2]

>
>> //  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 AŘ 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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