>
>Was testing the output of my AL80 (not A or B, or even C!) when the classic
>phfZzt sound echoed from the cabinet. When keyed, the grid current shows
>negative and little or no RF output power.
> Plate voltage indicates a slight
>increase when keyed, as well.
>
>AMPS 101: where does the novice start (after shorting the B+ to ground) to
>isolate the problem. I have already swapped out tubes. Results are
>identical with either set.
>
>What is the negative grid current telling me?
>
? Negative grid current indicates that the fil. is shorted to the grid.
(photo on page 15 of 9-90 *QST*). During an intermittent parasitic osc.,
considerable lateral force is apparently placed on the hot filament. //
The increased plate current is caused by 0V bias [the fil./grid short].
? cheers, John
- Rich..., 805.386.3734, www.vcnet.com/measures.
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