Update: resolved, and another gold star for Garey
In early February, I asked for ideas after finding my R4B audio output stage
running hot. Positive bias, climbing slowly to +3, would mysteriously
appear on the grid after a few minutes of operation, and the audio became
distorted. Cooling the tube with forced air mitigated the problem, reducing
but not eliminating the weird grid bias. I could find no defective
components (leaking caps, thermally-misbehaving cathode resistor, etc.) and
had tried three different tubes that behaved the same way.
Richard WB6KBL suggested I look for some way that screen voltage might be
leaking to the adjacent grid pin on the socket. I could find nothing by
inspection, and cleaning made no difference. I began a half-hearted search
through my junk box for a replacement socket, but didn't end up going that
way.
Garey K4OAH thought the symptoms clearly suggested a gassy tube. I was
skeptical, having tried several different tubes, both 6EH5 and 6CA5, all of
which tested OK (and not gassy) on a borrowed Hickok 600A tester. Garey
pointed out that 6CA5 is not an exact replacement and that in his experience
it has sometimes been necessary to go through several 6EH5s to find a good
one.
So I decided it was cheap enough, at $3 each from most sources, to obtain a
couple more tubes and see what happened. I've finally done that, and, lo,
it appears Garey was correct. One (and only one) of my new tubes behaves
properly.
Of course (sigh) the R4B has now gone of the rails for some entirely new
reason, but I'll open it up soon and see what's what.
73, and thanks for the help.
Randy WB4SPB
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