Out of curiosity, how many 6EH5 tubes did you have to go through to find a good one?
Al, n7ioh Payson, Arizona, USA On Fri, Mar 9, 2012 at 11:24 AM, Randy WB4SPB <wb4...@earthlink.net> wrote: > 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 > > > ______________________________**_________________ > Drakelist mailing list > Drakelist@zerobeat.net > http://mailman.zerobeat.net/**mailman/listinfo/drakelist<http://mailman.zerobeat.net/mailman/listinfo/drakelist> >
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