Well, Randy, I'm thinking I'll add the output tube to my list. I've been
thinking my 12AX7 (which tested good) is at fault, but I never considered a
bad audio output.
As you said - the tubes for either are cheap enough. Add to that the fact
that I fixed a ton of noise on an R-4C recently by simply substituting 6
EJ7's...
Steve Wedge, W1ES/4
"I can't complain, but sometimes I still do."
- Joe Walsh
If the above message appears, it came from Steve's Son of Laptop!
----- Original Message -----
From: "Randy WB4SPB" <[email protected]>
To: <[email protected]>
Sent: Friday, March 09, 2012 1:24 PM
Subject: Re: [Drakelist] R4B Audio Distortion
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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