--- Stan or Patricia Griffiths <w7ni@...> wrote:
Steve B. wrote:
Thanx Phil, here goes;
I am trying to understand the failure mode of an
8233/E55L in a 3A6.
Both cathode tabs were melted and the 47^ screen
resistor is smoked.
With great trepidation, I replaced the defective
tube after checking
the asssociated 6DJ8s, xistors, and diodes. The
replacement held and
the screen resistor stayed cool. I suspected
possible filament-
cathode leakage/short in the otherwise fresh
looking dead tube. No
way to check for that now. I definitely don't want
to risk a
presently good 8233/E55L. Any ideas on the
probable failure mode?
BTW, there is now a DC imbalance on both traces
that the controls
won't comp. Related?
The P/I came in a group w/a 564. It didn't fail
here. I have
entertained the possibility that fil voltage was
mistakenly applied
to the cathode pins in a tester, but this doesn't
explain the smoking
screen resistor observed with the defective tube
in place in the P/I.
Thanx in advance; Steve
Stan responds;
The reason I did not respond to this question (I
remember it now) is that I
have never seen or heard of this failure before. I
didn't think I had
anything useful to contribute, but I guess just
letting you know I have never
seen it in many years of observing would tell you
something . . . My guess
would be a random internal failure of the 8233 and I
would do what you did .
. . replace it and cross your fingers . . .
By the way, I believe the 8233 is subject to the
dreaded "cathode interface"
and I would keep 8233's with that symptom around
just for running such
tests. If you waste an 8233 with cathode interface,
you haven't wasted much,
IMO.
Thanx Stan for response;
My only present source for 8233/E55Ls is junker 3A1/3A6 P-Is (no
extra charge for cath interface laden & defective tubes), is there
another reasonable priced source? Last retail price I saw was about
the same as a whole surplus working modern scope, ca $85(?!).
Haven't had enough experience with them yet to recognize cath
interface probs, assume it is near-baseline vert anomaly on display?
Not good news for a scarce and pricey tube.
BTW, the internal structure of the dud looked good under a loupe and
direct sunlight, no burned or warped grids, no discolored plate or
lumpy/perforated cathode visible thru the envelope, just the melted
cath tabs and clean & shiny getter. Strange indeed.
Thanx again. Best Rgds; Steve
