I witnessed something recently that In nearly 50 years of working with
tube rigs, I had never seen.
I was working on an older hybrid rig with 6JS6's in it. Had a known
good set of finals and was getting unit ready to ship to a new owner.
Powered it on the bench and all was good..did a cold
neutralization...then reconnected filaments and powered up.
Its sitting there at idle and I start hearing "tink..tink...tink..."
like tubes getting hot. I look into the final cage and see both tubes
filaments on. sits there some more..and I start smelling it HOT. Look
again and one tube is bright red plate. Just as I reach and cut the
power, I heard some arcing.
Very strange I thought since it had all been working fine.
So I open it all up and check out everything. All looks ok EXCEPT..the
one tube that was bright has a small suck-hole thru the side of it. It
got so hot, the glass melted and a hole was sucked thru the side without
breaking the rest of the tube. YIKES>
So, smart fellow that I am, I'm thinking bad tube right?. Just to be
sure, I double check the bias supply right at the sockets..looks ok.
All else seems fine. Double checked the driver plate coupling cap since
that can put b+ on the grids of the finals if it shorts..OK..it would
have smoked both tubes anyway.
So..bad tube.. So I put back in a back-up set of finals..about 80% on
10M pair. Power on, quick set bias V , watch plate current, all
good........."tink tink tink..AWE CRAPP!...power off. Same tube
position is COOKING hot. What the....
So..smart fellow that I am, I plug in the bad tube and check the
resistance between the tube pin above the socket to the bias
bus...OPEN! Unbelievable...the grid pin connection in the tube socket
had been opened up to the point that it no longer contacted the pin on
the tube. Wide open throttle on one tube...even in standby....so no
indication on the multi-purpose, plate meter.
Quick correction of the socket...a quick look at all the empty shelf
space where the expensive spare finals used to be stored...and the rig
is ready to ship..
Life is an expedition...
Happy Holidays and controlled bias to all.
Curt
KU8L
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