[Boatanchors] tube question

[Bill Cromwell]

On 02/25/2014 01:39 PM, bonddaleena at aol.com wrote:
> Hi, I think I knew the answer to this question, but old age must be catching up to me!
>
> I am restoring an older tube-type VHF (FM) receiver. The kind that covers both VHF low and high. It was made by Sonar and during clean up, I ran the tubes through my Sencore tube tester. (I have several testers, but this is my 'go to' tester.
>
> I found 2 'bad' tubes, which I need help understanding. The tubes are a 6BE8, which is a High Mu Triode - Sharp Cutoff Pentode.
> The other is a 6KE8 which is also a dual device (Medium Mu - Sharp Cutoff Pentode).
> Anyway, when testing the tubes, one section of both tubes tests fine for emission (I know how that part works) and NO Leakage.
> However, when I set the tester to test the other 1/2 of each tube, the emission is fine, but the 'grid leakage' starts off low (not zero) and slowly climbs to the extreme right of the meter.
>
> I'm trying to understand 'what' is being tested and how 1/2 of each tube can be OK, and the other 1/2 'leaky'.
> It's NOT my tester, because I found a new 6KE8 in my stash, and both 1/2s test fine.......
>
> Excuse my ignorance....
>
> ron
> N4UE
> ______________________________________________________________
>
Hi Ron,

Are the tubes made by the same manufacturer? You may not know.

I am guessing if the tubes ARE bad that with heating something in the 
structure warps and touches something else that it shouldn't. Maybe it 
got *bumped* while it was very hot. Dilbert pulls the tube out of the 
socket live. Dilbert notices his fingers are being scorched. Dilbert 
drops the tube. Oooops.

The best way to resolve this is to put the tubes into a *real* circuit 
and run them at full voltage under real world loads (suitable for the 
tubes) and see what happens. Make sure there are appropriate fuses 
involved. Over the years I have encountered a few tubes that tested (on 
my Sencore) "flaky" but were okay and some that tested (same tester) 
okay and were absolutely scrap. The *real* circuits sorted them for me. 
Others with different testers have reported the same result. If the cold 
tube has dead shorts where there is no intended internal connection do 
NOT plug it into a circuit of any real value!! (for newbies who don't 
know that).

I look at tube testers as sorting out *gross* defects. That's useful but 
not the holy grail. If a radio works well with them they are probably 
good <wink>. Some of them will work well in one application and fail in 
another. You can sort them to use in the appropriate application but 
they usually won't stay sorted <evil grin>. Been there - done that.

73,

Bill  KU8H