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12BH7s That Slowly Climb Under Test

Posted by baileyler (A) on August 16, 2015 at 08:19:53

I got a group of older (mid 1950s) CBS branded 12BH7s with black rounded ladder plates and U getter elements.

I have now tested several, and for the most part they show the same behavior (on both a Westmore 501 and B&K 550) - they start out low (below reject, regardless of length of warmup) then continue to rise under test. After a 2-3 minutes under test they do level out into the "good" range.

I then take them out of test mode (but leave them in with filaments on) for a minute or two, put them back under test and it is the same behavior - start out in reject them slowly rise into good.

I will test them a day or two later, and the same behavior shows up.

I have had an oddball tube do that from time to time, which I usually throw in the reject pile.

I have never had a group of this many of the same tube show that behavior.

Should they go in the reject pile?

The printing and getter flashing are nice on most, with no unusual burns or discoloration. Most likely they are equipment pulls.

What would be a good explanation for this behavior?

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