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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?
Follow Ups:
Before you throw them out, try leaving them in the tester for a few minutes at an elevated heater voltage, say 20% higher than specified before you run the test. This has worked for me in the past for tubes that were in storage for a very long time. They seem to stabilize after that, but I don't know how long they will continue to perform at that level.
That just means they are weak/borderline tubes. When you hold the test button down the tube starts to draw more current causing the cathode to heat up more releasing more electrons than in normal conditions. This causes the readings to slowly increase.
Not sure if any models of the 12BH7 have slow warm-up qualities. Like the 6SN7GTB.
I don't think tubes with specified warm up time had 2 to 3 minute delays as the poster's 12bh7. Wasn't the controlled warm up times less than 30 seconds? regards, Dak
The tubes are not "A", and I can run the filaments for a long time and it does not change the test pattern.
If I had to guess, taking into acocunt appearance and test results, the group I have were run continuously for a long period of time under relatively mild conditions.
Just simply closer to end of life than I would like. The question is how close they are to end of potentially useful life.
It seems that you know you need to replace those tubes or if you are trying to sell them well, I would not want to be the one to buy them. If you bought those tubes and now knowing they are borderline as long as they are not microphonic they can still be useful. regards, Dak
The tube builders were more consistent in indicating controlled heater warm up time, with the 7 and 9 pin mini types. An "A" after the 4 character type indicates controlled heater warm-up time. 12AX7A, 6AQ5A, and 6AU6A are fair examples.
Controlled heater warm-up time was introduced as a cost saving measure in TV sets. Long series heater strings allowed the manufacturers to "cheap out" on power trafo requirements. Without heater warm-up time control, tubes in long series strings experience heater failure. ;> (
Eli D.
Hi Eli,
What you said is sort of correct but not the whole story.
Series string heaters for the reason which you mentioned have been around since the 1930s, long before controlled warm-up tubes were thought of.
It turns out the resistance of the heater is a function of the temperature and therefore changes as the heater warms.
Before controlled warm-up, watching the tubes in a series string TV warm up was scary. Some would be dark and some lit like light bulbs until the whole mess stabilized. As you might expect the ones which lit like lights had a short heater life. GE would put Glo-bar resistors in series with the heaters to slow things down and mitigate the problem. Finally it occurred to someone to design the tube heaters to warm at the same rate when fed with the rated current. This took care of the heater warm-up issue but the cheap TVs were still death traps with their "hot chassis".
Phil
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