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In Reply to: RE: 12BH7s That Slowly Climb Under Test posted by DAK on August 16, 2015 at 09:51:21
Not sure if any models of the 12BH7 have slow warm-up qualities. Like the 6SN7GTB.
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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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