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Old 2nd May 2011, 10:30 PM   #1
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Default Drifty tubes

So, I have a new set of 6LX6's with 500v on the plate and 150v on the screens. The tubes are extremely drifty. The tubes are NOS. Anybody have any experience in drifty tubes?

Thanks,
Ray
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Old 2nd May 2011, 10:34 PM   #2
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They may well settle down with some hours on them....
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Old 2nd May 2011, 10:44 PM   #3
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So, I have a new set of 6LX6's with 500v on the plate and 150v on the screens. The tubes are extremely drifty. The tubes are NOS. Anybody have any experience in drifty tubes?

Thanks,
Ray
Thermionic emission varies with the square of the temperature. An unregulated power supply for the filaments and/or substantial changes in ambient temperature will alter the filament temperature and therefore the cathode temperature. The cure is negative feedback in the amplifier signal circuit which works to stabalize gain. One of the many negative consequences of not using feedback is that the circuit quiescent operating point will drift around forever. Where stability is critical, a regulated dc power supply to heat the filaments and supply the electrode voltages and an enclosure to stabalize temperature can help so long as the tube does not overheat. If the tubes are not power output tubes, tube shields help too.
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Old 2nd May 2011, 11:02 PM   #4
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So, I have a new set of 6LX6's with 500v on the plate and 150v on the screens. The tubes are extremely drifty. The tubes are NOS. Anybody have any experience in drifty tubes?
How are they drifting? I had a couple of 6BQ6s where the plate current would not settle down, but kept creeping higher and right into red plate territory. That turned out to be due to excessive gas. A tube tester confirmed that, even if the blue glow wasn't yet visible.

If that's what's happening here, suspect gassy tubes. If it's just a case of the plate current not staying put, it's probably normal, and they should settle down after running 'em awhile. That could take a few hours to a few days.
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Old 2nd May 2011, 11:53 PM   #5
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I had a brand new EH KT88 tube that would drift up into the danger zone as well and that was with regulated bias, plate and filament voltages. It was a bad tube even though it was brand new.
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Old 3rd May 2011, 12:05 AM   #6
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All my supplies are regulated too, though they seemed have settled down. We see. I'm going to leave them on overnight idling at 38ma/ea.
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Old 3rd May 2011, 12:06 AM   #7
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Ray, time may help (gettering). Also, what's the grid resistor size, grid bias, and idle current?
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Old 3rd May 2011, 12:12 AM   #8
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Sy,
They are 6LX6's. Plate is 500v, G2 is 150v, and G1 is -45v. Idle Ib is 38ma/ea. They seemed to have settled down. The Ib was going up and down for quite a while, though never entered the "red" zone.
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Old 3rd May 2011, 02:22 AM   #9
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All my supplies are regulated too, though they seemed have settled down. We see. I'm going to leave them on overnight idling at 38ma/ea.

Personally,I wouldn't leave it running unsupervised, at least until it calms down, just in case anything 'funny' happens.
But,it's your call.
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Old 3rd May 2011, 10:18 AM   #10
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Ray, my 6lr6 tubes took 10hrs to stabilize and they wanted a hefty - bias, I had to add a couple 6.3 volt windings to get the neg bias low enough. One tube wanted -65 so I put it aside.... I'm running 160V on G2, just the draw of the zener stack. I also use it to power the nixie tube. I run 40ma through the output tubes..
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