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Old 12th January 2013, 02:18 AM   #1
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Default Re gettering?

After carefully going through an old pair of Altec 1570B amps I brought them up to full voltage and all was well except for that telltale blue glow inside one of the 811A tubes. I suspect it has gotten gassy as these amps have not seen the light of day for over 25 years. I have read about HAMs dealing with gassy 811's by just lighting the filaments with no B+ and letting them burn for a day or so and thus "regettering" them. Is there anything to this or are they just "blowing smoke"? (pun intended)
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Old 12th January 2013, 03:02 AM   #2
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Obviously, you can't add getter material. However, "cooking" the tube allows the getter material already present to react with unwanted gas. How much the vacuum will "harden" depends, in part, on the amount of getter material available.
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Old 12th January 2013, 03:05 AM   #3
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Filament alone will not bring down gas unless it is only very minor. You need to pull some plate current. Leaving it run in a tube tester under test is one way. Another way is to light the filament with a small transformer. Connect the grid to the plate making a diode out of it. Then connect AC mains voltage across the tube (filament to plate) with a 40 or 60 watt incandescent lamp in series. I use an isolation transformer and variac for this. As the gas recombines the lamp will dim. But even this may or may not bring down gas depending on how bad it is.
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Old 12th January 2013, 03:08 AM   #4
chip647 is offline chip647  United States
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Since the gassy tube is basically garbage without some kind of change, the trick is not only to run the tube with the heater lit, but also to raise the heater voltage 25 to 50% in the cooking process to get the tube hotter than normal.
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Old 13th January 2013, 02:33 AM   #5
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Perhaps even add a titch of thermal insulation around the glass where the getter is flashed - to raise the getter temp a bit more than if normally being cooled through conduction to the glass amd then convection to air.
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Old 13th January 2013, 03:25 AM   #6
AJT is offline AJT  Philippines
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anybody tried induction heating?
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Old 13th January 2013, 04:03 AM   #7
TheGimp is online now TheGimp  United States
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Stick them in the oven at 350F (166C) for four hours. Let them cool down slowly.

Bakelite bases discolor under this process.
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