The AM Forum

Hello all..just wondering if anyone else saw the article on pg 77 about rejuvenating 811's?..wud I be off course if I thought the same simple scheme cud be applied to it's big brother the 572B?..or for that matter other RF?AF triodes/pentodes etc..like maybe 6146's or such?..yah I did "experiment" with a pair of defunct Taylor 572B's..using my trusty and rather rugged tube tester by Sylvania..it sorta worked but maybe I needed more amperage as the article suggested..I was able to get the tubes up from "for sure-dead as can be" to well interesting in the yellow questionable area on the meter..they may have gone higher up into the gud region but I was a bit timid about flicking on the 12.6 fil pwr for more then 30 seconds..didn't want to lose my best tester of all even though the tranny is some beefy..perhaps those particular tubes did not sport thoriated tungsten filaments..??..yup Mr Curious(meself) did try this out on a 6V6 that was way down in the "throw it away" area on the meter..brought it up to just w/in the gud section and to my surprise it stayed that way..I'm sure guys like Patrick KD5OEI "Op-Com" have thought about this as well..I do not know if all tubes do have thoriated tungsten in their filaments..??  call me a dreamer but just wondering..???
PS:I am able to test 572B's by using the 811 specs and slightly adjusting one control BTW

Most power tubes use a "directly heated" filament as the cathode, also known as the " thoriated-tungsten filament." The major reason for the use of the thoriated-tungsten filament in power tubes is because it is more tolerant to ion bombardment than the oxide coated emitter. Example tubes are the 813, the 811, the 572B, 3-500Z, etc.

The "Indirectly Heated," or "Unipotential," or oxide coated emitter has the construction whereas an electrically insulated heater is inside a nickel-alloy cylinder and coated with barium-strontium oxides. It is sometimes denoted as the "any position,” or “dark cathode” tube. The major advantages of indirectly heated cathodes are higher electron emission efficiencies at lower temperatures, and reduced hum modulation of the space charge. Examples of indirectly heated cathodes are the 807, the 6146, 4D32, and most receiving tubes.

If the filaments are subjected to overvoltages for a short time this might boil off some oxides - of course assuming the tube still has a vacuum.

I did not read the QS? article. ;D

Phil - AC0OB

Tnx Phil..you know mucho more than myself about filaments in tubes..the hope was if it cud be done or achieved in the 811 then might there be hope for the very common 572B's?..my experiment with the 6V6 was maybe as you say burning off the oxides or perhaps rejuvenating the getter on what may have been a gassy tube..that tube and the 2 572B's are the only ones tried so far..my main aim is whether dud 572B's can be brought back to life as there appears to be success here based on the article with it's smaller cousin the 811..my only 811 in the stockpile tested very weak and was tossed out over a year ago..wish now I had held unto it..
              Agn tnx Phil for the enlightenment..73 de DAVE