cheating life out of old tubes?

[ke4mcl]

i was testing a batch of 12ax7's and there was one that tested low but balanced. for giggles i stepped up the filament voltage to 7 volts and the tube came up to beyond acceptable and all other tests were ok.

obviously they aren't happy running at higher than rated filament voltage but i'm thinking it could be a way to tap into a cheap source of otherwise pricey tubes. if a tube fails at normal voltage they have little sale value anyways right? there isn't much to lose.

how drastically is life shortened by running a filament at aprox 1v higher than rated? could there be any other dangers?

[soulmerchant]

I would cook them as Morgan Jones does. Nine pin tubes can take higher temperatures and don't quite smell quite as bad as phenolic based tubes...

Higher filament volts will simply bring about a quicker demise. Trust me.

[poynton]

This was common practice for TV tubes years ago.

I built myself a rig when I was repairing computer monitors in the '90s.



Andy

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[DF96]

The cathode will already be losing emission, so all you are doing is buying a little extra time. Worth doing for rare or expensive CRTs, less so for common valves.

[Osvaldo de Banfield]

Tube manuals suggest use VH plus or minus 10%, so 6.3 +10% is 7V aprox. I believe there isn´t nothing bad in use 7V.

[Mooly]

Just an off the wall thought... is your meter accurate at low AC voltages. Don't assume it is just because it might appear to be at high AC voltages.

Just look at the readings here,
High efficiency speakers - how much power do they really need?

As to over running heaters, well yes it was common practice as Andy and DF96 mention. Dedicated heater transformers were available for CRT's with I think a +20% tapping. Once run at over volts there's no going back though.

[ke4mcl]

thanks for the replies guys.

i'm sure it will bring quicker demise of the tube but keep in mind this would be a tube that is already testing poor to begin with at its intended heater voltage. it's basically on its way to the garbage anyways.

as far as testing to see if my voltage is accurate. i'm going by the heater voltage select on the tester. the only accuracy check performed is to adjust line voltage as described in testers instructions. a tube that fails at the 6.3v heater setting seems to come up noticeably when the 7v setting is chosen and filament has caught up.

it wouldn't take much on a small amp to put an LM317 on each tubes filament supply and dial in the desired voltage. not only would odd filament equivalent tubes suddenly be an option but you could also take worn tubes and drive more life out of them. you wouldn't feel soo bad about using up spent tele's on a guitar amp and such.

[v4lve lover]

quicker demise of the tube ? nah the cathode was just inactive . you need to put it through overload so it can form a new layer of barium oxides

try current sourcing it whit a LM317 put it 30% over ratings for some time they will tolerate it and emission will increase


ive cooked 8 volt pcf tubes just for giggles at about 25vdc they will start to smell after some time i couldnt kill it whit 30vdc

ive also tested several 6080s for emissions at low voltage they showed very low values at first . but i cranked up the heaters abit and measured increased emissions at normal voltages a week after i burned them in

[DF96]

Yes, sometimes you can reactivate a cathode. What happens is that the metal (not the oxide) migrates to the surface to form a new emissive layer. Basically you are repeating one of the final stages of manufacture.

[v4lve lover]

absolutely correct