Mouth-to-mouth for tired filaments
Over the years I've read that old final PA or modulator tube pulls (with several 1,000s of hours) and with thoriated-Tungsten filaments, such as an 813, can be rejuvenated by running the filaments at 10.5 Volts for 1 to 2 hours (in the 813 case), absent HV and excitation. The result apparently being significantly increased emission, and thus transconductance.
In the days of tube TV repair, it was common practice to put a booster on CRT filaments to brighten a dimmed picture and postpone CRT replacement for the customer. To my surprise, some of those boosted CRTs ran boosted for 5 or more years. Dunno if they were thoriated-Tungsten though.
Is what I'm describing (with an 813) wise in the long run? Is the short-term benefit worth the shortened life?
If there wisdom in the practice, I'd appreciate hearing experience related to the 1-2 hours cited. Too long, too short, other?
The CRTs had (have) oxide coated cathodes.
I've heard of cooking thoriated filaments as you describe. If the tube is low on emission and unusable what's the harm?
I see a notable difference between low on emission (but on the air), and unusuable. A tube that's functional, albeit low in emission, could keep a station on the air.
Originally Posted by WX7G
I'm asking what's the risk to destroying the tube entirely using voltage and time above?
After thinking longer, the advice to use 10.5V for 1 to 2 mins on an 813 is way too anecdotal for my tastes. I'm unaware of data to support the values given. Furthermore, I haven't a guess about how much increased emission I might expect, were I to do what I read.
What about the 4-400A? Is 5% overvoltage correct for it?
When I posted, I hoped to find someone who has done a similar operation, preferably numerous times, and could confirm that the suggested procedure is more than a best guess. None of my bottles were free. Trashing a couple that still function, they're just getting tired, just to find out if a scheme is valid, isn't appealing
I don't know anything about the 813. But, I have run some tubes at more than double their rated filament and heater voltages at times with no ill effects. I have never noted a difference in any recieving tubes. I've never had a tube that magically got better after running the filament overvoltage, but, I've never had one get worse either.
I have run picture tubes overvoltage (heaters only!) with incredible results. Nothing that permanently improves the tube, but a very incredible improvement in picture.
There are many references to this procedure for thoriated filaments. This article says to run it at 150% of rated voltage for one minutes followed by 5 minutes at 110%. This sounds harmless to me.
I've petted an old pony, but never, ever given mouth-to-mouth to a tired filament.
Thanks. The link seems useful. It shows he's done it enough times to have a handle on the Dos and Don'ts. I benchmark my tubes for emission using a test setup I found in QST 30+ years ago, so I'll have no difficulty judging how much emission has increased. Again, Tnx!
Originally Posted by WX7G
I believe "ill effects" is a function of the time you ran them at 2X volts. Many years ago I read that incandescant lamp filaments' life varied inversely with voltage to the 13th power. That explains why the so-called miracle lamp burning in the Thomas Edison museum (or is it an Edison Electric facility?) has run continuously for several decades. They operate it at significantly reduced voltage. I haven't a clue what the longevity vs. voltage formula is for thoriated tungsten tube. If I wanted to dispose of a tube, I'd probably put under the heel of my boot, not run it at 2X volts for many hours.
Originally Posted by KC9UDX
I have a project that I've been working on for about a year, and isn't finished (nor is it likely to ever be). I had several 6V tubes that for whatever reason I inadvertently connected to a 12.6V supply and when I double-checked it, I found it to be 12.6V which for whatever reason seemed correct. I probably had about 20 hours on the tubes like that before I smacked myself in the head and realised what I had done. One of them was a NOS tube, don't recall what type it was. None of them showed any sign of change from their initial test values. Now, again, these were receiving tubes, not transmitters.
Originally Posted by NM7G
Obviously, there has to be a point where the filament is damaged if not broken. But I don't think we can estimate where that point is without trying it out, something I'd not be inclined to do.