The AM Forum

I've got about 10 15E tubes that I'd like to bring up and check out.  I'm not exactly sure the best way to do it.  I am currently running 1 tube on dc voltage (filament voltage is 5.5V for these).  I figure I would run it for a day and swap the leads around and run it for another day.  I figure before I did anything else it's best to let it cook for a while.  What's the best thing to do next, how low of voltage on the plate and how long before bringing it up?  I've read some other info and not sure if there should be any special considerations for these.  Before any time I've had a NOS tube I've just threw it in the socket and let it rip and never had a failure.  I remember another member of the forum had a bunch of these and they said they all had lost vacuum. so I'm thinking it may be a common problem with such an old tube. 

Heres the info http://www.nj7p.org/Common/Tube/SQL/Tube_query.php?Type=15e

This is the coolest looking tube I've seen accept for the 250th's and the like.  I'd really like to use them in something.  BTW the filament is bright but not nearly as bright as the picture makes it out to be, and I've checked the voltage AT THE PIN and have it on 5.5V. 

Reversing the voltage?  What electrical justification is there for that?  Why not just use AC for a few minutes and be done. ;)

With receiving tubes I've found they come up to full power within 10-15 minutes at most.  bill

I've just always heard that with direct heated cathodes your not supposed to run dc because of greater emissions from one side causing issues.  But I guess if theres no plate voltage that doesn't matter.  It probably doesn't even matter with such a short ammount of time huh?  It was just easier to hook it to the power supply.  All my transformers are at our new place...the place we were supposed to be moved into a month ago.

Isn't reversing DC polarity once per day just very low frequency AC?

 :o ;D

Yes, however instead of an Oscilloscope, one must use NOAA Tide Tables to make measurements.  ;)

Bought 16 pieces 15E on a hammarket 2 months ago. one piece is ok , all others bad or no vacuum. The seller had 10 left and after a complaint i got those 10 pieces as well , but all also bad. All n.o.s. from an army surplus . If i come close with a highvoltage generator probe, some 50 kV,  the tubes light up blue , so there is gas inside. Next time a photo of the test.
Anton

This page from W8JI's web site is a good read on this topic:

http://w8ji.com/vacuum_tubes_and_vaccum_tube_failures.htm

I wonder Anton tried to getter these tubes by running the plates RED at zero bias (or positive bias for high Mu tubes), and low plate voltage? If no, perhaps some of those 15E's are salvageable?

Quote from link:

"Under some conditions a glass tube can be restored to operation by running low anode voltages and positive bias on the grid. This will sometimes allow full operating anode temperatures to be reached, and the tube can be "cooked" for several hours. I've had about a 50% success rate restoring old 3-500Z's that have sat for years without use. Even though they initially arced severely at full voltage, by cooking them at low voltage and positive grid bias to show anode color vacuum was restored."   

If there's too much gas inside including oxygen from air outside, then you might run the risk of burning out the filament long before a tube is gettered. You need HV on the plate for many thoriated filament tubes for gettering.  Double your explosion fun.

Wouldn't surprise me if these tubes suffer from the same brittle filament/lower grade metal used during WWII as the 100 & 250TH/TL etc. Many of the war surplus tubes have been found NIB to be gassy or dead. Been covered here numerous times.

Not sure how you successfully 'getter' a tube without a getter of some sort provided. I'm guessing the hope is that running the tube hot enough for long enough will allow the impurities to be re-absorbed.

Sometimes you just can't save 'em. But if it's a case of try it or toss the tubes, anything is worth a try. What's to lose?

I had a 100th tube that ran A FULL ENVELOPE purple glow but gave decent output power....I kept running it and gradually the purple diminished to almost gone...then the tube output started to decrease...a slow decease of out put until I couldn't stand it and I replaced the tube...I probably got 10-15 hrs of transmit time and 100 hrs filament time out of that tube...Your mileage may vary..

Those were made for torpedo bomber use.. ought to be rugged enough allright. but for age and air leaks, I don;t know.

I think the metallurgy of the plate material in the 15E is supposed to absorb gas molecules if run at high temperature.  Also part of the manufacturing process is to run them hot. I Forget the term used.  The plate or anode is the getter.

Whereas for oxide, indirectly heated cathode tubes, the plate never runs hot enough to act as a getter. So the getter has to be the hot cathode.  Also run at temperature for awhile to remove residual gas.

would this apply also to other high performance tubes like a 3-1000 that was found used, idle for 10 years perhaps, and when put in an amplifier only makes 350W? Would "burning it in" with high dissipation possibly bring it back? Or, is it likely a worn out thoriated filament?

Eimac in "Care and Feeding of Power Grid Tubes" outlines answers.  Written in an era of mostly thoriated tungsten tubes, but adequate mention is made for oxide cathode indirectly heated tubes, mostly by disclaimers.  There is an entire section devoted to extending life and rejuvenation.

You have to be careful knowing the beast your working with though. A lot of this has been covered in previous posts on this board.

Even though external anode ceramic tubes all look like the latest indirectly heated cathode types, many are just plain ol' glass thoriated tubes in disguise.  The 3cx1200a7 comes to mind.  Seems like a beefed up 3-1000z encased in metal.  Still has the few second warmup filament BUT has a relatively cold anode that is not the getter.

The promised photo's of the poor quality 15E tubes. Most of them have poor vacuum and others have no vacuum left at all.  With a external high voltage wire close to the bulb they will ionize , if the vacuum bad,  seen as a blue light. If the vacuum is sufficient or when there is no vacuum at all,  they don't light up. In the whole batch was only one still OK and usable.

The second photo.