advice on old tubes

[maryc27182]

Hi folks, Happy New Year.

I took a gamble and bought a pair of 45 globe tubes (the description had the usual stuff about them being from an estate sale and they tested OK but no great details. I bid pretty low and somehow won the auction. Red flag alert, right?)

Anyway, I already had one tube of this type that works fine, so I put in the better-testing of the newly-acquired tubes in the amp (along with my old reliable one), powered on, set the hum pots, and listened to music for about an hour. For about 10 seconds when I first started listening to music, I heard a slight variation in volume in the channel that was powered by the newly-acquired tube. I listened carefully, but it didn't happen again.

The next evening I powered up the amp, only to find the newly-acquired tube had a beautiful bright blue-purple glow filling the tube. This wasn't the blue glowing outline I've seen in good tubes; this was the oh-my-goodness-this-is-really-gorgeous-but-bad-bad-bad-are-we-arcing-yet? glow. Since it's a globe tube, the glow was incredibly obvious and bright. One loud pop occurred and I powered everything down, and threw in my ST Sylvanias (known to be good) to see if I'd done any permanent damage (all seemed fine.)

Anyway, this afternoon I put in newly-acquired-but-didn't-test-as-well tube #2 (along with old reliable.) No issues on powering up, got hum pots dialed in nice and low, and music is playing. Sounds great.

Is there anything I can do to possibly avoid this tube suddenly showing gassy behavior? Should I cook it (while watching it? Easy to do today--I'm home with a bad cold.) Or is leakage an inevitability, and just one that I need to accept if I want to try to possess/use old tubes?

Mary

[drewh1]

here is some info on burning in tubes from EML - he mentions this is necessary for old tubes that have not been used for years. Maybe this applies to your tubes?

Yes they do! Good Burn-in will assure maximum lifetime and develop the final sound. These tubes are burned-in initially, to do the factory testing, after tube data becomes stabile. However emission of the cathode is not homogenous at this moment. Emission is build by many small islands, overlapping each other. This will not give the final sound yet. The burn-in process will take place in the first 50-100 hours under normal use conditions. This means it is best to switch off the amplifier after each use, and in the beginning not use the tubes longer than 4 hours at one time. Many short use periods have a better result than few long periods. The tubes need the „cold“ periods in between for best formatting of the filament. Some blue glow effect on the glass will disappear during burn-in, or may take longer to disappear eventually. What you observe, is a fluorescent effect, and it is normal with new tubes, even so proving cleanliness of the glass. For good burn in, use different loudness levels from the beginning, and increase the maximum loudness gradually. If tubes with very little use were switched off longer than 12...24 months, it may be necessary to repeat the burn in. So tubes that were not used for some years, may sound unpleasant, and simply need a new burn in.

[maryc27182]

Thank you for the reply! I would guess that these tubes have been sitting for decades, and were probably only tested quickly.

I'm now wondering if there was a start-up procedure I could have followed on tube #1 (the one that gave me the light show last night.) I saw some stuff on some of the radio forums that I might try on tube #1 (and there's an old tube tester at work, so I might see what tube #1 measures before I rig anything up--there's also a full electronics lab there, too.)

Hopefully tube #2 behaves better, and I'll try the EML procedure on it. Thanks again.

Mary
p.s. there probably will come a day when I abandon my cheapskate ways and get EMLs. Which did you purchase? Did you have hum issues with them?

[Paul Joppa]

Is there a silvery patch on the tube surface? That would be the getter, which absorbs residual gas when the tube is hot. If it has turned white, then there was a major leak, the getter is used up, and the tube cannot be fixed.

But if it's still silvery then a break-in effort would give it time to do its work. It's possible that the first heating liberated some traces of gas from the internal metal, mica, and glass parts. Here's a picture where you can see the getter flash at the bottom of the glass envelop:

http://www.sophiaelectric.eu/us/b/45-mesh/0?r=167

[maryc27182]

Thanks for the reply. The getter is completely silver--no white.

That's why I was wondering if I could do something with tube #1. I know a slight bit about gas leaks in other applications (not electronics--aerospace) and figured that there was a chance the gas was from inside the tube, and not air coming from outside of the tube to the inside.

There was definitely a pop when I saw the blue-purple glow. I'd still like to try again--should I simply use good judgment and if the fireworks show is horrific, stop?

Mary

[Doc B.]

It popped because the high voltage arced over thru the gas. The first step would be to heat only the filament for a while, no high voltage. That will avoid the arc, though it may or may not get the tube hot enough to activate the getter.

[Paul Birkeland]

Disconnect the wire to "Start" on the green PCB and leave it hanging in the air.  Remove the driver tube and other output tube, then put your funky gasser in the amp, power it up, check for filament glow, then leave it running for 48 hours.

-PB

[maryc27182]

Thanks, Doc. Thanks, Paul.

I will try that. I appreciate the help, especially as you are helping me fix something that isn't even your hardware.

Mary

[maryc27182]

OK, it's going. Filament lit, fingers crossed. Thanks again.

Mary

[maryc27182]

It was an interesting experiment, but no joy. Thank you though, for the information about what to try.

I think Doc is right---it didn't get hot enough to drive sufficient number of molecules to the getter flash.

All is good, though. Of the three globes I have, two work just fine and I have ST backups which sound quite good as well. Cannot complain at all...

[2wo]

You say you have a tube tester at work. You could try using it as a burn in jig. If you can figure out which setting is the plate voltage, you could run at a lower voltage for a while to mimimize any arcing...John

[maryc27182]

Yep, we do have a tube tester. I'll talk to our electronics guy. (He is going to mock me, though. Last time he helped me test a Raytheon windmill getter 5751 (which failed), and he immediately got online and found a $10 replacement and looked at me funny. He's definitely going to laugh at the globe 45.)

Back to the tube: I read some stuff on the old radio forums, and there are anecdotes about these RCA 45 tubes having shiny getter flashing, even when the getters aren't getting anymore (though obviously not when there's a major leak into the tube.) I have no idea how much truth there is to that, but it was interesting to read about.

[braubeat]

I believe you can heat the tube in an oven. I am not sure the exact temperature to use.

Michael

[borism]

Hi Michael,
And if you add a chicken you'll have at last some roast chicken to eat.
Sorry for the joke, a tube in the oven just seems such a funny idea. But then again if you can cryo treat them, why not bake them?
Boris

[maryc27182]

I did some searching on the 'tube baking' concept. Morgan Jones has done it.

http://www.vacuumstate.com/fileupload/Baking_valves%20pix.pdf

He recommends 120 C (248 F) for 12-13 hours. He commented this will mess up phenolic bases, and guessed that 100 C (212 F) might be OK for phenolic bases.

There are also multiple-page-long fights over this concept on various forums.