White flakes in rectifier tube??

[Klimon]

Hello,

I found a Mazda 5y3g rectifier in an old frequency generator. It test as new with no gas leakage - optical condition is fine also; no darkened glass. When turning the tube upside down there is however a small heap of small white flakes trapped inside the glass envelope - Does any of you more experienced tube-addicts know the explanation for this phenomenon and if it impairs operation?

Thanks!

[cogsncogs]

Originally posted by: Klimon
When turning the tube upside down there is however a small heap of small white flakes trapped inside the glass envelope

I've seen this many times esp in directly heated rectifriers such as 5U4's, 5Y3's etc. I believe it's pieces of the cathode coating flaking off, such as the places where the filament have sharp bends like at the top of the mica 'spacer' at the top of the tube. The tube may test ok and be ok for the meantime, but will eventualy need replacing.

Cheers
Wayne

[pinkmouse]

Originally posted by cogsncogs
The tube may test ok and be ok for the meantime, but will eventualy need replacing.

Isn't that true of any tube?

[cogsncogs]

Isn't that true of any tube?

Well jeez yeah!

If it where me I'd go ahead and 'chuck it' and get a new one! Well that would depend on how much flaking was present.
I just didn't want him to think that the 4th of July was going to come early this year for him, heheh!

Truth be known I've got some very old tubes, small signal that is, that are still going strong and may never have to replace them. But VT rectumfriers (especially) and Power Tubes are a different story...

Cheers
Wayne

[tubelab.com]

The white stuff is indeed flakes of the cathode coating. This happens in all DHT tubes to some degree. A friend has a Fender Bandmaster that hangs the tubes upside down. I replaced his 5U4 because it contained a lot of the white stuff. He told me that his amp sounded worse, and he wanted his old tube back. That was over a year ago, and he uses the amp a lot.

I have some 45's that contain flakes, and I am certainly not going to toss them. They sound too good, and were "flakey" when I got them on Ebay a couple of years ago. I run them at 320 volts, well above the max, with no problem.

[cogsncogs]

Originally posted by: Tubelab.com
I have some 45's that contain flakes, and I am certainly not going to toss them.

I certainly would not either. I have a very old RCA 5U4 that so much of the oxide coating has flaked off you can see the bare filament metal, about 80% of it. Now that I would toss (painfully). But I have a beautiful Westinghouse (black plate) 5U4GB (circa 1963) that has a few flakes and it's looking for a home in some kind of a project. Now that one is a 'keeper'.
It really just depends on how much of the oxide coating has flaked off. Cathode emission.

Wayne

[Klimon]

Okay, I'll just use it and keep an interested eye on how it evolves. The Mazdas are reputed to sound quite pleasingly and the cathodes of this one are unlike anything I've ever seen: a smooth shiny dark grey. Thanks for spitting it out !!!