sparks

[mpnmatthews]

Hey guys,


Anybody know why my AO-28 would be throwing sparks in a BRAND NEW NOS 6X4 tube?


I examined the socket, looks good to me... and the tube actually works after the fireworks burn off... works like normal.


Anyone ever seen this before?

[KC9UDX]

Blue sparks in a 6X4 indicate oxygen infiltration, which means the tube is very much at the end of its life. It's also not good for the rest of the circuitry to let it do that.

Likely the reason it stops is that the getter is doing one heck of a job. But that will run out soon.

[mpnmatthews]

This is a brand new NOS tube from Tonewheel General. It arrived yesterday. How could it be at the end if it’s life?

Could there be something awry with the wiring in the AO-28? I checked through the preamp, and it is in original condition, and working fine.

[KC9UDX]

It's old. The glass to metal seals aren't perfect. In fact, it's possible for them to fail prematurely from disuse.

By the way, NOS≠Brand New.

There's nothing an electronic circuit can do to cause oxygen to form inside a tube without melting the envelope, in which case you still have a leaky tube. But I don't think your circuit is melting the envelope.

It takes oxygen to make blue sparks. They can't happen under normal tube vacuum.

[Silken Path]

Agree - Ages ago I was driving along on one humid night in Georgia in an old straight-eight Pontiac that had a tube-type radio. About midnight, I saw a strange bluish light dancing on the floor... and then the radio quit.

[KC9UDX]

By the way, if you pull the tube and inspect it, you'll likely see a white film appearing on various surfaces inside the tube. That is what happens when the getter absorbs large amounts of air. If you have access to a tube tester, most testers (almost certain any you'll encounter) have a test for gas and shorts. This test is supposed to be performed first, to prevent damage to the rest of the tester. My money says that your 6X4 fails the test. Unless otherwise noted, anything less than a perfect score in that test is an indication of gas infiltration. In ye olden dayes when tubes were plentiful, any such tube would be immediately discarded. I can't think of any reason not to do that today, but don't throw it in the trash. When these become rare enough, odds are that someone will start rebuilding old tubes, and that one may be worth something.

Contact Tonewheel General; I don't know what their policy is. They may tell you that there are no returns, or that you voided the warranty by using the tube. But maybe they'll take a gaseous tube in return if it hasn't been too long and there's no obvious sign of abuse. I have no idea.