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Silver spots on power tubes; sign of overheating?

JDW3

Member
Messages
3,637
I had tubes installed last year in my '68 Vibrolux. I noticed today that the power tubes had silver spots on either side of each tube. I'm kind of angry as I paid a tech to bias the amp.

The amp wasn't played much at all after they were installed. The tubes are/were strong GE 6l6s.

Anything to worry about?
 

doctord02

Member
Messages
1,061
Thats to be expected, many GE 6L6 tubes have the getter flashing on the sides instead of the top. In otherwords, it's supposed to be silver there - and it was silver there the day he installed them, you probably just didnt notice it.
 

Trout

Member
Messages
7,551
Thats to be expected, many GE 6L6 tubes have the getter flashing on the sides instead of the top. In otherwords, it's supposed to be silver there - and it was silver there the day he installed them, you probably just didnt notice it.

+1

Side Getter GE's are great tubes,
GE 12AX7 clear tops are pretty sweet as well.

 

phsyconoodler

Member
Messages
4,315
Quote"I had tubes installed last year in my '68 Vibrolux. I noticed today that the power tubes had silver spots on either side of each tube. I'm kind of angry as I paid a tech to bias the amp."

If they had no silver spots I'd be upset.The getters are heated as the tube gets vacuumed down to aid in getting a better vacuum.If the tube gets a good vacuum,the glass gets silver glazed where the getters are.If you look at a tube where the vacuum is lost,the glazing is gone.
 

JDW3

Member
Messages
3,637
Excellent. Thanks guys! The amp never changed tone or sounded bad. I just happened to look today and that's what I found.

Good to hear.
 

Timbre Wolf

Hyperspatial Gravity Surfer
Platinum Supporting Member
Messages
10,080
The getters are heated as the tube gets vacuumed down to aid in getting a better vacuum.If the tube gets a good vacuum,the glass gets silver glazed where the getters are.If you look at a tube where the vacuum is lost,the glazing is gone.
Not quite accurate, but functionally a close attempt. The vacuum process is separate from the inductive heating/getter deposition.

The silver coating on the inside of the glass is barium metal, which is highly reactive with oxygen, rapidly forming white barium oxide when exposed to stray oxygen that escapes as an impurity in the metals and cathode coatings inside the tube. In a way, the deposited barium getter material helps maintain a more pure vacuum, but really the important role is to scavenge damaging oxygen before it oxidizes the cathode materials.

Bottom line - yes, do worry if you don't see an area of silvery coating inside the glass of a vacuum tube.

- Thom
 

donnyjaguar

Member
Messages
4,199
Timbre has it. Think of the metal getter loop as a short circuit. when the tube is subjected to a near-field magnetic field of decent intensity, it heats up and flashes the material onto the glass. It removes remaining impurities in the tube's vacuum. A similar thing is done in incandescent lightbulbs except they coat the actual filament. When first powered up there's a puff of smoke inside the envelope. You won't see this unless the bulb missed final test during production.
 




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