Why is there a silvery coating inside a vacuum tube?

David Mellor

David Mellor is CEO and Course Director of Audio Masterclass. David has designed courses in audio education and training since 1986 and is the publisher and principal writer of Record-Producer.com.

Friday February 11, 2011

Have you ever wondered why there is a silvery patch inside a vacuum tube? Is it there for a purpose? Does it affect the sound of the tube?

A studio isn't a proper studio without at least one vacuum tube device. The sound quality of tubes is unique and hard to emulate digitally (and impossible to emulate using transistor circuitry).

You may have wondered however why most tubes have a silvery coating on the inside. This is usually quite easily visible. Just look in the back of your guitar amp.

Tubes work by controlling the flow of electrons through a vacuum. It has to be a vacuum because otherwise the air molecules would get in the way. That's why they are called vacuum tubes :-)

But creating a 'hard' enough vacuum inside a glass envelope is quite difficult. So the trick is to pump out as much air as possible, seal the tube, and then absorb what air is left in some way.

Absorbing as much as possible of the last remnants of air inside is the function of the 'getter'.

The getter is a small cup inside the tube containing a strongly reactive metal, such as barium. When the tube is pumped out and sealed, the getter is fired, producing a 'getter flash' inside the tube.

The silvery patch that is visible is created by the getter. The getter has performed its function by absorbing much of the remaining oxygen.

And the harder the vacuum, the better the performance of the tube. Sometimes the getter flash might be uneven in appearance, but that doesn't make any difference to the sound or how well the tube works. If it turns white however, that's a sign that that air is leaking into the tube. Time for a replacement I think.

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