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White getters are a sure sign of AIR in the tube. Silver getters indicate the tube is still under vacuum. Gases reacting with the getter material cause its color to change. The more silver the getter "flash spots" appear, the harder the vacuum. A good tube has bright silver getter spots, "soft" (gassy) tube will have dull grayish spots, while a tube that has virtually no vacuum at all will have white getter spots. This applies to 15GP22's just like it does to 6L6's, 5U4's, 6AS7's, etc. If the getter is white, the vacuum is gone.
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