Testing a 15GP22

[benman94]

The Sencore will work fine. Dave Pike and I did the final test of a 15GP22 I had, and subsequently sold, on a Sencore CR-70.

Note however, that a Sencore will only tell part of the picture. It is technically possible, but very unlikely, to have a tube that tests good on the Sencore, with a good, normal cutoff response, that is too gassy to use.

First test the tube for emission, allowing time for it to wake up as Tom alluded to.

Then, test cutoff. This is critical. If turning the cutoff control moderates the cathode current normally, you have a good tube. If the tube responds normally until you reach a certain grid voltage, and then all of a sudden cathode current runs away uncontrollably, the tube is gassy.

Also look in the neck of the tube while checking. A faint blue glow on the inside surface of the neck glass is possible, and actually a good sign. (In very high vacuums, the surface of glass is actually slightly conductive. This is why some 6L6s, 6W6s, 6Y6s, 6V6s, etc have graphite coatings in them... it's there to collect stray electrons that would otherwise light up the inside of the glass slightly.)

A purple, pink, or indigo glow BETWEEN elements of the gun assembly is bad and a sure sign your tube is too gassy to use. Getters are also sometimes indicative, but can not be relied upon. Anything white is clearly a gassy dud. Good looking getters could be a good or bad tube. Faint looking getters, even nearly invisible to the naked eye, are often a bad sign, but aren't necessarily so. I've seen one 21AXP22 with NO visible getter material produce gorgeous color pictures over the past 5 years, so it's possible.

The best check of gas is a nice, strong tesla coil. They can be purchased or built for reasonable cost. A gassy 15GP22 will light up like a Geissler tube...