Picked up this 7DP4 the other day
 

At Ben Tongue's estate sale. Looks good; I hope it tests good!

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Do you have a 621 ? If so, Kewl !

Sandy, I have a blonde 621 and a mahogany one. Also one of those GL-4592 7DP4 replacements from Fair Radio. Still, it will be nice having a spare original tube!

The bottom of the gun isn't blued... That's a good sign!

I'm not familiar with that. What does it mean for the bottom of the gun to be 'blued'? Thanks.

Many high hour CRTs have tell tale 'bluing' of the metal of the first grid aperture, which is the section closest to the base concealing the heater and cathode. Very high hour tubes will have a unique 'dinginess' with sort of a scaling effect upon that element, in addition to the bluing.

Having none of these visual 'tells' doesn't mean the tube is necessarily good, but it does indicate the tube wasn't used excessively.

Hate to tell you, but a grid cup that's not a shiny new metal color could mean high heater heat, or nothing at all. Yes, metal can turn colors. But what is the source of the heat? One of the things I learned from rebuilding CRT's was that the heat of the sealing torch often causes some elements of the gun structure to darken or otherwise change colors, let's remember that you have cherry red near molten glass very close to the gun during button sealing. A dark grid cup can't be considered a factor about the health of the tube, since it could mean anything from high heat during rebuild to extended use of a tube brightener or even the bomber heating the gun during bake out. Which one you'll never know, unless of course the brightener is still hanging off the neck.

Bluing of the cup pertains to oxidation of the metal. That would suggest that the tube is gassy. Don't forget that fresh rebuilds are bombed, and the entire
gun assembly becomes cherry red. If there were air in the tube, the whole gun assembly would turn blue.

All academic until you actually test the tube. Hopefully it's a good tube and will provide years of service. Finger's crossed.

-Steve D.

The tube is at atmospheric pressure during button sealing, so you're right about that. :D