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If you don't have a crt tester, there is an easy way to determine if the tube is very gassy. IT IS A DESTRUCTIVE TEST IF THE TUBE IS VERY GASSY!
1. measure the heater resistance... should be fairly low, like a few ohms.
2. apply 6.3 volts to the heater (Pins 1 & 12)... does it light up normal brightness? if so, the CRT may have a decent vacuum in spite of the crack and "used up" looking getter.
3. if it just barely lights up, it will likely burn out very quickly, perhaps you will see a flash. Tube is very gassy and now has a burned out heater.
4. re-measure the heater resistance... if the tube is very gassy, it is now likely "open".
Here is what happens...the heater resistance is much lower when it is cold than when it is hot. If the tube is gassy, air molecules will transfer heat away from the heater, so it will remain fairly cold. Since the heater resistance stays low, it will draw more current than it is rated for, and open up like a fuse.
Even if the tube is very gassy, continue to handle it carefully, as it still could have a partial vacuum, and be an implosion hazard.
jr
Last edited by jr_tech; 07-19-2010 at 03:24 PM.
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