I recently picked up a tube tester and a GE 6LB6 tube was inside. What would this be worth?

6LB6 is a Horizntal Output tube used in mainly color TVs. It has value to the right person. Your tube looks new. AES is asking $36.

Well since you have a tube tester, test it already. Worth more alive than dead.

Agree, old sweep tubes are used in some amateur transmitters, and supplies are drying up.

I would test it but the tester I bought doesn't have the appropriate socket to do so. That and said tester needs to be repaired.

Should have a nice silver area on back of the tube. This tube looks new. If used very long glass will darken and silver area will change color. Your tube matches the carton and has clean lettering.

Looks brand spankin new to me. Someday we will wish more of these were saved for audio amps