How to Identify Used Tubes?
Some of the things to look for that indicates that tubes'
have been used are burn marks,
which may appear as dark spots on the silver part of the glass tube,
called the "flashing" of the tube.
The tube’s flashing is the first place to look for signs of use.
Also look for a halo of colors around a spot of flashing.
If you see a tube that has very little flashing,
or any white parts of the flashing,
this indicates a tube with gas,
or a loss of vacuum.
Do not buy tubes with white on the flashing!
Also look for darkening around the base of tubes with light brown bases,
on the base of the tube where it meets the glass.
If the edge of the base looks darker than the rest of the base,
it’s caused by heat, which indicates many hours of use.
Look for scrapes on the tube bases as well that are left from tube holders,
or butterfly clamps that hold the tubes securely in place in amps.
Look for heat cracks in the base as well.
Heat cracks will appear as very fine hairline cracks in the base,
which may be hard to see so look closely.
Heat cracks usually indicate a very well used tube.