Most tube stores will help you get mesa "spec'd tubes .
Here is the GT to mesa spec chart .
The bottom line here is;
Lower numbers will start to distort at lower volume settings on your amp, and have more dynamic touch and feel.
These are the GT 1-3 range, and the Fender "blue" painted tubes. Many Jazz or Blues players like this range, and
they work well for recording where levels are going to be lower, or in smaller venues.
Medium numbers are the most versatile, will drop into most amps and be very close to most factories bias settings,
and are the best general choice for most players wanting a versatile well rounded amp. These are the GT 4-7
range, and the Fender "white" painted tubes. The range of 4,5,6 are able to be used in Mesa Boogie fixed bias
amps, and on the Mesa scale convert roughly to:
Mesa scale Groove Tubes scale
Red 4
Yellow 4
Green 5
Gray 5
Blue 6
White 6
Nice to keep handy
/cheers
