G-2000 dated 6/4/73 penciled in inspection initials. tube question

Hello people in here! I am so glad this sight exists.
What is the difference between RCA 6L6GB and 6L6GC ? I changed preamp tubes and it quieted it down alot and now the vibrato works great. The amp doesnt sound like 100 watts loud like it used too or maybe it is my ears, I dont know. I am considering changing out power tubes with old tested tubes instead of new groove tubes. All comments apprecaited. I have had this amp since 1979. without evere changing a tube. I know some of you guys used to work for earth. This site has been very helpful.

Anyone claim to have made this amp? It has always worked great. Loudest amp of any band I was in!!

Cheers folks Frank