What is the difference, and how do I tell what I have? I am going to change all the tubes in my Marshall. I am going to put the Tung Sol 12AX7's in the preamp. They say there are real good for a linestage preamp but not for a phono stage.

Does anyone know what I have in my JCM2000 DSL100 head, or how I can tell?

I know this might be a stupid question, I am still learning.
Thank you.
its not a stupid question cos i dont have a clue am interested in finding out though