SVT crackling :(

[ThePoloHobo]

I bought this SVT new about a year and a half ago. Recently I'm starting to hear this sporadic crackling coming through the cab after 20-30 minutes... I'm thinking its a tube out.. but which one? I'd rather not pay for a diagnostic for a bad tube if possible. Help!

 

[JimmyM]

One way I'll do it is to take off the back screen and slide out the preamp, then plug it into a cab and turn it on, then gently tap each tube with a chopstick or similar wooden stick (don't use metal for obvious reasons). Bad tubes often expose themselves that way.