My little vintage Mesa combo started buzzing and making spitting noises, so time for a tube change. I went to my supplier, tube depot in Memphis and got a whole set and spec'ed upgrade to low microphonic and all matched. JJ's by the way, not some super pricey NOS Telefunkins sprinkled with fairy dust.
I didn't take the scientific approach and change one tube at a time to find the trouble maker. I suspect it was a preamp tube in the number 3 position. (it uses 4) Power tubes are EL-84's, though the amp sounds more like a Fender than a Vox. (the AC30 and AC15 use this power tube.....Fender tends to use 6V6. I did leave an EH AX-7 in the #1 spot.....it was fairly new, and I always liked EH preamp tubes, ran 'em my old Conrad-Johnson audio pre.
Holy cow batman. Everything better! Much lower noise floor and much more headroom. I like higher headroom as I get my 'dirt' sound from pedals. Touch sensitivity is fantastic.
Do yourself a favor and retube your amp, especially if they are been in awhile. JJ's are fairly cheap too. , My amp tech (hey it's 20 years old and need a bit of work last year) said JJ's are one of the most consistent brands of current production tubes, and uses exclusively unless customer request something else.
Ok, I'm gonna go rock out now.