sesroh said:
I would start with replacing the power tubes only, since pre-amp tubes generally last much longer in a guitar amp. You will need to get a quad of EL-34's which could cost anywhere from $50 - $100. You'll also need to get the amp biased if you are not able to do that yourself...might be looking at a $30 - $50 bench fee there too (I do my own tech work, so am not really up on those costs)
If any or all of the pre-amp tubes need to be replaced then you are looking at $10 - $15 per tube. I beleive there are 3 12ax7's in that amp.
Worst case looks like about $200.00 assuming you don't want to put pricey NOS tubes in there.