I almost have enough money to retube my Marshall JCM900 High-gain Dual Reverb 50watt head. This is my first tube amp and my first marshall and i really want this amp to have a good texas/chicago blues sound while keeping a decent rock sound....so I guess the first thing i should do is retube it because from what ive herd it hasent been retubed in a long time so i what would i want to put in it? i might be willing to pay for extra modifications if need be
(I have a les paul..if that matters)

Just in case you need it.......

I'm in a similar boat. My 4501 needs some different power tubes, I think. I'm looking at JJ KT77's, and I might be waiting for the new TechTube 12ax7's to come out.
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gear in profile
For a Texas sound you're looking more towards Fender design with 6L6 outputs. Your Marshall is a fine amplifier but not the first choice for this type of genre.
As for re-tubing, good quality tubes can be ridiculously priced. Believe it or not some may still be perfectly fine even if old. If you can, try to find somewhere/someone who can test your amp's tubes first and concentrate on replacing the bad ones before just changing them all unless you're loaded with extra cash. As for mod's, tred carefully. There's more to changing tubes than just biasing!
Moving on.....