I've recently purchased a used 5150 head that has some pretty worn stock tubes(the original owner had it for 15 years or so and never replaced them). So, I purchased a new set of 12ax7's pre-amp tubes and 6l6-R Power tubes, all Groove Tube brand(guitar center stock). Would there be a need for electronic work to the amp itself? I've read online that the pre-amp tubes can be changed without modifications but the power tubes would be a different story. Can anybody help? Thanks!
The preamp tubes can be just dropped in. As long as the amp works you should be able to just drop in the power amp tubes without any issues. Now it is general practice to bias your amp when you put in new power amp tubes but with an amp like the 5150 I believe they are usually biased real cold and don't absolutely have to. Actually I don't even know if you can easily bias a 5150 without opening the amp.
Last edited by Sputnik1 at Jun 21, 2013,
For the 5150, just get 6 12ax7 tubes and put them in. Get what is called a "matched quad" of 6L6GC power tubes and replace all of the power tubes at once. Other than that, unless the amp isn't working it shouldn't need any repair.

What you misunderstood is that preamp tubes can be changed one at a time whereas power tubes should be changed in pairs or quads.

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