Quick question guys! I just bought a 1968 Ampeg B-15N and I think the power tubes are bad. They are the 6L6 GC's and I want to know if I can just go and buy a pair of matched 6L6 GC's and put them in (the B-15n has a fixed bias...but has it wavered?) or do I need need need to take it to a tech and have them install the new power tubes and check the bias? It has a set of Electro Harmonix 6L6 EH power tubes...so I know that they were changed within the last decade...but I really just want to buy a new set of power amp tubes and just put them in myself without having to pay for a tech and his bench fee. Let me know what you guys think!!! I appreciate it!