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I have a friend who wants to run his JCM 900 with 6550s instead of the stock 5881s (6L6). Would that require anything special other than biasing the amp of course.
The 6550 tubes will pull 1.8 amps each of heater current. A 5881 pulls .9 amps per tube. This is twice the load of the design spec of the power trans heater winding. Your bud is likely to wind up needing a new power transformer. If he wants tighter tone he might try a very clean 6L6 such as the Svetlana 6L6GC or the Tesla/JJ 6L6GC.