Modding a JCM 900 with 6550's

[LesPaul59]

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.

 

[Big Cookie]

BAD Idea

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.

Best Blues,
Cookie

 

[LesPaul59]

Thanks for info, Big Cookie! I will let him know!