Audio Research Ref 3. 6L6GC or 6550C

Take a look at the Ref 3 section of the arcdb.com database. They state that the 6L6GC was in the earlier models, replaced later by the 6550C. I wouldn't think the two are interchangeable. Be safe and check with Audio Research customer service.

The 6L6 is probably close to the KT66 than the 6550. The 6550 was derived from the earlier 6L6 and has higher output and plate voltage than the 6L6. As Mistycat advised, check with Audio Research because the earlier models might have had different circuitry values which could might not properly handle the 6550.

Thanks for your answers. Mistycat, you know if the sound with the 6550C is much better?