Revisions to the original metal 6L6 such as increased power handling and glass envelopes yielded new designations such as 6L6G, 6L6GA, 6L6GB and finally 6L6GC (introduced by GE in the late 1950s). Today, the 6L6GC is one of the most common power tubes used for electric guitar amplification and there is an almost overwhelming supply of current ... More @Wikipedia
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