[Ron Ramirez]

I'm frankly surprised the 6L6G isn't in your manual for the reasons I mentioned in my previous post (used in 38-116, 38-690, 42-1015 and 42-1016).

You may sub the 6L6G with 6L6GA, 6L6GB, 6L6GC. Also 6L6WGC, 5881, or 7581 (the 7581 is equivalent to a 6L6GB; the 7581A is equivalent to a 6L6GC).

The KT66 has a higher filament current (1.27A) than the 6L6 series (0.9A).

The 6L6G has the largest envelope. 6L6GA is also taper-top or "Coke bottle" shaped but smaller than 6L6G. Starting with 6L6GB, the envelope became tubular, tapered at the bottom just above the base.