That MOV/GEC KT66 data sheet is dated 1963, which is fairly recent.
This 7581A data sheet states that the type is a unilateral replacement for the 7581 and 6L6GC. I read the remark as stating it's OK to use the more capable type in locations calling for other variants. With the exception of the KT66, the 6L6 variants all draw 900 mA. of heater current. At 1.3 A. of heater current, the KT66 is "the odd man out".
This 7581A data sheet states that the type is a unilateral replacement for the 7581 and 6L6GC. I read the remark as stating it's OK to use the more capable type in locations calling for other variants. With the exception of the KT66, the 6L6 variants all draw 900 mA. of heater current. At 1.3 A. of heater current, the KT66 is "the odd man out".
I found a datasheet showing 30W dissipation for the 7581, it showed identical specs to the 6L6GC. It states it has different zero bias characteristics.
http://www.tungsol.com/specs/7581-tung-sol.pdf
The "A" version has the 35W plate and came later.
http://www.tungsol.com/specs/7581-tung-sol.pdf
The "A" version has the 35W plate and came later.
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