see this link for descriptions http://www.vacuumtube.com/FAQ.htm
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Hi. The 12AU7 and 12AY7 are both dual triode tubes. They are not the same tubes. Characteristics are not the same: transconductance, etc. If memory serves me, the AU dissipates quite a bit more than the AY. Get a copy of the RCA Tube Manual (RC-19) from Antique Electronic Supply and look at the stats on each. As far as Mullard is concerned, they may have made a 12AY7, but virtually all the US made versions were manufactured by General Electric. You can tell from the number etched on the glass.