05-11-2008, 10:28 PM
I think they are exactly the same but I stand to be corrected. I often see XXLs labelled as "XXL/7A4" which wouldn't be the case otherwise. I don't know why the XXL moniker was chosen unless it was for marketing purposes or maybe it had not been "accepted" by the tube-numbering convention at the time it was released into the field.
There's a number of 'merged' numbers in the tube world. As example, in old radios the 39 and 44 were combined since any differences between them were negligible. In that case it was a matter of different manufacturers submitting their new tube somewhat simultaneously and they turned out to be the same for all practical purposes.
12AU6 and 12BA6 are only closely related, not at all the same tube. One is a sharp cutoff and the other is a remote cutoff. Something like that wouldn't normally be noticed except in difficult AVC situations. Then they behave quite differently.
-Bill
There's a number of 'merged' numbers in the tube world. As example, in old radios the 39 and 44 were combined since any differences between them were negligible. In that case it was a matter of different manufacturers submitting their new tube somewhat simultaneously and they turned out to be the same for all practical purposes.
12AU6 and 12BA6 are only closely related, not at all the same tube. One is a sharp cutoff and the other is a remote cutoff. Something like that wouldn't normally be noticed except in difficult AVC situations. Then they behave quite differently.
-Bill
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