Manufactured for the military.
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Timrhu, have you tried the Mullard 12AU7 10 M tubes?, can you elaborate more on the Sylvania 12AU7WA tubes?, what did the y cost, what did you compaire them too?, what is the sound and presatation like?
Not sure about the 10 M designation but I did try pair of Mullards. They were delivered with the Unico and seemed to have a very smooth sound, kind of what I expected tubes to sound like.
The Sylvania tubes were much more lively with more top end bite and detail. They also excelled with bass presentation. As I said, the Sylvania 12AU7WA ended up as my favorite. I don't remember what I paid for them but it was in line with many other NOS 12AU7 tubes I was looking at. And again, this particular pair had not been used.
There were many other 12AU7s auditioned. RCA, GE, Philips and a others I don't recall. Most of these were obviously used but all worked. They were mostly purchased on ebay so the fact they all worked well was nice.