Mullard cv4003 & 12au7 Blackburn

Good deal, @msommers . I've bought from Andy four times and all his tubes have been of outstanding quality.

msommers, Perhaps I shouldn't write this, since you've already spent your money, but IMO the best sounding 12AU7/ECC82/CV4003 is actually a 6CG7 (in 9-pin miniature format) or a 6SN7 (octal base).  Neither of these tubes is in the 12AU7 family but both of them exhibit the electrical characteristics of a 12AU7 and can be substituted, if you know what you're doing with the tube socket.  You just have to fiddle with the filament connections vis a vis the 6CG7/6FQ7. 6SN7 would take a lot of work and probably is not an option.

Amperex 7316 is not exactly a 12AU7 but can be installed into the same socket with no mods and is close enough electrically that there are no problems.  Nice tube, too.  I admit I have a bias; I have never liked the 12AU7 in a gain stage. OK for driver or phase splitter.  If one must, I liked the Amperex Bugleboy Ecc82/12AU7 best amongst a bunch of others I tried in a gain stage, back about a decade or more ago.  Real Bugleboys are scarce now, sadly.

It's no problem honestly. I bought these knowing full well that I might not even like them! Such as it goes in this hobby. I figure with a reptuation like Andy if I got to sell them at least people can know it wasn't from some random person. I'm not much into tinkering with sockets and such anyways. 

What I was really after were Brimar 13D5 (lots on eBay from India!). Apparently Brimar CV4043 are the better version according to a seller on eBay, which might be true, but I actually looked through various documentation available for CV4003 and CV4043 and nothing came back that they were compatible, even trying to decipher datasheets. I'm a complete amateur so I might be out of line but something just didn't seem right.

In the event someone comes along looking for 13D5 info, I asked Brent from Brent Jessee Recordings about this and his thorough, gracious answer was:

"The 13D5 is one of the best British 12AU7 tubes ever made. It was designed from the ground up to be low noise, uniform output, and long life. The CV4043 is a military version of the industrial 6061, which is not compatible with the 12AU7 and is not a recommended substitute.

 

CV tubes are British military spec tubes. They are not designed for audio use but often are very good tubes because they were built to withstand impact and vibration. The 13D5 is still a superior tube. We have them in stock. They are listed on our 12AU7 page if you are interested in singles or pairs."

Even for just four I think they're beyond my budget for such a thing but they could be the best for anyone else looking.