Mullard cv4003 & 12au7 Blackburn

msummers, 12AU7 and CV4003 are two different names for the same tube. 12AU7 is the American designation and CV4003 is the British designation. ECC82 is the European designation. There are also many variations with other names, such as the Amperex 7316 or the Mullard M8136, that are basically the same tube.

Most Mullards were made at the Blackburn plant, with the Mitcham plant coming in second. Tubes made at other Phillips factories were often labelled Mullard, though.

Brent Jessee has a lot of information about tubes on his website: audiotubes.com, including how to read Phillips tube codes (see the yellow box entitled "Euro Tube Codes"), which will tell you where and when your tube was made, whatever it is labelled.

Check out his website and if you have more questions, which you probably will, feel free to ask. I would start out by reading up on the 12AU7 by clicking on the the yellow box with that title.

I spoke with Andy at Vintage Tube at length regarding Mullard, since I was looking for the very best, most holographic tube that Mullard offered. He stated that the ECC82/12au7 Boxplate and the Longplate O-Getter 1950's had the finest sonics and measured as lowest noise.

The CV4003, also Blackburn, is not as extended and he thinks is a lesser grade tube; (1970's). Kevin at Upscale disagrees.
Google Joe's Tube Lore and read his findings after testing many brands and tube types.

In addition, Brimar has similar sonics to Mullard...warm, liquid mids and a smooth top-end. I have various 1950's Mullard Longplates, the low noise sound exceptional.
But I found that the Brimar CV4003 have excellent depth, and upper and lower extension.

Edit...My bad, in looking thru my notes, it is the cv4004/12AX7 that Andy said was a lower grade Mullard. The CV4003 is a military spec box plate.

Thanks for all the info,Lowrider!