Mitchum or Blackburn for the Mullard 6922?

Do Mitchum production and Blackburn production sound different in the Mullard 6922? I believe I once read somewhere that Mitchum made tubes for the military, whereas Blackbutn did not, so, presumably, with better materials and/or tighter tolerances or testing. 
i believe all E88CC/6922/CV2492/CV2493 were made at Mitcham factory (code R). Blackburn (code B) made ECC88, though having the Master facility as well. I never traced a Blackburn made E88CC even to back 1961 made ones, if ever made at that factory is very rare and prior 1960.
Mitcham ones after 70 have different construction and sound different.
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Just bought a Blackburn e88cc on Ebay. I'll provide feedback by and by, though without a Mitchum to compare.
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Branded IEC Mullard?
B5H3 is only the manufacturing date and plant, should have more digits for type of tube before those.
Are these numbers etched or same lettering as Mullard?
I have seen such tubes on eBay and they are Mullard made ones but not sure if rebranded or not.

I'll look for the other numbers in a while. The tube's fermenting in my dac at the moment and sounds rich, detailed, 3-dimensional and quiet. 
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He's just a little north-west of Croydon, between the A23 and the A24.  Where the best action movies are made.
Go to and buy a set of their super low noise tubes and you will never look back.
The Blackburn Mullard is fully burned in. The weakness of the tube is the anemic bass and midbass, producing an unbalanced sound spectrum. The upper mids and highs are glorious, however, detailed, harmonically layered, 3-dimensional and natural sounding -- among the best I have heard. This tube is great for a narrow range of music (in my system), but not for general use. For instance, a soundtrack of nature sounds from the forests and savannas of Africa is the most layered and natural sounding that have heard thus far. I use this recording as a test for soundstage and imaging. Another downside of the Mullard is a more restricted dynamic range compared to a Siemens 6922. I wonder if the Mitcham production fills in the bottom frequency range, offering as detailed but more balanced sound.
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Tvad... On the contrary, I'm "satisfied" with most of the 6922s I've tried. Each has its own character and I appreciate their respective ways with music. On occasion I change tubes when listening to particular types of music. My generalist tubes thus far are the Amperex USA 6922 D-getter and the early Siemens e88cc. There are still mnay I haven't tried. What fun. Re the Mullard, maybe I got an inferior tube...
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Ah, yes the "magical midrange." I agree. Yet in my Blackburn, the mids and upper mids seem over-emphasized. I prefer to tone it down a notch or up the bass and midbass. Could be a mismatch with my system, I suppose. Does the Mitcham have as much detail and air? 
I use RAM 6922's in my ARC phono amp and they are by far the best tubes I have ever used. 
Wow, it took a long time --150 hrs or so -- for my Mullard e88cc to break in. Now the bass is fleshed out, the highs smoother and soundstage more dimensional. Altogether a more balanced sound. Yes, magical, richly satturated midrange, if without the razor sharp delineation of the Siemens -- a wetter, more romantic sound. It's certainly a keeper, at least for certain kinds of music..