Audiofanboy
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More EF91 goodness
Thorn/AEI CV138 KB/EN, made in 1951 in Sunderland, grey glass, tubes were a few months apart, one had dark grey military markings, the other one off-white, but markings were the same; same construction. These have a fairly wide soundstage, with some audible reverb that makes everything sound like 80s music (like a large room DSP effect). They have a very unusual sound signature, like a laid-back EF91 with unobtrusive bass and mids. The treble lacks the shimmering quality of the best EF91 tubes, and they aren't quite as resolving. Again, like the Brimar, the bass has a rather specific and unusual signature, that -might- work well with jazz. Overall, they should be very forgiving of poor recordings with their weird effects and loose bass.
CV138 KB/FB, made in STC, Footscray in 1951 (?), by Brimar (?), at least that's what google points to. These tubes have silver on the side of the glass (like the Brimar) in different surface coverage. Both tubes have an O-getter, one is medium sized, the other one oversized. These have a pleasant bass, yet another flavor (kind of like the Brimar -again- CV4014, but not as strong) that oscillates between being loose and boomy. Mids are -again, must be a EF91 specificity- unobtrusive. Soundstage is average for an EF91, and highs are tame and rolled-off with just enough detail to feel natural. The frequency response sounds fairly flat for this family of tubes, no U-shaped curve here (no elevated treble, even the bass while fat-sounding isn't even that loose).
MWT CV138 Z77, from letters one the tubes, these could be from late 1958 or 1959 (?). Not perfectly identical in markings but I suspect markings have been erased on one tube since they look and sound the same. One tube has the MWT logo (circle/spiral thingy with MWT) and aforementioned CV138 and Z77, while the other tube does not have the MWT logo. Both have grey glass and O-getters. The highs on these sounds fairly thin and a bit sibilant, mids are unobtrusive if useful at all. Bass is loose-ish with no particular special flavor. Soundstage is average for an EF91 tubes, instrument separation is good and resolution is OK but nothing exceptional. Treble quickly gets annoying on these tubes, hardly a very pleasant listen (which is what I'd read about MWT tubes).
Z77 mystery tubes, one might have been made in 1950 (not really sure for this one?) has a sticky paper "OSRAM" tag on it, other tube might be from 1952 (?) and has no tag, they were probably made later though. Both have silver glass in slightly different amounts and Z77 written in a square, and a two letter code that I assume is a datecode. They look identical but one flashes when switched on (like the late Mullard CV4014 whereas the other one doesn't, so... let's say these are similar. They sound identical though. These have a fairly wide soundstage and a fairly neutral sound. Bass is actually quite realistic for an EF91 tube and not too loose. Mids are OK, treble dominates the presentation just a little bit without being spiky. Just airy enough to feel detailed; but other EF91 tubes do this even better. Still, I could live with these if I had to. Pretty good tube for jazz recordings with having that ultra shimmering over-detailed treble. Pleasant overall.
So, my conclusion is -how surprising- that the EF91 with the most potential are the CV4014, especially the Mullards, but the Brimars call for more investigating; The old CV138 (the ones with 6AM6 also written) are pretty decent too.
Thorn/AEI CV138 KB/EN, made in 1951 in Sunderland, grey glass, tubes were a few months apart, one had dark grey military markings, the other one off-white, but markings were the same; same construction. These have a fairly wide soundstage, with some audible reverb that makes everything sound like 80s music (like a large room DSP effect). They have a very unusual sound signature, like a laid-back EF91 with unobtrusive bass and mids. The treble lacks the shimmering quality of the best EF91 tubes, and they aren't quite as resolving. Again, like the Brimar, the bass has a rather specific and unusual signature, that -might- work well with jazz. Overall, they should be very forgiving of poor recordings with their weird effects and loose bass.
CV138 KB/FB, made in STC, Footscray in 1951 (?), by Brimar (?), at least that's what google points to. These tubes have silver on the side of the glass (like the Brimar) in different surface coverage. Both tubes have an O-getter, one is medium sized, the other one oversized. These have a pleasant bass, yet another flavor (kind of like the Brimar -again- CV4014, but not as strong) that oscillates between being loose and boomy. Mids are -again, must be a EF91 specificity- unobtrusive. Soundstage is average for an EF91, and highs are tame and rolled-off with just enough detail to feel natural. The frequency response sounds fairly flat for this family of tubes, no U-shaped curve here (no elevated treble, even the bass while fat-sounding isn't even that loose).
MWT CV138 Z77, from letters one the tubes, these could be from late 1958 or 1959 (?). Not perfectly identical in markings but I suspect markings have been erased on one tube since they look and sound the same. One tube has the MWT logo (circle/spiral thingy with MWT) and aforementioned CV138 and Z77, while the other tube does not have the MWT logo. Both have grey glass and O-getters. The highs on these sounds fairly thin and a bit sibilant, mids are unobtrusive if useful at all. Bass is loose-ish with no particular special flavor. Soundstage is average for an EF91 tubes, instrument separation is good and resolution is OK but nothing exceptional. Treble quickly gets annoying on these tubes, hardly a very pleasant listen (which is what I'd read about MWT tubes).
Z77 mystery tubes, one might have been made in 1950 (not really sure for this one?) has a sticky paper "OSRAM" tag on it, other tube might be from 1952 (?) and has no tag, they were probably made later though. Both have silver glass in slightly different amounts and Z77 written in a square, and a two letter code that I assume is a datecode. They look identical but one flashes when switched on (like the late Mullard CV4014 whereas the other one doesn't, so... let's say these are similar. They sound identical though. These have a fairly wide soundstage and a fairly neutral sound. Bass is actually quite realistic for an EF91 tube and not too loose. Mids are OK, treble dominates the presentation just a little bit without being spiky. Just airy enough to feel detailed; but other EF91 tubes do this even better. Still, I could live with these if I had to. Pretty good tube for jazz recordings with having that ultra shimmering over-detailed treble. Pleasant overall.
So, my conclusion is -how surprising- that the EF91 with the most potential are the CV4014, especially the Mullards, but the Brimars call for more investigating; The old CV138 (the ones with 6AM6 also written) are pretty decent too.
Just because of the CV4015 being a really good tube... 
