As promised, and after getting a metric ton of tubes in the mail, here are my thoughts on different EF91 tubes.
Mullard CV4014 KB/D 6064 made in Mitcham, O-getter and open structure below getter, blue side glass and inside visible (no cage like element between anodes). Both tubes are identical in construction and flash when turned on (they dim shortly afterwards), one only has military markings and was made in 1972, the other also has the old Mullard logo and M8083 and was made in 1968. They sound exactly the same. They have a very large soundstage, a very airy treble forward presentation with great instrument separation. Highs can be a bit spiky and borderline sibilant but are always extremely natural and spot on for vocals. Bass has weight and quantity despite being quite loose (but if it's in the song, you'll definitely hear it loud). Mids are fine but not exceptional. This tube feels like a looser bass-ed CV4015 with more space and treble, useful for specific types of music. They really wake up the HD650, but are a bit spiky on the HE-400 (and bassy). My impressions match what's on the first page.
Another Mullard CV4014 KB/D 6064, made in Mitcham in 1958, identical tubes with same date code, blue glass, a cage like element between the anodes, D getters with a cage underneath; they don't flash when turned on. These sound quite neutral, balanced across the frequency range, and quite tame for EF91 tubes. Highs are not spiky or sibilant, detailed but not spectacular like other EF91 tubes; at least the highs aren't fatiguing or painful. Instrument separation and resolution are very good (maybe not quite as good as the later CV4014 reviewed above, but it's hard to tell without ABing immediately). Mids and mid-bass are pretty good and don't detract from the rest. Bass isn't too loose and just rumbly enough to be entertaining. Overall, this is a very versatile tube that sounds like a tame crossover between an EF92 and EF91 (which is weird since the CV4014 from '56 I tried and reviewed two weeks ago weren't like that at all and had the treble-centric triangle presentation I hate, maybe factory improvements or better burn-in? Those were the first production years for CV4014, so different batches with different sounds are to be expected). Another pair of tubes I could live with if I had to!
Brimar CV4014 (only markings on the tube with "Made in England B.V.A." and indecipherable factory codes), silver glass all around the anodes that hides the internals, identical O-getter tubes, I'd assume from the getter that these are 60's production or later. These are different from anything else I've heard before. The bass is quite extreme and other-worldly. Sure it reflects what's in the music, but it renders the bass in such an interesting and fun way that you just want to keep on listening. Although the bass may or may not be perfectly hi-fi like (debatable), it is still very well controlled and powerful (drives the schiit out of the HE-400 and makes listening to the HD650 an absolute riot!). The treble is very detailed and airy; still a bit spiky on the HifiMAN but perfect on the Senns, pleasant overall (doesn't make my ears bleed), not quite as refined as the CV4014 though. Mids could be a bit more emphasized but don't detract from the global presentation; this gives these tubes a fun U-shaped sound signature. The soundstage seems boundless which pairs very well with the extreme bass that flies in all directions (think fun fat controlled bass like the CV4010/M8100 but in a grown-up version, not perfect but fun). One of the easiest tubes to listen to in the EF91 family in my opinion (and many EF91 tubes are hard to listen to for over 30 minutes...), great listen on the HD650, awesome bass kick. This is one pair I'm sure to burn in a bit.
Mullard CV138 KB/D, made in Mitcham in 1952, blue glass, D getter with an open top part (flying copper leads and black rectangle below the getter), no cage between anodes, internals look like the basic Mullard EF91 of the same time period. They sound the same too, that is SS-like and a bit sterile. Detailed and over-emphasized highs that quickly get grating. Recessed mids and thin loose bass. They might improve from a long hard burn in like many basic Mullard EF91s do but knowing the results in advance, I don't feel like it; the sound signature wouldn't change that much anyway.
Mullard CV138 6AM6 KB/D, made in Mitcham (no factory code but KB/D) in 1954, blue glass, cage element between anodes, D getter on top of an open part. These are like the (noisy) CV138 I reviewed 2 weeks ago, but they seem to have had an easier life (they may be NOS or close to new). They have an extremely wide soundstage. The bass is loose, slightly punchy, pleasant and unobtrusive. The presentation is basically treble-centric, but not sibilant; mids are a bit recessed. Overall, these tubes are fairly detailed, but not quite as much as the best EF91 tubes. Again, I could probably live with these if necessary, and a bit of burn-in might help.
That's it for now, there's a few more pairs I've tested, I'll post impressions tomorrow. I have to admit these EF91 really have quite a lot of treble. After a whole day of listening to different tubes, my ears feel like they're just about ready to give out, and my head kind of hurts... This is when I miss the 6AK5 love a little.
Edited by Audiofanboy - 1/30/13 at 6:15am