Mullard CV4024 12AT7 Tubes

Discussion in 'Amps and Cabs' started by Adam79, Mar 23, 2011.

  1. Adam79

    Adam79 Member

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    I've been looking into buying a CV4024 for the Phase Inverter spot in my preamp. I have a couple questions..

    -What's the difference between a regular Mullard 12AT7 and the CV4024?
    -One tube I've noticed (that has a fair price tag) is an '80s KQDD/K branded Mullard made at the Mitcham factory (the "R" in the code denotes Mitcham, right?). Pictured here:
    [​IMG]

    -I've also saw a Phillips branded Mullard CV4024, but haven't been able to find out it's code; I just know that it's from '78. Here's a picture:
    [​IMG]

    These are two of the more reasonably priced CV4024s I've come across. Any opinions on these two tubes? Is there a certain Mullard CV4024 that is considered to be the one to get?

    Finally, I was reading up on 12AT7 tubes on audiotubes.com, and saw this:
    "The 12AT7 is a high-frequency twin triode in a small 9-pin miniature glass package. This tube has a mu factor of 60, as compared to the higher gain 12AX7 with a factor of 100, and the 12AU7, with a mu of only 20. As you can see, it shares similar pin connections with it's cousins, but these tubes are not interchangeable."

    Is this true? The preamp section in my amp take 12AX7s, but I always thought that you could use a 12AT7 in it's place.

    Thanks,
    -Adam
     
  2. DC1

    DC1 Supporting Member

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    They interchange fine, but the gain changes. What they mean is they are not direct replacements for each other.

    Don't know about those Mullards, but might be worth a try if the price was reasonable.


    dc
     
  3. Jahn

    Jahn Member

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    http://music-electronics-forum.com/t8710/

    from that, it looks like a 12AX7 would be ok as a phase inverter, but a 12AT7 drops the gain in the preamp if it wants to see a 12AX7 (which you might like) but also ups the current (which the amp may not like?)
     
  4. stratovarius

    stratovarius Supporting Member

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    The 4004s, 4024s, etc. are military spec'ed, but should otherwise be the same.
     
  5. inca_roads

    inca_roads Member

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    The 4024 is a later 1980s tube as you know. Quite linear (maybe even bright) in its response...

    A fine phase inverter, but I've lately been digging the earlier Blackburn 1960s Mullard AT7s (ECC81) for the PI spot in my Fenders. Rounder, mid-range bump with more harmonic content...
     
  6. Adam79

    Adam79 Member

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    I found a GE branded Mullard 12AT7:
    [​IMG]
    The code is VF3 B0L3. This means that it's the 3rd 12AT7 production series from the Blackburn plant, 3rd week of December, 1960, right? Can someone explain to me what the production series means. If I was to guess, it would be the third production run of this tube in 1960. Am I close?

    Would this tube be considered superior to the two CV4024s I mentioned at the top of the thread? I know that one person's "superior" is another's "inferior." I guess a better question would be what the sound differences are. I've read from a couple different sources that the Mullards from the '80s aren't even in the same league as the early ones.

    Just to confirm, anything that begins with "CV" (i.e. CV4024, CV4004, etc.) means that it's military spec? Is that the reason why mostly all the CV tubes I've seen have the "WA" suffix (i.e. 12AT7WA)?

    Also, what negative consequences are there when upping the current of the amp by using a 12AT7 where a 12AX7 is expected?

    Thanks,
    -Adam
     
  7. Timbre Wolf

    Timbre Wolf GoldMember Support Group Gold Supporting Member

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    Adam -

    The term "better" is only applicable by you, for whatever suits your taste.

    I have two favorite Mullard 12AT7 types, both from the Mitcham plant ("R" factory code), and I favor them for different reasons

    late-'50s Tk1 type ECC81/12AT7, with square-getter wire... bright, thin tonal character, like most other classic-era 12AT7, but with the most exquisite clarity.

    late-'70s/early-'80s 654 type CV4024/M8162/12AT7WA, with round getter wire... fattest-tone of any 12AT7 I've ever tried (that is many), with a kind of smooth, viscous liquid feel. Good clarity, but less than the Tk1 above.

    You choose which one you feel would best suit your taste. The CV4024 is more common, and less expensive. Take note: there are '60s 654 types, made in Mitcham, that are not like the last production ('70s/'80s) version, but are more akin to all other 12AT7 (tonally thin, bright). The last-production CV4024 really stands apart from the rest, if you like that fatter character.

    - Thom
     
  8. Roe

    Roe Member

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    yes, its true that some are fatter than others
     
  9. Adam79

    Adam79 Member

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    My goal is to improve the amp's clean sound without sacrificing it's brutally heavy distortion. From what I've read about how phase inverter tubes interact with an amp, I'm pretty confident I can achieve this with the right PI tube.

    I was reading an article on phase inverter tubes by Myles Rose.. his conclusion was this:

    "The bottom line here is simple. The phase inverter is one of the most important tubes in your amp and the hardest worked tube in the preamp section of your amp. It is how this tube breaks down that provides your output stage distortion tone, character, and amp feel."

    I had a Mullard 12AX7A in the PI position in an old Music Man HD130 amp I recently sold. It turned a boring clean sound into that classic Fender gangley chimey tone. Looking back, I should have replaced the tube before selling it. Although I probably wouldn't have gotten as much as I did without that tube included..

    Maybe this means that I don't necessarily need a 12AT7 in the PI spot. I think I'm gonna have to find the tube locally, so I can hear it in my amp (to make sure it does what I want), instead of blindly buying it on eBay. The PI position can be a tricky one to fill.

    -Adam
     
  10. Deadduck

    Deadduck Member

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    I've got several of the KQDD code tubes that I got from Mike at KCA, and I really like them in the PI slot of a Deluxe Reverb.
     
  11. Timbre Wolf

    Timbre Wolf GoldMember Support Group Gold Supporting Member

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    Adam - you're just going to have to try a few swaps, find out what you prefer, and adjust from there. Check out the feel, as well as the sound.

    I highly recommend the fatter-sounding late version of Mullard CV4024; they're not too expensive, and you will probably be able to use it somewhere, even if not in the PI. That will help you understand if you like a 12AT7 in the PI at all, or if you want a 12AX7. Who knows - you may end up with a 5751!

    Best of luck!

    - Thom
     
  12. DC1

    DC1 Supporting Member

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    Hey Thom,

    Do you know of a good source for the late version Mullard you describe above? My Hiwatt has been asking for one...

    dc
     
  13. Timbre Wolf

    Timbre Wolf GoldMember Support Group Gold Supporting Member

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    There are many possible sources - I believe TGP dealers Doug's Tubes and KCA NOS Tubes both carry these at a reasonable price, for example. Look for the white military boxed CV4024 with the "KQDD/K" and the black plastic pin protector.

    - Thom
     
  14. DC1

    DC1 Supporting Member

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    thanks!

    dc
     
  15. espinete

    espinete Member

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    Finally i tested that, Timbre Wolf. I completly AGREE !!
     
  16. Timbre Wolf

    Timbre Wolf GoldMember Support Group Gold Supporting Member

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    Right on! :beer

    - Thom
     
  17. studiodunn

    studiodunn Supporting Member

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    A CV4024 is PI heaven for me.
     
  18. seattleman1969

    seattleman1969 Member

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    Spot on description. I just put one of these in my PI coupled with a NOS JAN GE 5751 in my V1 and it tamed my out of control OD and really rounded out my sound, far more than I expected. So much so that I am going to, for the moment, forgo any other mods I had planned and play it for a while!
     
  19. Timbre Wolf

    Timbre Wolf GoldMember Support Group Gold Supporting Member

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    Glad to hear it !:aok
     
  20. Adam79

    Adam79 Member

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    What type of amp you using? My Peavey VTM-120 calls for all 12ax7s. I rolled a lot of 12AT7s and 12AU7s to tame down the sound; a '50s CBS Hytron "JHY" label in the PI slot was a game changer.. the amp is no longer a one trick ("hi-gain") pony. Leaves all the Mullards, Brimars, Telefunkens, etc. I've tried in it's wake. The '60s Sylvania black long plate in the v1 also helps things. I switch between that and a Raytheon 5751 windmill getter; one of the most underrated tubes out there.

    I got Mullard 12ax7 f91, f92, MC1s, Telefunken 12ax7 smooth long plate diamond bottom, as well as 12AT7 Mullards (one '60s, one '80s (NIB/NOS) for sale. PM me if interested; I'll hook ya up.

    -Adam
     

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