12AT7 balanced for Phase Inverter?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Phishy4, Dec 26, 2009.


  1. Phishy4

    Phishy4 Member

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    So I've read a few places that you want a balanced 12AT7 for your Phase Inverter position in Fender amps. I'm ordering some NOS JAN 12AT7WC's, and was going to get a few of them as I use one in v4 in my Accomplice to calm it down a little, plus I'm getting a '73 Super restored and I figured I could use the other 2 in there. Question is that I see you can order them "balanced" or "matched" or "matched and balanced" for a slight upcharge. What would I want if only one or two of them will be in the PI position in my amps? And if I order them all the same way, it doesn't hurt anything to have balanced 12at7's in other positions does it?

    Tubes in question btw: http://www.tubedepot.com/nos-12at7wc.html
     
  2. Roe

    Roe Member

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    I wound pay extra for a matched tube. just order a few and try which sounds the best.
     
  3. GearHeadFred

    GearHeadFred Member

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    Realize that all this is is hand selection of tubes based on measurements of their performance. Nothing is being done to the tubes.

    I've not heard of "matched" preamp tubes before. (I don't stay on the cutting edge of vacuum tube marketing hype, though). A matched PAIR of output tubes makes sense in a push-pull circuit.. But I'm not sure why you would want/need matched preamp tubes.. Consistent performance when you swap tubes perhaps?

    "Balanced" refers to the two triodes inside the preamp tube. Again, some claim this is important but I don't buy into it personally.

    But the short answer to your question is no, it won't "hurt" anything for the tubes to be matched/balanced, or anything else. The only pain will be felt in your wallet!
     
  4. Wilbur

    Wilbur Guest

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    Balanced triodes are largely snake oil. If the tube is in spec that's good enough. Balanaced triodes are one thing, but is the rest of the amp balanced? Your output tubes and output transformer? How is the PI socket set up? Both sides of the PI socket might not be set up the same any ways depending on the amp.

    It doesn't hurt anything but your wallet to have balanced tubes in any positions in your amps.

    Also, IMO 12AT7s sound like ass in 12AX7 sockets - especially in preamp sockets in front of the PI. They ususally thin out the tone and don't sound anything like a 12AX7. Not talking about the gain factor. Talking about the tone.

    If you want to tame an amp with 12AX7s, try 5751s or 12AY7s.
     
  5. VaughnC

    VaughnC Supporting Member

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    While it's a good idea to order matched power tubes so their idle currents are close, it's a waste of money to pay extra for a balanced PI tube because the circuitry around a PI tube in a guitar amp is rarely balanced. In fact, an unbalanced PI tube just might make the PI stage more balanced....but the bottom line is liking what you hear.
     
  6. Timbre Wolf

    Timbre Wolf GoldMember Supporter Gold Supporting Member

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    Consistent performance in a replacement is one possible use. But HiFi folks buy matched preamp tubes so they can use one each in the left/right channels, and have the same (matched) performance.

    In my opinion, matched pairs of preamp tubes are completely unnecessary in the guitar amp world.

    - Thom
     
  7. Phishy4

    Phishy4 Member

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    That all makes sense. I'll be using the 12AT7's in the reverb driver and phase inverter spots in my super reverb, and then in my Accomplice I'm running one in the V4 (PI) spot where Allen usually has 12AX7's, which helps me get more headroom from a fairly gainy little amp. I do really like 5751's in V1 (used one in my Deluxe Reverb) so I'll probably do one of those in the Allen as well.

    KCANOS seems to really like the Mullard CV4024 alot for a PI or reverb driver, so I may try those. The Jan/Phillips stuff is $20 a tube cheaper and seems to me it would have to be better than the current russian stuff anyway, so maybe that is the way to go. As far as 12AX7's, I'll need to replace them in my Super as well. I've got some 50's Mullards and RCA's around but I know at least a few are microphonic. Do you guys have recommendations for reasonable NOS 12ax7s or new production ones you like? I've heard the new Tung-Sols are good for around $15 each.
     
  8. GearHeadFred

    GearHeadFred Member

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    That's exactly the combo I have in my Super Reverb right now.. CV4024s in V3 and V6, and new Tung-Sols everywhere else. The CV4024 in V3 did improve reverb quality noticeably (just as Mike from KCANOS said they would)
     
  9. Phishy4

    Phishy4 Member

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    I think I'm going to get 3 of the CV4024's and probably the new prod tung sols for the rest if my old mullards and rcas act up in the 12ax7 slots. What do you use for a rectifier? Looks like KCANOS has alot of really reasonable NOS 5U4G/B's (like $28 for GE's I think)?
     
  10. Roe

    Roe Member

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    the Mullard CV4024 is absolutely great in the pi - I use it in my rockford reverb, comet 60, express, and jmp50
     
  11. Phishy4

    Phishy4 Member

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    I just got 3 of the CV4024's (2 for the super reverb and 1 for the Allen) and 2 Tung-Sol 12AX7's for the other positions in the Super to try out (they're cheap enough if I dont' love 'em I won't be heartbroken). Any suggestions for good, reasonably priced NOS 12AX7's? I still need to find a 5U4GB rectifier for the Suepr as well, looks like NOS RCA's are fairly cheap still.
     
  12. Timbre Wolf

    Timbre Wolf GoldMember Supporter Gold Supporting Member

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    The short gray-plate RCA 7025 will be the classic V1 tube for that amp, and will definitely bring a smile to your face. Don't know about reasonably priced - you'll have to shop around.

    - Thom
     
  13. Phishy4

    Phishy4 Member

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    How would these be? http://kcanostubes.com/products/11/ANOS-RCA-12AX7A.htm

    Looks like about $50 for used/tests like new RCA.
     
  14. Timbre Wolf

    Timbre Wolf GoldMember Supporter Gold Supporting Member

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