Tung Sol 12ax7 robust enough for phase inverter?

Discussion in 'Glowing Bottle Tube Amp Forum' started by Les H, Jul 15, 2018.

  1. Les H

    Les H Tele-Holic

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    I have read that the Tung Sol 12ax7 is a finicky tube that will die fairly quickly in the cathode follower position which I found out first hand before reading that. I was curious about its durability in the phase inverter section. I really like that tube in the phase inverter slot in my amp along with one in V1. Hard to explain but the difference in my amp was obvious.

    I order my tubes from Doug's Tubes and he didn't think it would be a problem but he's never heard of anyone using that tube in that slot when I talked to him.
     
  2. archetype

    archetype Fiend of Leo's

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    I would think that the PI position would wear a tube more than any of the positions ahead of it.

    What amp do you have? The 12AX7 isn't typically used in a push-pull PI, where a 12AT7 does the job it's supposed to do. If you have a push-pull circuit, an NOS 12AT7 can be had cheaply and will likely outlast you. In a split-phase PI, the gain is supposed to be 1 in some circuits, but in my experience one of the triodes does matter if it's used as a driver stage. A 12AX7, 5751, or 12AY7 is sometimes used for more gain at that stage.
     
  3. Les H

    Les H Tele-Holic

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    The amp is a carvin belair and has 12ax7s throughout the whole preamp. The tung sol in the PI adds some brilliance to the soak channel oddly enough.
     
  4. JD0x0

    JD0x0 Poster Extraordinaire

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    PI's usually don't run the tubes terribly hard, and I've used Tungsols in TONS of PI's I'm genuinely surprised your tube source never heard of anyone using one here.


    It really depends on the voltages, but even run near max it shouldn't be an issue. The cathode follower is an issue with (not just Tung sols) but many tubes due to the high voltage on the cathode, which will apparently damage the cathode over time. Most PI have under 100V on the cathode, so it's generally much less of an issue.

    I can't really see a 12AX7 being overworked unless the slot was biased for a high current driver tube (like a 12AU7), instead of a 12AX7
     
  5. JD0x0

    JD0x0 Poster Extraordinaire

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    Huh? 12AX7 is arguably the most popular PI tube type. Some older Fenders and stuff used AT7's, which IMO, can work better, but there's literally hundreds of amp designs successfully using 12AX7's in that position, as a PI for a Push/Pull power amp..
     
  6. Inglese

    Inglese Tele-Meister

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    The only problem with cathode follower I'm aware of is due to cathode-heater voltage limit but it's a well know issue for tubes and it's specified in datasheet.
    All tubes manufacturer states the same spec as found in original datasheet but who knows if you can rely on them?
     
  7. Les H

    Les H Tele-Holic

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    I may have misspoke. I don't think anyone has inquired about the tung sol in that position like I did. I don't know the ins and outs of the preamp stages or any stages for that matter so the term cathode follower means nothing to me as I don't know what it is or where it's at in the circuitry. So I asked him about the tung sol in the PI position and he said it would be fine but nobody has asked him specifically about that tube in that position before. Most people use that tube for V1 specifically and generally prefer the long plate Sovtek in the PI which I have tried and didn't necessarily feel the Hype in any position to be honest.
     
  8. archetype

    archetype Fiend of Leo's

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    You could be right. It was my understanding that Fender’s classic designs derived from Western Electric and RCA circuits that used the 12AT7 for push-pull.
     
  9. uriah1

    uriah1 Telefied Gold Supporter

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    12AT can handle PI better. IMHO
     
  10. zombiwoof

    zombiwoof Tele-Afflicted

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    My original Princeton Reverb ('65) has the circuit that calls for a 12AX7 in the PI spot.
    Al
     
  11. archetype

    archetype Fiend of Leo's

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    AFAIK the 12AX7 is spec’d for that position because one of the 2 triodes is used as a driver and the other triode is a PI.
     
  12. Andy B

    Andy B Friend of Leo's Silver Supporter

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    Try a Sovtek 12AX7LPS as a PI. The hi-fi guys love them. I've been using them for a PI for the last year or so in my own amps and repairs, everyone loves the sound.
     
    Last edited: Jul 16, 2018
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  13. uriah1

    uriah1 Telefied Gold Supporter

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    Ah..then no 12at

    Long plate 12AX7 sb good in there.
     
  14. bluesholyman

    bluesholyman Friend of Leo's

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    I have used a TS 12ax7 in the PI slot on a Vox TB18C1...it started to give me problems there so I pulled it out and went with a Sovtek 12ax7lps. A decent modern production tube you might look at is a JJ ECC803 (yes, 803) - their long plate version of an ECC83/12ax7.

    I like Tung-sol tubes in certain positions, but not in the PI slot - I personally don't think it will survive too long...

    Agreed...
     
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