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Phase inverter, 12AT7 or 12AX7?

Discussion in 'Amp Central Station' started by JohnnyCrash, Jun 15, 2019.

  1. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

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    What PI tube do you use? Why? What differences do you hear (feel free to be as descriptive as possible).

    I just buttoned up an amp build, essentially a blackface type Super Reverb...

    It currently has a 12AT7 tube in the phase inverter socket, like the old ‘60s Fender schematics call for. Sounds big and clean, but not necessarily as warm or dynamic as some people prefer (opinions being subjective and all of that).

    I’m primarily using this amp for vintage cleans (soul, surf, country, rockabilly), so I’m not necessarily interested in grinding gain (I have plenty of other amps for that).
     
  2. ahiddentableau

    ahiddentableau Tele-Meister

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    I'm pretty sure this is entirely a matter of personal preference. I usually prefer the 12AX7 because it usually produces more power amp and OT saturation and therefore more compression. I like that. But if you want a harder, cleaner sort of sound, then by all means stick with the 12AT7.

    I also find tube rolling can make a noticable difference on PI tubes. More so that any other tube other than V1. Oddly, one of my favs is the Sovtek 12AX7LPS. I don't like Sovtek tubes in general but I really like that one for the PI. Lots of compression.
     
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  3. Dacious

    Dacious Poster Extraordinaire

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    I find using a 12AX7 in that slot sounds warm but it has unwanted artefacts in the reverb on a BF style amp.

    +1 on using a quality AT7 in that slot. Not balanced but solid.
     
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  4. BobbyZ

    BobbyZ Doctor of Teleocity

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    Try a few of each and see what you prefer. Used to put 12ax7s into blackface type Fenders as a matter of course think I'm pretty much back to 12at7s now.
     
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  5. MickM

    MickM Poster Extraordinaire Platinum Supporter

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    Glad to see you're back around JC. By now I bet you been shocked enough times to really know what yer doin. AT7 cause that's what some smart amp guy told me 'bout ten years ago. Still got the Sig too, cool.
     
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  6. JD0x0

    JD0x0 Poster Extraordinaire

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    AT7.

    Why? Well I can say more than "I like it"

    The 12AT7 has some advantages. It can drive more current than a 12AX7, and the output impedance is significantly lower.

    If you use this calculator, swap out AT7 and AX7, you see in the Phase inverter, the difference in gain is completely negligible, it barely changes.. (come on, raise your hand if you thought the AX7 gave a big gain boost ;) ) https://www.ampbooks.com/mobile/amplifier-calculators/long-tailed-pair/


    The reduced output impedance improves the bass response and 'feel' of the amp IME. You can really whack the amp with a ton of bass, and it holds together very well. Much less chances for blocking distortion (ie. 'Farting out')
     
  7. bftfender

    bftfender Poster Extraordinaire

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    GZ34/5AR4
     
  8. brokenbones

    brokenbones Tele-Holic

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    I read somewhere the noisiest tube you have should be in the PI spot.
     
  9. JD0x0

    JD0x0 Poster Extraordinaire

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    That's a rectifier. Nothing to do with the phase inverter.

    Not quite.. It's just that the Phase inverter is one of the last stages in the amp, so it's one of the least sensitive stages to noise.
     
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  10. bftfender

    bftfender Poster Extraordinaire

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    ooppss..12ax7 then..got em mixed up. Black flag has SS & tube rec pretty cool to flip that switch
     
  11. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

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    This is probably why Leo changed from tweed era 12AX7 phase inverters to 12AT7s in the first place (and therefore NOT into the Marshall gene pool via the Bassman/JTM-45 connection).

    I really do appreciate this amp’s controlled and very low bass response. I can get fat twang from it on a Tele’s low E string. I imagine it would easily be happy with a baritone.

    Which brings me to another misconception: the idea that 10”s don’t have as much bass as 12”s. Like bass cabs aren’t loaded with 8”s and 10”s.

    This Eminence 1058 has a nice and wide frequency response.
     
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  12. kbold

    kbold Friend of Leo's

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    The phase inverter in my amp is a 6922. A better choice for PI in my opinion, but a different genetic pool to the 12A*7 series.

    In your amp, I would favour a 12AT7 over 12AX7 for a phase inverter.
    12AT7's have more output power to drive the power tubes than a 12AX7.
     
  13. BobbyZ

    BobbyZ Doctor of Teleocity

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    Also think people believe 15"s have no treble.
     
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  14. Greg_L

    Greg_L Tele-Holic

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    That's a pretty cool trick.
     
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  15. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

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    I have a new Jensen (“reissue,” not really) P15N 15” that sounds EPIC.

    I use it with a 40-50 watt tweed type amp head (essentially a tweed Twin/Pro circuit).

    It’s not a true and exact reissue, but it is one of the nicest sounding new AlNiCo speakers I’ve heard. One of my favorite speakers!
     
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  16. kbold

    kbold Friend of Leo's

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    Or: Phil Jones Bass cabinets, using 5" speakers

    Or: the Bose 901-II speakers in my stereo, chock full of 4" speakers which can output an obscene level of bass if I choose to.
     
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  17. Nick Fanis

    Nick Fanis Poster Extraordinaire Ad Free Member

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    12AT7
    I need clean low E and A strings especially on the low wattage amps I use.
     
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  18. kbold

    kbold Friend of Leo's

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    I like to have a clean G string as well.
     
  19. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

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    :D
     
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  20. 11 Gauge

    11 Gauge Doctor of Teleocity

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    Not only do I insist on a 12AT7 for the BF/SF amps that came with one, but it's also an instance where I'll splurge and spend a few extra bucks for a NOS Sylvania or Philips. The last time I bought any (last year, IIRC), the price wasn't that much higher than current production. I have a Sovtek 12AT7 or two that aren't bad, but the NOS ones seem to be a bit better, IMO.

    Along with the tighter low end vs. a 12AT7, I like to tweak the circuitry at the PI and power tube inputs, especially if it's a lower powered BF/SF amp (DR being the best example). Increasing the PI's tail resistor value and decreasing the power tubes' grid leak resistors seems to have the most positive effect.

    ...I (heavily) tweaked an AB165 and did the above changes to the PI and power tube inputs. Even when running lower output power tubes like Tung Sol 5881 RIs, it's still got a fantastic amount of clean on tap. I couldn't ever imagine the need for something like a Twin Reverb, with its quad of power tubes, ever again (unless someone personally insisted on the need for internal reverb and trem).
     
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