how important is the phase inverter tube for the overall sound?

Discussion in 'Glowing Bottle Tube Amp Forum' started by braveheart, Jun 16, 2019.

  1. mabley123

    mabley123 Friend of Leo's

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  2. 3-Chord-Genius

    3-Chord-Genius Poster Extraordinaire

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    I thought those rack mount tube power amps had tube phase inverters.
     
  3. Frodebro

    Frodebro Doctor of Teleocity Ad Free Member

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    Both of mine do (Peavey Classic 50/50 and Marshall 9100).
     
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  4. archetype

    archetype Fiend of Leo's Silver Supporter

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    I just looked at a Musicmaster Bass schematic and that's one of the wackiest things I've seen. I can understand how a center-tapped transformer could function as a PI, but can't identify what's ahead of the transformer that makes it change phase.
     

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  5. JD0x0

    JD0x0 Poster Extraordinaire

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    Depends on the amp and how it's being used.

    If you're using your amp on '3' as a clean pedal platform, it's probably not as big a deal than if you're cranking the amp for output stage distortion. The PI balance alone can have a pretty significant effect on the clipping of the PI
     
  6. Rick330man

    Rick330man Tele-Holic

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    I've posted elsewhere on TDPRI that the 1980's production JAN GE 5751s are my favorite PI tubes. In every amp in which I use them for PI service they do a noticeable job of bringing out a clarity and note detail that I just don't hear with other tubes in PI service. YMMV, but my ears tells me that nothing beats a 1980's JAN GE 5751 as a PI.

    That said, I have heard amps where the PI tube seems neutral when it comes to impacting tone.
     
  7. Dacious

    Dacious Poster Extraordinaire

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    It's actually very clever - the only drawback is you get no gain increase (neither do you from a cathodyne stage like a Princeton/5E3) but you save one tube/socket/associated components and lets them use one dinky P/T for the 12AX7 and 6V6s/6AQ5 heaters.

    If you run signal into the primary from the 2nd preamp tube stage, each lead of the secondary is 180deg apart. Ingenious. It's like an output transformer in reverse, and it doesn't need to pass big current so it can be tiny. It doesn't stop them being great-sounding amps.
     
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  8. Andy B

    Andy B Tele-Afflicted Gold Supporter

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    Sovtek 12AX7LPS is a great PI tube.
     
  9. asnarski

    asnarski TDPRI Member

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    I agree, but can’t them anymore!
     
  10. telemnemonics

    telemnemonics Doctor of Teleocity Ad Free Member

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    What amps exactly?
    Modded non master or modern post non master?
    What I find is that adding only the PPIMV to a vintage Marshall doesn't really get much more crunch at low volume the way a complete "MV" amp does.
    This is AFAIK because vintage non master Marshalls are low gain amps, and distort when turned up above clipping.
    So the low gain clean preamp doesn't really drive the PI into distortion simply by adding the MV.
    When Marshall brought us the MV amps they also cascaded the first two preamp stages that were formerly run parallel, so the distortion at lower volume came from the first gain stage overdriving the second gain stage, then hitting the tone stack recovery, before finally hitting the PI.

    Just put the PPIMV in and leave the low gain preamp stock, and what I hear is a little bit of crunch with regular volume cranked and master down, but not the crunch we get with later Marshalls designed for lower volume dirt not involving the power section.

    Of course plenty of vintage low gain Marshalls that got PPIMV added also got gain added while they were on the bench.
    AFAIK the early Tony Iommi modded Marshalls had the same basic crude cascaded bright and normal channel tube stages that became the Master Model MKII and then the JCM800.
    Many Modded Marshalls got an extra 12ax7 too, but the first mods used the stock three 12ax7 complement.

    The point being that it's not as much the PPIMV that makes the distortion as the added/ cascading gain making preamp distortion.
    And (sometimes) driving the PI into clipping, but it's the gain rather than the MV.
     
    Last edited: Jun 20, 2019
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