Phase Inverter questions

Discussion in 'Ampage Area' started by calebaaron666, Sep 12, 2018.

  1. calebaaron666

    calebaaron666 Synchromatic

    524
    Aug 15, 2018
    Portland, Maine
    Howdy y’all.
    I traded my Vox night train combo for a 50 watt peavey valveking. First thing, after playing it once, was experiment with different compatable tube strengths. After all kinds of tedious testing and resting, testing and resting, I replaced all three 12ax7’s with 5751’s in V1 and V2 and a 12at7 in the PI. It sounded so damn good! Really! Like, really good for a cheap “entry level” tube amp. I figured this would be the tube set up i’d stick with. With a 5751 in the V1, the natural breakup was awesome! V2 ain’t important to me cuz I don’t bother with the gain channel. I don’t like it.

    Okay, long story long, i switched it on the other day after not playing it for a few days and..... no sound!
    I unplugged it and flipped over and saw the the 12at7 in the phase inverter burnt out. Probably just when i had switched it on.
    I wanna put another one in, but why bother if it’s gonna burn out!
    I used JJ’s.
    Meantime, i put a spare JJ 12ax7 in it’s place, and it still sounds good. Just beefier than with the 12at7.
    Any thoughts or suggestions?
     
    Last edited: Sep 12, 2018
  2. calebaaron666

    calebaaron666 Synchromatic

    524
    Aug 15, 2018
    Portland, Maine
    Nevermind. I see what i did. I took the speaker out and when i put it back in i commected the neg/pos wires backwards (idiot!) and that probably fried it. Coulda been a lot worse!
     
  3. stevo

    stevo Country Gent

    May 1, 2012
    Atlanta
    Neg/Pos on the speakers? That only matters if you have two speakers and need to match phase, but either way it shouldn't fry anything. It's a signal that's going into your speakers, so there really isn't a positive/negative orientation.
     
    drrohle likes this.
  4. drrohle

    drrohle Synchromatic

    725
    Jan 3, 2014
    Hays, KS
    AS stevo says, should not matter. Not even close to the phase inverter. Probably just bad luck with a new tube. I'd try again. The 12AT7 has nearly 7 times the plate current rating as the 12AX7 just not the same gain, which is why it sounds different.
     
    Last edited: Sep 13, 2018
  5. markeebee

    markeebee Country Gent

    The AT should be a little more robust than an AX in the phase inverter position, in theory. First guess is its just a dud tube......but that is a guess.
     
  6. calebaaron666

    calebaaron666 Synchromatic

    524
    Aug 15, 2018
    Portland, Maine
    Oh, well i did have the + attached to the - on the speaker and vice versa, thought that was the culprit.
    Yeah, maybe it was just a crap tube.
    I’ll try another one. Can’t really make a peavey sound like a fender, BUT i can pretend.
    It really did sound killer.
     
  7. swivel

    swivel Gretschie

    216
    May 13, 2018
    PNW
    Probably just a bad AT7. Did it get milky looking? loss of vacuum. BTW, NOS Mil spec AT7's are cheap.
     
    stevo likes this.
  8. Uncle Daddy

    Uncle Daddy Country Gent

    Jan 19, 2012
    Maldon UK
    "Plate dissipation for a 12AX7 is 1.2 w Where the 12AT7 is 2.5 W

    Voltage ratings are also different."

    Once I'd read this, I stopped using the AT 7 anywhere that it wasn't designed to be.
     
  9. calebaaron666

    calebaaron666 Synchromatic

    524
    Aug 15, 2018
    Portland, Maine
    It WAS milky! It was a JJ. I’ll try a better caliber next time.
     
  10. calebaaron666

    calebaaron666 Synchromatic

    524
    Aug 15, 2018
    Portland, Maine
    Fender tube amps use AT7 in the PI. From what i’ve read, they are totally interchangeable with a AX7. I’m pretty sure mine was a dud. The kind men at my local music shop educated me on %’s of power. And let me know specifically what would work in my amp.
     
  11. markeebee

    markeebee Country Gent

    Yeah, the AT will draw more current, so the plate voltage will be correspondingly lower and consequently other valves in the string may get a lower voltage too. In real life, though, its not much of an issue.

    On the other hand, I know that you're far less slapdash than me....
     
  12. stevo

    stevo Country Gent

    May 1, 2012
    Atlanta
    A lot of people seem to love JJ tubes, but that's more often the power tubes. I've not had good experiences with their preamp tubes. What are you getting instead? As Swivel mentioned, Mil Spec NOS AT7s are inexpensive and my experience with mil spec NOS tubes is that they last a very long time but aren't always optimized for tone. But in PI, you don't care about tone, just about longevity and balance depending on your amp's design. So I always try to have a Mil Spect NOS tube in my PI.
     
    calebaaron666 likes this.
  13. calebaaron666

    calebaaron666 Synchromatic

    524
    Aug 15, 2018
    Portland, Maine
    Well, i’ll probably try a NOS.
    I have another JJ in v2, but i really don’t use the gain channel. I may replace it anyway because i don’t want anymore milk.
    In V1 i have a groovetube 5751. Groove tubes have treated me ok over the years.
     
  14. stevo

    stevo Country Gent

    May 1, 2012
    Atlanta
    As an aside, the Tung Sol reissue 12AX7 variant sounds great in V1. It might very well be a re-branded EH or some other Russian tube, but it sounds big and full. I've had three so far - they tend to fail as if on a 6 month schedule, but they really sound good.
     
  15. swivel

    swivel Gretschie

    216
    May 13, 2018
    PNW
    Yep just a failed tube that lost vacuum. It happens to NOS tubes too.
     
  16. loudnlousy

    loudnlousy Friend of Fred

    Age:
    52
    Oct 18, 2015
    Hildesheim, Germany
    No problem with JJs in my preamps here.
    Sounds like bad luck.
     
  17. calebaaron666

    calebaaron666 Synchromatic

    524
    Aug 15, 2018
    Portland, Maine
    Yep, cracked tube. First i’ve ever had do that. Then again, i never used to mess with tubes myself. Always just left stuff well enough alone.
    I picked up a Russian EHX AT7 today at the local shop, and popped it in. The amp is back to sounding how i like it.
     
  18. Synchro

    Synchro The artist formerly known as: Synchro Staff Member

    Jun 2, 2008
    Sorry Vista
    Admin Post
    Should be good to go. The 12a?7 tubes seem to interchange pretty well. I use 12ay7s up front in Fender amps and have for years. The 12at7 P.I. should be in a happy place.
     
    calebaaron666 likes this.
  19. calebaaron666

    calebaaron666 Synchromatic

    524
    Aug 15, 2018
    Portland, Maine
    I’m too chicken to do a 12ay7 in v1. Too much headroom for me. I was thinking of trying one in v2 to see if it’d make the gain channel sound more natural and then maybe i’d use it. I wanna just get a bunch of tubes and switch them all around and see what i get.
     
  20. Synchro

    Synchro The artist formerly known as: Synchro Staff Member

    Jun 2, 2008
    Sorry Vista
    Admin Post
    It’s an easily reversible mod.
     
    calebaaron666 likes this.