Running a Vox AC15c1 with an unbalanced PI tube

Discussion in 'Glowing Bottle Tube Amp Forum' started by Dune736, Feb 22, 2020.

  1. Dune736

    Dune736 TDPRI Member

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    Picked up a used Vox AC15c1 for a great price. The amp still had the stock Chinese tubes, and they were on the way out. Ordered a JJ tube kit for my Vox from Tubedepot, but none of the 12ax7 were marked as the balanced tube. If I plug in a unbalanced 12ax7 for the phase inverter, could that cause issues with the amp?
    Thanks for any info.
     
  2. muchxs

    muchxs Doctor of Teleocity

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    It won't hurt a thing. You might not notice a difference.
     
  3. archetype

    archetype Fiend of Leo's

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    We've been running PI tubes with unbalanced triodes since guitar amps used PI tubes. The same with unmatched output tubes. No worries.
     
    Fretting out likes this.
  4. gusfinley

    gusfinley Tele-Holic

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    Even if you had a phase inverter tube with balanced triodes and balanced output tubes, there would likely be imbalances in the plate load resistors, bias resistors, coupling caps and even the output transformer.

    I sometimes use this to my advantage to balance the output by matching the output tube with the highest current draw with the OT primary winding with the lowest resistance.
     
  5. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    What @gusfinley said - balanced PIs are a myth in guitar amps. In fancy hi-fi land they definitely take the time to make sure resistors, output transformers, and whatever else are closely matched. In guitar land, close counts, throw it in, and it's likely going to sound just fine.
     
    tubedood likes this.
  6. 3-Chord-Genius

    3-Chord-Genius Poster Extraordinaire

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    I've heard a lot of things, but I've never heard of that being a problem.
     
  7. SPUDCASTER

    SPUDCASTER Poster Extraordinaire Silver Supporter

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    Good to know. Been wondering about this for a while.

    Happened on to some old Mullards a while back. Left the balanced JJ in.

    Will now switch that out. TDPRI comes though again!;)
     
  8. muchxs

    muchxs Doctor of Teleocity

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    My take on "balanced" tubes:

    Hi-fi guys like 'em because balanced tubes make for balanced channels on their hi-fi rigs.

    About the only place they make sense for guitar players is in the PI. Not all amps are built balanced, anyway.

    New production tubes drift so fast and so far that fancy "balanced" tube you paid extra for over at Ream Ola tubes probably won't stay balanced for long.

    That has been my experience with so- called "matched" pairs these days. With few exceptions, matched pairs aren't matched like they used to match 'em and they sure don't stay matched for very long.

    If you had a slight imbalance in a 12AX7 and a slight mismatch in your power amp tubes you could tube roll your way out of it. If you're monitoring power just move your tubes around until you get a slight increase in output. That's where your tubes are "happy". It shouldn't take too many tries.

    A slight imbalance may make for richer tone albeit at very slightly diminished output.
     
    SPUDCASTER likes this.
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