Question about Cross-Line PPIMV

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by gproud5150, Jun 2, 2008.

  1. gproud5150

    gproud5150 Member

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    I've been perusing all around the internet for the answer to a simple question regarding the Cross-Line PPIMV. What value/type of pot is used in it? There is a thread on here with the picture of it from John Phillips, but hasn't been on here in quite a while and hasn't responded to a PM I sent him. So if anyone has the answer I would very much appreciate it. Thanks.
     
  2. Swarty

    Swarty Member

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    I think Matchless uses a 1MA, but the few times I've used them I found 500k to be best.
     
  3. gproud5150

    gproud5150 Member

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    Would it be possible to use a dual 500k pot, but only use one side of it? The reason I ask is that I have one sitting around, but if it's advisable to get a single one I'll try to hunt one down in this jerk-water burb I live in. Thanks for the response.
     
  4. Structo

    Structo Member

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    No reason a dual gang wouldn't work just using one pot on it.
    Not familiar with a cross-line.
    Got any links?
     
  5. gproud5150

    gproud5150 Member

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    This is from another thread in here that John Phillips posted this in:

    [​IMG]
     
  6. BenH

    BenH Member

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    It depends on the amp set up as to what works best. Is it fixed bias ? Then don't forget to add two more capacitors to keep the D.C.from the poti and stop you shocking yourself or messing up the bias. If the coupling caps are say .01µ , I'd unsolder the phase inverter side of the caps, pull them up a bit, add caps of double the value - .02µ to where the others was lifted, and join the new .02µ's to the old .01µ's where they are reared up at each other. These two new junction points are where the cross line master volume poti will be joined.
    As to which is the "correct" value for the poti, I'd sugest trial and error untill you find what sounds best to you. I like get the whole useable range of the poti working. If you just stick a 1MA poti in there you will probably find that upwards of six or seven not much is happening in the getting louder department, maybe a bit of tone cancellation will be takeing advantage of large resistance. At the other end of the scale, turning the poti right down leaves only the scratchy leftovers of your likely not perfectly balanced phase inverter, and a hard to controll jump of sound at around 2 and a bit.
    How to solve this? Easy.......
    Start by trying a 500k resistor as a bridge between the new junction points we've added. Drop in volume big? - use a 1M poti. Drop in volume tiny? - stick to the 500K. Try a 270k as well. You dont have to solder each resistor in, just strum a loud chord and carfully press the resistors across the two junctions.

    Looking at the back of your chosen poti with the solder lugs facing down, start by soldering a resistor of 4k7 to the right-hand lug. This is a set minimum volume resistor. Change later to a value that suits your taste. Conect this resistor to one of the junctions, and the joined left and middle lugs to the other. Test it out - Turn the volume up and see if there is a point where turning up furthur has no effect. If so set the poti to this setting, turn the amp off, measure how many ohms the poti has between the right lug and the other two. This is the true value of the poti that you really need, which you will now make by adding a resistor between the left lug and the cap side end of other resistor on the right lug. (I hope you know the calculation needed for working out parrallel resistors.)

    As a practical example, on the last amp I did this to the reading I got of the 500K poti plus the 5K tail was 180K, so I used a 270K resistor 270 X 505 = 136350 / 270 + 505 ( 775 ) = 176K. This works just fine.

    I hope this has been of some help. Basicly it's a case of fiddling around untill you find a set-up you like.

    While you are at it, try out the treble conrol in this same spot, like the Matchless and Vox amps have it, I really like it right at the end of the chain there, very smooth and usefull.

    Just another thought, make sure your phase inverter tube is good before you start. Now that you want to have a master volume this tube is going to have a lot more effect on the tone than before. I favour the AT7 in this set-up for a full powerfull sound, finding the AX7 a bit edgy and thin at lower volumes.

    Ben.
     
    Last edited: Jun 3, 2008
  7. Wakarusa

    Wakarusa Member

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    When you add coupling caps to isolate the DC bias you can tweak the values to preserve frequency response, but you can't cure the phase shift ('least not easily). This can be a problem in an amp with global NFB. The question is which is more objectionable -- a bit of scratchiness, or the phase shift and finding room for the extra coupling caps?

    As for pot value, a lot depends on the values of the bias feed resistors since the pot is in parallel with them.

    An alternative, and I'd argue more elegant, solution is to use a variable resistance like a vactrol in the actual PI/power grid circuit and use the front panel knob (and a few bits of circuitry) to control the vactrol. This way you skip scratchiness and phase shift altogether.
     
  8. donnyjaguar

    donnyjaguar Member

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    This PPIMV circuit is such a kluge I'm astonished people are still doing it.
     
    matt1969 likes this.
  9. gproud5150

    gproud5150 Member

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    Really? I'd love to hear your comments about it. I'm assuming you've done it and used it thoroughly enough that you can give me some specifics to your reply. Thanks.
     
  10. CitizenCain

    CitizenCain Member

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    Any comment about the PPIMV that has a huge thread over at the Metro Amp forum?

    http://forum.metroamp.com/viewtopic.php?t=4390

    I've installed this in a couple of amps and it seems to work well.

    PS = the thread starts off discussing the "Rich Mod", then develops into a modified version of the Fischer Type 2 MV, start around page 4-5.
     
  11. BenH

    BenH Member

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  12. CitizenCain

    CitizenCain Member

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    Sorry [​IMG][​IMG]
     
  13. RedRock

    RedRock Member

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    In "The Trainwreck Pages," Ken Fischer discusses 4 different master
    volume controls. Three are post-phase inverter, and one is pre.
    Reading it can help clarify references to master volume types and keep
    a discussion focused on a particular one. They are all different circuits
    and each has a unique sound.
     

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