matched phase inverter tube???

Discussion in 'Amps and Cabs' started by ccoker, Feb 27, 2006.

  1. ccoker

    ccoker Supporting Member

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    I have read the verbage, makes sense...

    who has done it?
    impressions?
    which one and where did you get it?
    worth it?

    etc..
     
  2. Chiba

    Chiba Gold Supporting Member

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    I put a Groove Tubes MPI in my 18W combo and noticed a decent increase in volume. Noticeable to me, but probably not to somebody who hadn't played the amp before with a 'regular' tube in the PI spot. I couldn't find one locally, so I got it off eBay, no biggie. With shipping it cost me the same as if I had got it at GC or something.

    It made enough of a difference to my ears that I'll do it again when I re-tube the power section.

    --chiba
     
  3. ericb

    ericb Silver Supporting Member

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    Cool... The interesting thing for me though is that I like higher wattage amps , so typically when I'm changing something with the amp ,I never want an INCREASE in volume.. It's typically to decrease it!!! :) ERIC
     
  4. Matt H

    Matt H Guest

    a more balanced output section leads to more cancellation of harmonics generated in said output section.

    i'm not a fan.

    give me asymmetric waveforms, baby.
     
  5. atomicmassunit

    atomicmassunit Member

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    Right on, Matt H... I agree with that. The whole mojo of these old amps we love (and the new ones that sound like old ones) is from harmonic content and inherent imbalances. The closer you get to perfection, the closer you get to sounding like you plugged direct in to the board. Matched phase inverters is something guys that sell tubes came up with.
     
  6. 85db

    85db Member

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    I've been buying Groove Tubes Matched PI's from Todd Richards, www.RevolutionGuitar.com. He's the nicest guy and he honestly told me that he doesn't hear much differences with these GT Matched PI's and advised that plain vanilla JJ 12ax7 or an EH 12ax7 be used instead.

    I bought a few anyway and I do hear a significant difference (and I tried many other tubes in PI position). The tone becomes... ehhh... I think "balanced" would be the right word. I really like what I'm hearing and I hear no loss of harmonics or gain.

    It's possible that the magic is in the tube GT using and not in the fact that the triods are matched. But I personally could care less about the technicalities. The GT Matched PI's sound fantastic to my ears.
     
  7. BozoTone

    BozoTone Member

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    My Goodsell has a "long tail" phase inverter. I'm not sure yet what this means, but had just found out in the last few months what a huge difference this makes in tone vs. other PI configurations. The Super 17 only has one preamp tube and a PI before the power tubes. When I first started I wasn't really trying out unbalanced "properties" of a tube per se, but the brand of tube. I had read that an old Vox/EL84 guru had reccomended a Heerlen Amperex (Bugle Boy) 12AX7 for the PI spot. I got an old, unbalanced, noisey Amperex from Richard to mess with, and for the hoot of it, popped it into the PI position...O.M.G...I am not sure if it is the Amperex or the fact that it is unbalanced , but the change is very dramatic. The cleans seem richer and the overdrive has more harmonic content...3D...I don't think I will ever use another tube for the PI. Now I have to get a good balanced Amperex to compare it with to elminate whether it is the manufacture or the unbalanced properties that acheives this effect...Me thinks it is the Unbalanced Amperex...
    BZT
     
  8. riverastoasters

    riverastoasters Member

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    I never heard of matched PI until this thread, and my reaction is "why would I want both sides to match?"

    If you have dissimilar power tubes then I suppose matching the PI might make sense.
     
  9. Blue Strat

    Blue Strat Member

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    How do you know that the one you're using isn't balanced?
     
  10. fullerplast

    fullerplast Senior Member

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    Exactly what I was wondering too.

    If nothing else, matching gives you consistency. If you don't have the ability and equipment to match the halves, then at least when you change PI tubes, you will be able to replicate what you had before. Otherwise, it's a PI balance crap shoot everytime you swap the PI.

    Probably the best option of all is to find a PI tube you really like, based strictly on trial and error. Benchmark that tube with a tester and write down the exact balance of that tube. That way, you've defined what your ears tell you in a way that you can duplicate later.
     
  11. John Phillips

    John Phillips Member

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    The interesting thing about this is that the PI circuit - whether it's a long-tailed pair or a split-load - is inherently unbalanced in operation anyway (that's why the two plate resistor values are usually different in the LTP, to try to compensate for this, but the compensation is quite crude, even not allowing for resistor tolerances). Then you have the power tube matching and OT (which is also not always perfectly matched) to consider. So a matched tube does not necessarily produce a balanced output waveform - in fact, I would expect that it would produce a definitely unbalanced one in most amps. It may take a particular unmatched tube to create a truly balanced output waveform, even if it's possible at all.

    I'm not saying that a matched tube might not sound better, and if it does, as fullerplast says at least it should give you consistency since the degree of mismatch in the final waveform is then easily repeatable. But it's not because matching the tube gives a balanced output. I'm sure some of you will think that's splitting hairs, but I think it's important to be aware of the difference.
     
  12. Lord Valve

    Lord Valve Active Member

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    It's ********.

    LV
     
  13. Chiba

    Chiba Gold Supporting Member

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    How do you figure? It's my understanding - which, obviously could be mistaken - that the whole reason there's a phase inverter in a push-pull circuit is to hit both sides of the push-pull circuit with PHASE INVERTED SIGNAL on ONE SIDE (as compared to the other) to AVOID harmonic cancellation.

    That is, after all, why the thing is called a PHASE INVERTER, right? If the phase doesn't need to be inverted on one side, why is that even in the circuit?

    All a matched phase inverter does is hit both sides of a push-pull power circuit with signal levels as close together as possible. That's why I talked about the VOLUME INCREASE - the GT MPI tubes are very hot in addition to being balanced. I'm sure not every 12An7 tube is balanced, or every 12AX7 as hot as these, but I'm just as sure as there are examples of each out there among 'normal' or 'regular' tubes.

    Obviously any volume increase coming before the power section is going to affect your tone as it relates to power tube saturation - but tone is completely subjective, isn't it?

    --chiba
     
  14. big mike

    big mike Moderator - EL34 Emeritas Staff Member

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    Put one into my Z28. 5751 JAN. In place of a 5751 Jan. Didn't notice a diff really. Course the one that was there could've been matched as well. <shrug>
     
  15. badtoneno

    badtoneno Supporting Member

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    If LV says its ********, thats enough for me.
     
  16. fullerplast

    fullerplast Senior Member

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    Yeah, well his answer is ******** if left like that.

    The fact is that any PI tube will either shift the inherent balance (or lack of balance) of the PI circuit one way or the other. If matched it will retain the circuit imbalance. If mismatched, the tube can make the circuit balance worse if the halves are one way, or it will either cancel the circuit imbalance (effectively balancing the circuit) or send it the opposite direction as the natural circuit imbalance. You will hear the difference in many amps. Whether you care or not is another issue. Whether you want to spend any time messing different PI tubes to see what kind of imbalance sounds best is another issue. But to call it ********, and leave it like that is ******** IMHO. YMMV.
     
  17. riffmeister

    riffmeister Gold Supporting Member

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    The thing is, what you like or dislike in a PI tube due to the "balancing" aspect will likely change as the power tubes age and when the power tubes are replaced.......as John stated, PI, Power Tubes, OT, all affect the state of affairs in this regard.

    My unscientific opinion is that balanced triodes may be a good thing for high fidelity applications, and it is probably less important for Rock Guitar. That said, I buy tubes with balanced triodes for my amps.....just for the consistency factor. It's probably cost me $30 over five years. :)
     
  18. John Phillips

    John Phillips Member

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    Not quite.

    That is the purpose of the phase inverter - to drive each side of the push-pull output section with equal and opposite signals - and how it should work in theory; but in practice the two sides of the phase inverter circuit aren't the same. Although the two outputs are out of phase with each other, they aren't necessarily the same.

    In the standard long-tailed pair PI, the first half is a conventional grid-modulated amplifier; the second half is driven by having its cathode (which is commoned to the first side) modulated and the grid referenced to ground. This gives them different characteristics and different gain - the first side is stronger. So to compensate for this, its plate resistor is a smaller value, usually 82K to the second side's 100K in many guitar-amp circuits. But these values aren't 'exact' - they're simply the nearest 'prefered' resistor values which give approximately the right result. Not only that, each has a tolerance, usually 5% - so the real values could be as far apart as 78K and 105K, or as close as 86K and 95K. (In fact, in your Marshall 20W, the values are 100K and 150K, which is clearly not the same ratio as 82/100.) In other words, you don't know how balanced or unbalanced the PI output is anyway even before you get to the tube characteristics.

    Using a matched PI tube makes the balance of the PI more easily repeatable, but not balanced. Then add to that the probable mismatch of the power tubes - even if they're matched, it won't be exact - and possibly the OT windings, which rarely are identical either, and you really have no idea what the result is. I don't doubt that replacing a particular PI tube with a particular matched one made the amp sound better, but it's probably not because the output from the PI is more symetrical. It's almost certainly either because that particular tube is simply a better tube; or you happen to like the particular imbalance it gives better than the other one.
     
  19. fullerplast

    fullerplast Senior Member

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    Of course, everything changes and drifts slowly with age. That doesn't mean you need to completely ignore the effect. It would be more unusual for the two halves to drift in opposite directions, or for identical power tubes to drift dramatically in opposite directions, than to drift in the same direction. If you find that a PI tube measured transconductance of 1500/1100 really makes your amp sound sweet, then it simply makes sense that when you replace that tube you would be ahead of the game if you went for that relative imbalance as opposed to putting in a 1200/1600 PI tube.

    As for using a balanced PI tube:

    Pros:
    -Consistency when swapping PI tubes
    -Puts a limit on the degree of imbalance

    Cons:
    -PI circuit will still not be balanced
    -Cost
    -Limits a "luck of the draw" chance of accidentally finding a really good sounding imbalance or accidentally balancing the PI circuit

    Make your own choices and when in doubt, use your ears.;)
     
  20. Roccaforte Amps

    Roccaforte Amps Member

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    Transconductance measurements are meaningless, its all about current draw. A 12AX7 that measures 1600/1600 doesn't mean it has matching current draw. ( I hate when I see GM test results on ebay auctions suggesting the tubes are matched).

    Long tail PI circuits are self adjusting to compensate for tube wear,
    but are not balanced. Using a tube that has matching sections in a VTV tester (which is what the tube dealers are using to test) won't be matched in an amplifier circuit due to circuit values, global feedback,
    and the signal going through the tube.
    Which brings up another point; Matched output tubes.
    If you monitor current draw on a set of output tubes with a complex guitar signal running through them you'll see all of the tubes going different directions.
    The only place a matched PI tube could benefit an amplifier circuit is if it was put into the early WE PI circuit used in early amplifiers up to the mid 1950's. However, still no guarrantees. You'd still have to juggle tubes until you get one that gives you equal drive. Once you find it, it really won't be a matched tube anway.
    Adding a balance control to the PI circuit is the only way to achive balance, FWIW. Doug
     

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