Phase Inverter Tube Changes and Impacts

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Axemeister, Mar 27, 2008.

  1. Axemeister

    Axemeister Member

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    I have been experimenting with my Preamp tubes, and I am curious about the importance of the Phase Inverter. Excuse so many of my newbie questions...

    How much will a change from a 12ax7 to a 5751 or 12at7 influence tone? Will a higher gain tube in this position have a strong influence on power tube break-up? Or, will a lower gain tube pass more clean wattage to the power tube section, which would result in power tube breakup or tone changes when the power section is pushed harder with clean input.
     
  2. mark norwine

    mark norwine Member

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    You're asking for comparative data, yet you don't tell us what the stock tube is. Heck, you don't even tell us the amp make / model.

    Kinda hard to provide answers.....
     
  3. jcat

    jcat Member

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    PI tubes should have matched Triodes. Ask for them when you buy the the tube.
    Class A & A/B amps will behave differently as will the various power tube platforms.
     
  4. Blue Strat

    Blue Strat Member

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    In general, PI circuits don't produce very much gain so changing from a tube with a gain of 20 to a tube with a gain of 100 doesn't change anything about the gain of the circuit.

    There are a lot of other things different than gain in preamp tubes of different types. It really doesn't matter, technically, what they are. The tonal difference and how you like it is what counts.

    Some things can only be found out by trying and listening. This is one of them.

    With more specific info an a particular amp, users may be able to provide insite into their tonal experiences in changing tube types.
     
  5. Axemeister

    Axemeister Member

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    Thanks, that is the general answer that I was seeking. I will just experiment with a few tubes and see the results
     
  6. rockon1

    rockon1 Member

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    Not sure if Im following what your saying here. I notice a significant volume reduction when going from a 12AX7 to a 12AY7 or even more so to a 12AU7 in the PI. Bob
     
  7. Timbre Wolf

    Timbre Wolf GoldMember Supporter Gold Supporting Member

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    I agree, Bob - Mike's statement didn't make sense to me either. What's up Mike?

    - Thom
     
  8. VikingAmps

    VikingAmps Member

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    With all the feedback in this circuit there is very little difference in gain. Circuit gain is more a function of the configuration than the device. Even tho most people believe 12AX7's "have a gain of 100" I don't know of any vintage amp that gets too much more than half of that number out of one in any stage. The PI has way less gain than that even with a 12AX7 installed.
     
  9. rockon1

    rockon1 Member

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    Again Im not sure Im talking about the same thing. If I put a significantly lower gain tube in the PI the over all volume is sinificantly reduced also. Dont know about the theory but in practical application that what happens with my amps.

    Another interesting impact is tone. Often its said the PI has little affect on it. In practical application I hear a significant change with different tubes -even more with different tube types. :)Bob
     
  10. slider313

    slider313 Silver Supporting Member

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    I have to agree. I can hear a difference between brands of 12AT7's in my Vibrolux Reverb's phase inverter. I also hear a volume drop when going to a 12AY7 or 12AU7.
     
  11. Axemeister

    Axemeister Member

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    I started this thread as a newbie, but spent the last couple of days experimenting with tubes in various preamp positions. I learned how much all of the preamp tubes impact the overall tone and nature of the gain in a subtle way.

    I really found the right nuance for my ears and my amp using 5751 in PI position..The volume change is neglible with the change from a 12ax7 to a 5751, but the gain structure is just a tiny bit less grainy
     
  12. Timbre Wolf

    Timbre Wolf GoldMember Supporter Gold Supporting Member

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    This is how it really needs to be approached. Experiment away, and find your own truth. Good going! :BEER

    - T
     
  13. Blue Strat

    Blue Strat Member

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    Hmmm, tried going from a 12AT7 to a 12AU7 in my DR (the center of my guitar amp universe) a while back thinking that the AU would act like a master volume control turned down. There was no apparent loss of volume.

    The Fender PI circuit has low voltage gain (which is what we're talking about) and I'm pretty sure that they all do as voltage gain is not the objective in a PI circuit. A tube with gain > X in a circuit designed for a gain of X or lower will not produce higher gain.

    Maybe it's something to do with the fact that the Fender PI is designed for 12AT7s and the resistor values involved.

    What you're probably hearing is the effect of different Rp (internal plate resistance) and the effects of incorrect (cathode) bias for some of the 12A*7 tubes in circuits that weren't intended to be used in them.
     
  14. EarlTX

    EarlTX Member

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    Seems to me that the actual gain of the DR PI is ~9, with NFB. The open loop PI gain is about 20.

    All you need out of the PI Plates is enough signal amplitude to get the output tube G1 voltage to zero (from it's G1 voltage). If the amp has a -35V G1 bias voltage, the PI needs only to generate a 35Vpeak Plate signal.

    Tubes with different AFs will certainly give you various Plate signal amplitudes, but depending on where your G1 voltage is, it may not make much of a difference.
     
  15. VikingAmps

    VikingAmps Member

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    I would think if anything a lower gain lower plate impedance tube like an AU7 would have the ability to give you more volume out of an amp since it has a greater ability to drive the grids a bit positive. I can see a slight difference in gain between it and an AX7 in the same circuit but not volume. Unless you're pulling enough extra current to drag the plate voltage down significantly or the PI input isn't even getting hit with a few volts. Otherwise any of them should be able to give you 40 volts or more peak (about 28V on a true RMS meter) to drive the grids to zero.
     
  16. Timbre Wolf

    Timbre Wolf GoldMember Supporter Gold Supporting Member

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    I've gone from 12AX7 to 5751, 12AX7 to 12AT7, and 12AT7 to E180CC, in several different amps, with both long-tailed pair and split-load phase-inverter circuits represented. Each change "down" in gain was accompanied by changes in breakup in the amp (cleaner sounding with less gain). I wouldn't say that the volume was significantly dropped, though, so - if you're equating gain to volume, then I can see where a misunderstanding developed. Actually, when I seconded Rockon1's post, I was hesitant in talking about "volume" drops, rather than gain drops.

    I haven't tried 12AX7 to 12AU7, nor have I experimented with a DR.

    - T
     
  17. doublee

    doublee Member

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    What do matched triodes in a PI do?
     
  18. doublee

    doublee Member

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    Thank you I just wish I knew what you are saying!
     
  19. Blue Strat

    Blue Strat Member

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    Volume = gain, essentially. The notion that "gain" = overdrive (distortion) is a misconception that many guitar players have. They acquired this from "high gain" amps that produce preamp distortion and have "gain" and master volume controls.

    The "Gain" control, which does control (real) gain of a circuit (which is amplification, or how much larger a signal becomes when amplified by a tube or active device), passes more and more CLEAN signal which slams the next tube stage into distortion, hence the misconception that more gain = more distortion.
     
  20. Timbre Wolf

    Timbre Wolf GoldMember Supporter Gold Supporting Member

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    Volume does not equal gain, just the same as gain does not necessarily equate to overdrive. Gain relates to amplitude, which can equate to volume or to overdrive, depending on the circuit. What I'm describing in the phase inverter "gain" changes is a change in power section overdrive (i.e. cleaner signal, when going from 12AX7 to 12AT7, for example) and not a change in overall volume.

    I do agree that most guitarist mis-use the term "gain" to mean only "overdrive" - usually when discussing what their preamp is doing when they change the knob marked "gain." I believe that I'm using the term gain correctly to describe what I've experienced, many times, when changing the type of phase inverter tube.

    - Thom
     

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