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MU Follower LTP What am I missing?

Discussion in 'DIY' started by mashaffer, Jul 10, 2018.

  1. mashaffer

    mashaffer Super Member

    Messages:
    3,341
    Location:
    North Central Indiana
    Why the different response shape.

    MuLTP.png

    MuLTPOut.png
     

     

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  2. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    36,774
    Location:
    Southern NJ
    Why is there a sine wave input to the plates of the follower tubes? I'd expect that to just be B+ input, unless that is what is meant to represent the DC + ripple. The chain of resistors over on the left I'm guessing is your voltage divider to elevate the heater supply?

    I'd also expect output to be off the plates of the diff pair, not the cathodes of the follower arrangement, but I'll admit to not being very familiar with this topology. Usually its just a diff pair, maybe with a CCS if you feel the need.
     
  3. mashaffer

    mashaffer Super Member

    Messages:
    3,341
    Location:
    North Central Indiana
    Right on both counts... Supply ripple and heater supply. The same reaponse shows up at the plates of the lower tube. This is for an application where I need the lo Zout.
     
  4. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    36,774
    Location:
    Southern NJ
    can you use a standard LTP and feed it into a cathode follower in the usual way? That should give your LoZ out without any particular funny business to the circuit. Likely could even direct couple it depending on the plate voltage at the diff pair plates.
     
  5. mashaffer

    mashaffer Super Member

    Messages:
    3,341
    Location:
    North Central Indiana
    Maybe. I will run a few more sims and see. I will be driving a filter network with about 4k Zin. Will see what happens.
     
  6. mashaffer

    mashaffer Super Member

    Messages:
    3,341
    Location:
    North Central Indiana
    Just to note the actual response curve in the audio band is not horrible but the rising reaponse on each end does seem odd. The difference in gain can probably be handled in the normal ways though it may not be a huge issue in this particular application.
     

     

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  7. mashaffer

    mashaffer Super Member

    Messages:
    3,341
    Location:
    North Central Indiana
    Oddly enough just going to the CCS largely eliminates the issue.

    MuLTPOutCS.png
     
  8. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    36,774
    Location:
    Southern NJ
    same circuit but a CCS in place of the tail resistor? Seems like quite a dramatic change in performance for just that change, unless it was just the "tail" wasn't long enough. Off the top of my head I forget what my one LTP arrangement uses, but I want to say its at least a 47K. I've been meaning to experiment with a CCS on that amp just to see what it does.
     
  9. mashaffer

    mashaffer Super Member

    Messages:
    3,341
    Location:
    North Central Indiana
    Yes, just changed out the tail resistor for an ideal current source. Indeed it was probably more of a medium tail pair. :D Incidentally this might make a nice PI for a PP triode amp as it should be able to provide good current and voltage drive.
     

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