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Grid voltage / lack of grid stopper on LTP inverter, should I be concerned?

Discussion in 'Amp Tech Center' started by SSL9000J, Apr 26, 2018.

  1. SSL9000J

    SSL9000J Tele-Meister

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    Greetings all! I'm trying to iron out a few proverbial kinks in a recently completed build. Presently, my attention is on the phase inverter. It's a garden-variety LTP, identical to the schematic on the Aiken Amps site, minus the NFB & presence control:
    My questions to the experts are
    1. Does the PI need a grid stopper resistor? (It occurred to me that if the the .5meg volume pots on either channel gets dimed, there's essentially no resistor between that channel's coupling cap and the PI grid.)
    2. Is the grid voltage on pin 2 too low? (-.16v between the grid & cathode.) Everything else seemed about right.
    The amp actually sounded pretty good at low volume. But when I had the opportunity to crank it... not so much. Thanks in advance!
     
  2. clintj

    clintj Friend of Leo's

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    Couple of things may be going on here. For your grid voltage, what do you get if you probe the other end of the grid leak resistor, at the junction of R5 and R6 on Aiken's schematic? Directly probing the grid of either a cathodyne or a long tail greatly upsets the bias and gives a false reading.

    From a pure theory standpoint, a 12**7 type tube doesn't need a grid stopper. There's some old amps without them at all on the inputs, like one of the tweed Twins or Bandmasters IIRC. It comes into play as a device for setting bandwidth, or for limiting blocking distortion in hard driven preamp stages. It can also form part of a divider network to attenuate signal to a stage if needed.

    Do you have a means of comparing input and output signal from your PI? Scope, DMM, etc? Comparisons of waveform and amplitude really help when looking for a PI issue.
     
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  3. SSL9000J

    SSL9000J Tele-Meister

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    Clintj, thanks for the response! I have a basic Fluke DMM & an analog Simpson meter, but no scope or signal generator, unfortunately. This afternoon I'll get back to the workbench & check the voltage across the grid leak resistor. Interesting that you mention blocking distortion, as I suspect that may be one of the issues I'm hearing as I increase the volume. (There's no master volume, just preamp volume & simple tone controls for each channel.)
     
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  4. clintj

    clintj Friend of Leo's

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    Do you have a full schematic? Unpleasant distortion can also be an earlier preamp stage, as a long tail can usually handle some pretty significant signal voltages without issue. Blocking distortion comes from grid current, and it takes quite a bit of signal to push a long tail's input stage to that point if it's biased right.
     
  5. Ten Over

    Ten Over Tele-Holic

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    Do you have a capacitor between those pot's and the LTP?
     
  6. SSL9000J

    SSL9000J Tele-Meister

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  7. SSL9000J

    SSL9000J Tele-Meister

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    There are coupling caps before the pots, after the plates of the preamp tubes, but not between the pots and the PI tube.
    Thanks for looking!
     
  8. Ten Over

    Ten Over Tele-Holic

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    It's not going to work until you put a capacitor between the pot's and the LTP.
     
  9. printer2

    printer2 Poster Extraordinaire

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    +2

    With both pots you will get interaction between them, you can have one at zero and it will load down the other channel, most likely short it out. We usually put a couple of mixing resistors inline from the pot's wiper to the junction with the capacitor to the LTPI. Without the capacitor the pots are loading down the bias and tail resistor voltage.
     
  10. SSL9000J

    SSL9000J Tele-Meister

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    I think I understand:
    The resistor & cap values are lifted from a GA-30 schematic. (Lest anyone mistakenly assume any actual science might have been involved in their selsction.) Printer2, you mention placing the resistors inline with the wiper of their respective pot. Should they, meaning the pots, be turned around so the wipers face the PI? I vaguely remember reading somewhere that the orientation of the pots does, in fact, make a difference. Though it's difficult to discern if there's any consistency when I go through different schematics.
    Thanks again, I really appreciate the help!
     
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