Push Pull math question

Discussion in 'Shock Brother's DIY Amps' started by Jewellworks, Oct 17, 2021.

  1. Jewellworks

    Jewellworks Tele-Meister

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    im building this amp that i have dubbed the RicknDevil. based on a rickenbacker model 59 circa 1938. its an input directly feeding a phase inverter to a push pull output. this is my first P/P build and ive got some questions...
    ive been basing my design on the vtadiy.com/loadline-calculator. im using a pair of ECL86 triode/pentode tubes. most of my voltages are really close to where i expected, except the biasing of the pentodes. looking at the vtadiy.com/loadline-calculator its pretty straight forward with SE designs, but im a little lost on what i should be seeing looking at PP. specifically:

    Image11.jpg

    these are my actual measured plate and screen voltages. but as for the grid and mA, how do i know where im at when i have 2 tubes?
    it shows 31.4 mA with a grid voltage of -6.8.
    when i add my V1 and V2 mA together i get 31.4mA. as shown above.

    what im really measuring is V1 grid at -4.1, and V2 at -3.8
    if i add these up, its -7.9

    Image13.jpg

    and when calculating the plate dissipation, this is a lot closer to where these tubes are running.

    im sure there is a Merlin chapter somewhere that explains this, and im sure it will help. but ive been relying on this vtadiy.com/loadline-calculator.

    any guidance is appreciated
     
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  2. 2L man

    2L man Tele-Holic

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    That loadline calculator default is only ONE tube! Then for two parallel tube amps the OT impedance must be chosen 50%, for three 33%, for four 25%...

    Designer used to design circuit first for certain tube and then build or order correct OT impedance and PT voltage iron. We hobbyists often buy OT and PT first and then can adjust practically ONLY Bias operating point. We can do few tricks to make voltage lower if loadline does not look "right". Or change tube type, or double or half the OT impedance changing loudspeaker load.

    Possibly you have seen loadline drawings where another is drawn to same and other is turned 180 decrees around Operating Point, the red dot.

    What Giuseppe's loadline calculator does for PP output stage is it make "kink" to where all the load change to one tube, where operation change A/B and loadline begin to rise steeper. And it doubles the impedance when PP box is ticked.

    When you use two tubes and use cathode bias you need to select shared cathode resistance also half resistance when both cathode currents make the voltage loss to it which then becomes the g1 Bias voltage.

    In that Calculator you still adjust only one tube variables!

    I measure lots of things but very rarely control grid voltage. Only if I suspect leaking coupling capacitor. I know I change Vg1 when I adjust fixed bias stage but I measure only Ic. When I did study tube technology more than 40 years ago, mechanical needle multimeter internal resistance was much smaller than current digital have and putting it to control grid could have screw that stage bias.
     
    Last edited: Oct 18, 2021
  3. Jewellworks

    Jewellworks Tele-Meister

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    So if I'm reading this right, this load line calculator is for 1 tube. 1 pentode. If I can get each of my pentodes to adjust to the same bias voltage (-6.5 or so at 270 plate volts) to around 30mA then my pentodes will be equal push pull, at just under 9W PD, -Correct? This isn't for both tubes added together or anything. It's the values needed for each pentode.
     
  4. 2L man

    2L man Tele-Holic

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    Yes your calculus is right :) If you use cathode biasing the resistor value comes: 6,5V : 0,06A = ~108 ohms.
     
  5. Jewellworks

    Jewellworks Tele-Meister

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    .06A? Shouldn't that be .03A? this is for each pentode, correct?
     
  6. 2L man

    2L man Tele-Holic

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    If both tubes share same cathode resistor then both current flow thru it and thats why 60mA is used.
     
  7. Jewellworks

    Jewellworks Tele-Meister

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    I have the Pentodes on separate cathode resistors, so i can equal out any tube mismatch, but point taken.
    That said, I DO have my TRIODES sharing a cathode resistor. When I look at my triode current, it's through both triodes. So if I'm looking at a load line chart, (which is 1 triode at a time) do I half it? In other words, if I'm reading 3mA with them combined, do I chart is as 1.5? Seems logical...
     
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  8. 2L man

    2L man Tele-Holic

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    Yes!

    With Pentodes the Screen Grid voltage effects to Control Grid voltage and with Tetrodes the Screen voltage seems to have even more effect.

    That loadline calculator is not logical when Tetrodes seem to have Grid curves which few highest have positive values but for Pentodes the top curve is zero volts. Voltage value shows when pointer is on line.

    Control Grid drive signal up to zero voltage is called AB1 class and when Grid is driven to positive it is AB2 class.
     
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