Fisher 50A power tube static value

Discussion in 'Fisher' started by xinrun, Jan 6, 2018.

  1. xinrun

    xinrun New Member

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    3
    Asking for the static value of the power tube(the value among the green circuit), which is the cathode current sum. It is very important to me. Pls~
     

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  2. whoaru99

    whoaru99 Epic Member

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    It depends on the actual voltages you have, but per the schematic my thought would be something on the order of 100mA, give or take.

    This is rough estimate based on datasheet for triode-connected 807 at approximately that plate voltage and grid bias.

    Perhaps an expert will be along in time to give an opinion too.
     
    Last edited: Jan 6, 2018
  3. xinrun

    xinrun New Member

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    Thank you so much. :thumbsup:
     
  4. xinrun

    xinrun New Member

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    Hi whoaru99, could you tell me the way of the estimating. I want to calculate it. Thank you again for you kindly replying before.
     
  5. whoaru99

    whoaru99 Epic Member

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    Look at the 807 tube data curve/graph for triode connection. Put a vertical line on the graph at the plate voltage, then read across to the milliamps where the plate voltage line intersects the grid voltage curve. You will have to interpolate the grid voltage intersection on the curve as I think they were spaced every 5 volts.
     
  6. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Who; The schematics for the 50A show a 1614(a 21-25w 6l6 depending on who you believe) in the output position. What's with the 807 info??? Wouldn't a 6l6GB be closer to the 1614? Or did the OP have the sockets changed and is running 807's.
     
  7. whoaru99

    whoaru99 Epic Member

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    I looked at 1614 data sheet but there were no curves on the one I saw. On the datasheet it said curves for 807 could be used as long as you heed the 1614 ratings.

    48-50mA per tube at -42 grid bias, 440V plate is what I see, considering interpolation.

    807 Triode.png

    Again considering interpolation I see about the same from a 6L6GC datasheet.

    6l6GC triode.png
     
    Last edited: Jan 10, 2018
  8. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    On the Fisher schematic for the 50A, they specify an output tube plate voltage of 420 vdc. If that is in fact the voltage your unit produces, that backs the quiescent current down to about 40 mA per tube -- based on the excellent curves Who has provided. With today's higher voltages however, Who's graphical representation is probably more accurate, if you hold grid voltage as a constant. But then there is the ideal value based on the actual circuit conditions as well. This takes into account everything from the load impedance the tubes operate into, actual B+ voltage, manufacturing tolerances of the tubes, and even power supply regulation as well. Setting the quiescent current by way of schematic indicated voltages is surely a good start, as normally, Fisher has already done all of this homework. But adjusting them for the least amount of distortion based on the actual circuit they are operating in is always best.

    I have a pair of 50A's here I'm in the process of finishing up for a client, and will be posting on them soon. Since Fisher did not provide the quiescent current draw value on the schematic, I'll be happy to provide that value based on the lowest distortion operating point. These units are operating with 6BG6 tubes installed, but again, these are just another variation of the same 6L6 family of tubes that the 1614, 807, etc., etc., are all based on. Stay tuned.

    Dave
     

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