101 st bias

Discussion in 'Fisher' started by beemerbob, Feb 4, 2019.

  1. beemerbob

    beemerbob Member

    Messages:
    81
    Location:
    Vancouver wa
    20190204_143720.jpg 20190204_143737.jpg 20190204_143745 (1).jpg i have 1 power tube glowing red ever so slightly...
    i am assuming this would be a bias issue...
    i am wondering how this amp is biased

    and any ideas on how to stop the red plating


    Tks
    Bob B
     

     

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  2. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,905
    Location:
    Ball Ground, GA
    Bob -- Regarding the red plating, you have three issues to juggle here:

    1. Of course, first make sure that the coupling caps into the output stage have been replaced. Even the better type of cap that Fisher used when the last version of this unit was produced have been known to leak and cause output tube red plating over the years. Your unit has the earlier wax type that are a recipe for leakage, and have not been changed out.

    2. Under normal conditions, the quiescent current of the output tubes is pretty high as designed -- this because the current passing through the output tubes is also used to light the heaters of the 12AX7 phono preamp tubes.

    3. As a result of #2, this unit must use matched output tubes so that they will all draw a similar amount of quiescent current. If unmatched and one tube inherently draws notably more current than the other three, the current hog will then surely red plate simply because the tubes are not matched closely enough.

    When one tube is red plating brightly, then the problem is invariably due to #1 above. But if one tube is only slightly glowing, then it could certainly be due to a coupling cap starting to go out (particularly in your case), but more typically, it is due to #3 above.

    It would also be a good idea to replace the cathode bypass cap for the output stage as well (big orange cap located near the side wall in the power supply section).

    This amplifier is a cathode biased design, and was not offered with any user adjustable bias controls. Check the issues noted here first. Then, if you want to get into installing bias controls, that can be done too, but of course if much more involved.

    I hope this helps!

    Dave
     
  3. thornev

    thornev Well-Known Member

    Messages:
    777
    Location:
    Mid Hudson Valley, NY
    Dave G - Your #2 above... Are you saying that the preamp tubes get their current from the output tubes, or that the preamp tubes get their current from the same place as the output tubes, the power transformer? I'll have to take another look at my schematics. Thorne
     
  4. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,905
    Location:
    Ball Ground, GA
    In the most basic sense, all current flowing within the amplifier originates from the power transformer. But specific to my comments above, the current ultimately emitted from the cathodes of the four output tubes in the X-101st is drawn from the negative terminal of the power supply (ground), through the two 12.6 volt heaters of the phono preamp tubes, to the cathodes of the four output tubes, out the plates, through the primary winding of the output transformers, and back to the positive terminal of the power supply (B+).

    The high voltage (B+) to operate phono preamp tubes (plate voltage) comes directly from the B+ supply in the conventional manner by way of the B+ distribution circuit.

    Dave
     
  5. beemerbob

    beemerbob Member

    Messages:
    81
    Location:
    Vancouver wa
    Very much so thanks for sharing your wealth of knowledge
     
  6. tubeactive

    tubeactive AK Subscriber Subscriber

    Messages:
    322
    Location:
    NJ
    Yeah, those wax caps have got to go, especially when they connect to the opt tubes. That is a great sounding amp, btw. Note the opt transformers, with their 926 EIA manufacturer #. That means Todd Electric trannies, same mfr. of Fairchild opt iron ! Great iron !

    Furthermore, note the different part numbers on that iron. See the different wire exit orientation ? The earliest stereo era 101 amps actually used two, different style, tranny lamination stacks ! When I was "gathering" amps, I actually bought three type 101s to get a matched pair of opt trannies !

    The phono preamp has lots of potential. Read dcgillespie's posts on Fisher preamps to optimize the RIAA EQ circuitry. Be careful with the selector switch, though, when replacing caps. I remember having to draw out the wiring to/from the switch in order to remove the switch for access to the parts and the solder points. It was well worth the effort. That amp embarrassed many others ! Brass faced Fisher gear is sweet !
     

     

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