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New Tubes for a X1000

Discussion in 'Fisher' started by postman56, Sep 28, 2018.

  1. postman56

    postman56 New Member

    Messages:
    29
    I've picked up a quad pair of new matched version Gold Lion KT77's for my Fisher X1000. The old Mullards are showing some decline on two of the four unfortunately.
    I've read how to set the bias, just was wondering if anyone had any tips with this procedure and being you set the left and right pair, not individually since there is not separate dials?
    Much Appreciated.
     

     

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  2. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,673
    Location:
    Ball Ground, GA
    Post -- There are not individual bias controls for each tube, but with the controls provided (Bias and DC Balance), you can still set the bias correctly for each individual tube. In the X1000, the Bias control for each channel adjusts both output tubes for each channel at the same time. The DC Balance control then varies the two tubes of a given channel up or down individually as required at the point set by the bias control. Think of the DC Balance control as a teeter-totter: when adjusted, it moves the bias for one tube up, while at the same time, moving the bias for the other tube down. It is therefore adjusted so that the current draw of both tubes are made equal. When that is done, then the Bias control moves both tubes up or down equally as needed to achieve the correct set point.

    Fisher's procedure has you adjusting the DC Balance control for minimum noise output using a sensitive AC voltmeter connected to the output of the amplifier. Today's DVMs do a poor job in determining a minimum noise level, as the display jumps around too much. It is better to attach a pair of headphones directly to the speaker output terminals (not the headphone jack), and adjust the DC Balance control for minimum hum/noise that way. Then adjust the Bias control for the correct reading at the output tube cathode connection.

    I hope this helps!

    Dave
     
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  3. postman56

    postman56 New Member

    Messages:
    29
    Thanks Dave for taking the time to explain it to me, I expect the new quad KT77's to arrive tomorrow so I look forward to hearing how great this receiver can be now that's it's been recapped with new cans from
    Hamseed.
     
  4. Kingbleu

    Kingbleu Active Member

    Messages:
    180
    Location:
    Southern California
    I am a bit confused about the purpose of the Headphones. Would I be able to adjust the DC balance using a multimeter. That is to say can I hook up the red probe to pin 8 and black probe to pin 1 then adjust the DC balance/voltage going back and forth like the teeter-totter from V11 tube to V12 tube until the voltages on both tubes are balanced (matched). Then after that, I could adjust the DC bias to the correct values and both tubes would be balanced and biased? And of course the same procedure for the other channel. Would this work or am I missing something?
     
  5. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,673
    Location:
    Ball Ground, GA
    Hi King -- Your process would work -- if there were separate cathode sampling resistors installed at each output tube. But in the X-1000 as designed, there are not. That is, the cathodes are connected directly together, and then grounded through a common 10Ω resistor by which to assess the current that both tubes are drawing at the same time. Therefore, in your scenario, if you move your meter probes back and forth between pins 1 and 8 of the two tubes in one channel, you'd simply be measuring the same thing electrically at both tubes -- which is the current flow through both tubes. The next best approach then is to use the headphone approach as described. With it, you are balancing the two tubes by using the residual ripple in the power supply to act as an indicator. This can be done because the noise is injected in common mode into the output stage (that is, into both tubes at once). When the DC Balance control is then adjusted for equal current flow through the tubes, the ripple noise will cancel in the push-pull connection of the OPT, resulting in a null in the ripple noise heard in the headphones.

    I hope this helps!

    Dave
     
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  6. Kingbleu

    Kingbleu Active Member

    Messages:
    180
    Location:
    Southern California
    Thank you very much for that response, It all makes sense to me now. I'm glad I installed the separate cathode sampling resistors on my Fisher X-1000 back when I did my restoration because now I just set each tube individually to 0.43v and I'm done. Whereas before I added the separate cathode sampling resistors I was using the headphone method you describe above, but since I had the headphones plugged into the headphone jack it proved difficult. Yes that's right I had the phones plugged into the headphone jack. I never knew they were supposed to be connected to the speaker terminals.
    Thanks again.
     
    Last edited: Oct 4, 2018

     

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  7. postman56

    postman56 New Member

    Messages:
    29
    Dave, do you strip away a pair of headphone cables in order to do this, red and ground ? Not sure if I want to do that with high quality headphones?
     
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  8. baconbadge

    baconbadge New Member

    Messages:
    36
    Location:
    Queens, NY
    Alligator clips would work
     
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  9. postman56

    postman56 New Member

    Messages:
    29
    Great idea, much appreciated.
     
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  10. postman56

    postman56 New Member

    Messages:
    29
    I assume you just do one channel at a time with the rear speaker connector pins, hot to one and neutral to the other speaker pin?
     
  11. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,673
    Location:
    Ball Ground, GA
    I have a special cord I use that has a stereo female 1/4" plug on one end, and three alligator clips on the other end -- Black for the common connection, red for the right phone, and green for the left.

    Watch out when connecting your phones to the X-1000. Yes, you connect directly to the speaker connector boards, but you connect the common phone lead to the CHASSIS -- not the Com output terminal of either channel. In the X-1000, the Com output terminal does not represent ground level. If you assume it does, and use it as a ground connection for the phones in this case, you will get very uneven sound from the the L and R channels. Connect the phone common lead to the chassis, and the respective L and R phone hot leads directly to their respective 16Ω output terminals.

    Dave
     
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