1. Time for some upgrades in server hardware and software to enhance security and take AK to the next level. Please contribute what you can to sales@audiokarma.org at PayPal.com - Thanks from the AK Team
    Dismiss Notice

Setting bias question

Discussion in 'Fisher' started by walyfd, Oct 10, 2013.

  1. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,678
    Location:
    Ball Ground, GA
    Walyfd -- Perform the following measurement with the unit turned off and unplugged:

    1. On pin #1 of each 7247 phase inverter tube, there is an 18K 1 watt resistor connected (brown-gray-orange). Use your ohm meter to measure each of these resistors, marking their exact value down, and noting which resistor goes with which channel. Connect your red lead to pin #1, and the black lead to the other side of the resistor being measured when making the measurement.

    2. Locate the two phase inverter controls. Each will have three terminals of which two are connected together. Select a control for one channel, connecting the black lead of your ohm meter to the two terminals that are connected together, and the red lead on the third terminal of the control.

    3. Adjust the control so that the resistance reading exactly matches the resistance of the 18K resistor associated with the channel for the control you are setting.

    4. Perform the same exercise for the other channel.

    5. Enjoy!

    Dave
     

     

    Please register to disable this ad.

  2. Patrice B

    Patrice B AK Subscriber Subscriber

    Messages:
    2,219
    Location:
    Montréal, Canada
    Sorry to hijack the thread, but would this technique work for an X-100-2 Dave?

    Thanks.

    Patrice
     
  3. Patrice B

    Patrice B AK Subscriber Subscriber

    Messages:
    2,219
    Location:
    Montréal, Canada
    Last edited: Oct 16, 2013
  4. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,678
    Location:
    Ball Ground, GA
    Hi Patrice -- Yes, it absolutely will, but the values and measurement points would likely be slightly different. If you can supply a schematic of the phase inverter section of that unit, I'll be happy to check it out for you.

    Dave
     
  5. Patrice B

    Patrice B AK Subscriber Subscriber

    Messages:
    2,219
    Location:
    Montréal, Canada
    See my previous post. The X-100-3 schematic is almost identical to the X-100-2, at least the PI section is the same.

    Thanks Dave,

    Patrice
     
    Last edited: Oct 16, 2013
  6. Patrice B

    Patrice B AK Subscriber Subscriber

    Messages:
    2,219
    Location:
    Montréal, Canada
    BTW, R41 and 42 are 100k 1W
     

     

    Please register to disable this ad.

  7. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,678
    Location:
    Ball Ground, GA
    Patrice -- In your case, you would measure the precise value of the 68K resistor attached to pin #3 of each 12AX7 phase inverter tube. Measure with your black ohm meter lead connected to the chassis, and the red lead to pin #3. Measure and record the value for each channel.

    Then, connect your ohm meter across the 100K resistor attached to pin #1 of each phase inverter tube: Red lead to pin #1, black lead to the other end of the resistor. With the meter so connected in each channel, adjust the phase inverter control for the channel the meter is connected to, to equal the exact value of the 68K resistor measured at pin #3 for the channel being adjusted.

    And there you go!

    Dave
     
  8. walyfd

    walyfd Well-Known Member

    Messages:
    722
    Location:
    Northeastern PA
    Great! I'll try it tonight. Still can't seem to get any changes on the DC Balance adjustments. Should my test leads be DIRECTLY hooked to the speaker terminals (VTVM procedure) or my ground to chassis?
     
  9. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
    21,796
    Location:
    Glen Burnie Md.
    Ground to chassis. With DC Balance you are measuring MILLIVOLTS. Meter to appropriate setting. If set too high you won't get a good reading. Put it on 20mv.
     
  10. walyfd

    walyfd Well-Known Member

    Messages:
    722
    Location:
    Northeastern PA
    I can't wait to get this all back together! If not for this site, I'd never have attempted this project.
     
  11. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,678
    Location:
    Ball Ground, GA
    Remember too that Fisher's instructions to set the DC balance via a low scale volt meter connected across the speaker output connections was based on using a conventional VTVM, the most sensitive type meter of the day. Due to the fact that a VTVM is inherently an analog device, it's much easier to see a slight average rise or fall in the output voltage when making the adjustment, versus trying to do the same thing with a modern DVM, whose digits merely want to float all over the place. It is for this very reason that I recommend making the adjustment using headphones or sensitive speakers, and adjust the DC balance controls for a point of minimum hum in each channel.

    Dave
     

     

    Please register to disable this ad.

  12. walyfd

    walyfd Well-Known Member

    Messages:
    722
    Location:
    Northeastern PA
    Precisely what I noticed using the DVM. All it does is flash 2,3,2,3,2,3...

    All three adjustments should be done tonight. I think the Bias is the most critical and was really the easiest to do.
     
  13. Patrice B

    Patrice B AK Subscriber Subscriber

    Messages:
    2,219
    Location:
    Montréal, Canada
    A big big thank you Mr Gillespie. Much more easy than the other methods.

    If I understand correctly, it balances equally the resistors going to the plate and the cathode of the second half of the 12AX7. Correct?

    Patrice
     
    Last edited: Oct 17, 2013
  14. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,678
    Location:
    Ball Ground, GA
    Patrice -- You are exactly right. When the plate and cathode resistors in the phase splitter stage are exactly equal, the stage is then attempting to produce signals that are exactly equal in amplitude, but opposite in phase at the plate and cathode elements.

    Of course, if everything else down stream from that point is not perfectly balanced (coupling caps, output tube grid return resistors, the output tubes them selves, and even the OPT as well), then the setting cannot represent the most optimum setting, which is the main reason for using a distortion analyzer when making the adjustment. With an analyzer, all the small imbalances between each side of the push-pull signal path can almost be perfectly compensated for by simply setting the control for minimum distortion, which equals maximum distortion cancellation in the OPT.

    Still, if matched or close tolerance components are used throughout the push-pull signal path, the method I've outlined for setting the control will still produce extremely good results.

    Dave
     
  15. walyfd

    walyfd Well-Known Member

    Messages:
    722
    Location:
    Northeastern PA
    SUCCESS! We're in phase, biased and balanced! This has been such a great learning experience. I'd never be afraid to attempt another recap after this!

    I had it running for close to 2 hours last night with a phono and it just cruised right along. I measured the power trans temperature at about 125 degrees(thought it was a bit hot but my 500 gets hot as well). Tonight, it goes back in the console and some serious listening is in order this weekend!

    Is the Phase Inverter adjustment procedure applicable to the 500B? I'd like to check the settings on that as well!

    Thanks Dave and Larry! You've been more helpful than I could ever expect!
     
  16. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,678
    Location:
    Ball Ground, GA
    Walyfd -- Glad you've got it all squared away! The X-202 is a wonderful unit, with the X-202 through and including X-202B being Fisher's best integrated unit that was also the most practical unit for upscale home installations. The X-202 is basically a marriage of a 400C preamp and SA-100 power amp, while the B version was a marriage of the 400CX preamp and the SA-100 if it had been outfitted for more power with 7591 tubes. (Anybody -- did Fisher make a dedicated stand alone 7591 power amp only unit? I would think they did, and if so, what is it?) The X-1000 of course had more power, but basically has the same features as the 202 series. At any rate, you've got one of their best, so enjoy!

    As for setting the Phase Inverter adjustment on your 500B, the same process can be used on it as well, but just as with Patrice, the values recorded and meter connection points will vary slightly, as they will from any given Fisher design to another. However, as long as any design from any manufacturer includes a cathodyne type phase inverter with an adjustment provided in either the plate or cathode circuits of the inverter stage, then this process can be use with very good success.

    Enjoy your 202!

    Dave
     

     

    Please register to disable this ad.

  17. walyfd

    walyfd Well-Known Member

    Messages:
    722
    Location:
    Northeastern PA
    I finally feel as though I CAN enjoy it! It's running on all 8. I've heard the X202 is regarded as anemic against some of the other Fisher amps but it has enough power for me. And it's also one of the coolest and most elegant designs. The brass faceplate, the brass ringed knobs and the indicator lamps ooze quality.

    I've heard a lot of negatives regarding Fisher gear--it's complicated; it's over engineered; it has too many Fisher specific parts... So do Duesenbergs. After this project, it's clear that that "over engineering" is what makes a Fisher a Fisher. I can't make any adjustments on my RCA--it is what it is. It's reassuring to know that I can recheck and adjust as time goes on and it ages gracefully.

    All the Best!
     
  18. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
    21,796
    Location:
    Glen Burnie Md.
    Dave. Is this phase inv. adjustment feasible on an 800C? If so where do meter leads go.

    My Standalone is a 33000 series (30001-39999 manual)
    [​IMG]

    And the Executive 800-C is a 10400 series.. (Executive VIII manual 10001-19999 series).
    [​IMG]

    The 30001-49999 manual is identical to the 30001-39999 manual So the procedure for the standalone would work for all 800-C's from 30001 and up and possibly for 500-C's with the same schematic for the amp section.

    Thanks

    Larry
     

    Attached Files:

  19. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,678
    Location:
    Ball Ground, GA
    It will work Larry, but first you've got to lift one lead of the 120K resistor attached to the cathode (pin #3) of the phase inverter section, and measure it out of circuit, as you will get a false reading with it completely in circuit. Record the value. Then:

    With this resistor still removed from the circuit, measure the resistance from the cathode of the phase inverter to ground. Record the value. Then:

    Determine the effective value of these two unequal resistances when connected in parallel, as they effectively are in this circuit. The formula is (R1 X R2) / (R1 + R2) = Effective Resistance. Let's say the 120K resistor actually read 121.6K, while the cathode terminal to ground read 69.1K. Substituting these values in the formula above gives 44.062K, so the effective cathode resistance when the 120K resistor is reattached is 44,062 ohms. Reattach the 120K resistor.

    Now, connect your red meter lead to the plate (pin #1) of the phase inverter tube, and your black lead to the two terminals that are connected together on the Phase Inverter control for the channel that you are adjusting. Adjust the control for 44.062K, or as close as your meter can resolve to this reading.

    This will cause the inverter itself to develop two signals that are exactly equal in amplitude, but opposite in phase to drive the output stage with. In a pinch, the theoretical value of the cathode load resistance of this circuit (if all the associated resistance values were perfect) is 43.489K, so you could use that setting for a quick set. But for the most accurate setting with regards to the phase inverter itself, the full procedure should be used.

    If this exercise shows anything, it shows that while from appearances, the plate and cathode circuits of this design are wildly unequal in value, but in reality, with a fixed 33K plate resistance, only another 10,489 ohms is needed from the phase inverter control itself to produce a theoretically equal setting in both circuits. Since the control is a 25K control, you can see that the proper setting will reside well within the mid range of this control.

    I hope this helps!

    Dave
     
  20. walyfd

    walyfd Well-Known Member

    Messages:
    722
    Location:
    Northeastern PA
    This should be in a "STICKY"!

    Moved my amp back to the cabinet and gave it a good workout. Never skipped a beat all weekend.
     

Share This Page