Testing performance of "noose mod" on a Fisher 500-c

Discussion in 'Fisher' started by vintelectra, Aug 1, 2018.

  1. vintelectra

    vintelectra Active Member

    Messages:
    467
    Hi Fans: The following experiment is an attempt to observe objectively the effect of releasing Dave"s noose
    mod on a Fisher 500C

    Material: Restored Fisher 500C, All new balanced Tungsol 12ax7 s New matched quad of 7951A Tumg-sol.
    Noose mod on only one channel (left) , mod as per Dave G, removing R 115 (180K) and decreasing plate B+ of amp stage #6 to 104vdc. Rigth channel untouched. Audio feed 1K @Aux 200mV. PI re-adjusted to minimum THD.

    Observations: Max audio Power; Left ch (mod)-----21.9 wt rms

    Right ch (stock)---22.8wt

    The amp becomes unstable above 1 wt output pwr both ch
    THD@ 1wt Left (mod)----- 0,26%
    Right------------0.04%
    Frequency domain: !st order of harmonics more prominent on Right ch (stock)

    Conclusion: I will return left ch to stock. Martin
     

     

    Please register to disable this ad.

  2. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    36,100
    Location:
    Southern NJ
    What do you mean by the amp becomes unstable?

    Worth a question, did you try swapping tubes between the channels to see if the results changed? Less than 1w output power difference could just be tube differences.
     
  3. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
    21,706
    Location:
    Glen Burnie Md.
    And using Russian tubes is counterproductive. They don't respond to the noose removal as well as Domestic or Euro OLD STOCK TUBES. USE OLD STOCK DOMESTIC or EURO 12AX7's rather than Russian 12AX7 tubes. If you read thru the whole thread on this Dave DOES state that Russian tubes do NOT fare well with or without the noose. Domestic tubes will work better without the noose rather than with the noose.
     
    Tim D likes this.
  4. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,558
    Location:
    Ball Ground, GA
    Power output is considerably lower than normal in both channels. Was the performance of the unit the same in both channels before you made the modification? Also, please post the voltages for the modified driver stage if you will. The modification has no impact on the stability of the amplifier.

    Dave
     
  5. vintelectra

    vintelectra Active Member

    Messages:
    467
    OK! Becomes unstable as in self oscilation, before the natural clipping of normal high pwr. Clean sine under 1w both ch.
    No , I did not swap tubes, a tightly matched new quad.biased at 30 mA.
    I understood Dave made clear that the purpose of removing the noose was to operate the tubes in the linear part of the curve. which does not happen when t
     
  6. vintelectra

    vintelectra Active Member

    Messages:
    467
    when the noose is applied to new high biased tubes Tungsol.

    No , I did not test it before the mod. Voltages coming up
     

     

    Please register to disable this ad.

  7. vintelectra

    vintelectra Active Member

    Messages:
    467
    V13 MOD: #1---246vdc, #2-----120 #3----104 #6-----104 #7--0 #8--0.9 #9---0
    V14 stock: #1----303 #2------120 #3---123 #6------120 #7---0 #8---1,1 #9---0
     
  8. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,558
    Location:
    Ball Ground, GA
    How can you have 104 vdc on pin 6, but 120 vdc on pin 2? There is a direct coupled connection between these two pins. If it was a typo and supposed to be 104 vdc on pin 2, and you have 104 vdc on pin 3. then the inverter stage has no bias applied to it, and of course would perform poorly in that instance. Same if both voltages are 120 vdc.

    The modification was developed and offered on the Fisher 400 receiver (not the 500C), where the current passing through the AF amplifier stage is typically almost double that of the same stage in the 500C. As a result, in a 400 receiver, the typical plate voltage of the AF amplifier section is 85-90 vdc after the modification is installed, which in that receiver, allows the inverter stage to then bias itself quite nicely with the noose removed, producing very good performance with driver tubes of all make and manufacture known to date.

    In the 500C, the benefits of removing the noose still exist, and applying the modification should still be quite similar. But you also may need to tweak it a bit to fit the conditions of the 500C before categorically dismissing its potential for improvement out of hand. One rather obvious tweak needed will be to increase the value of the 33K inverter plate load resistor to about 56K. Otherwise, it will be impossible to balance out the plate and cathode resistance in the inverter section with the noose removed. Once that is accomplished, it also may be that in the 500C, the AF amplifier stage needs to operate from a lower B+ supply voltage yet, than that offered at the next lower B+ tap -- but that can't be determined until the plate and cathode loads are first balanced out in the inverter stage.

    Dave
     
    Tim D likes this.
  9. vintelectra

    vintelectra Active Member

    Messages:
    467
    Thank you for your interest, I understand your design was meant for the 400, but my question was: should I do it in the 500c, and the answer so far is no.

    Reducing the B+ by moving the tap to A ( lowest B+) would also reduce the plate at the inverter.
    But the elephant in in the mod is the self oscillation, While the whole unit is restored , I did not come around yet to measure the audio section with detail as I usually do, but sounded ok when using modulation when aligning RF/IF/MPX sections.
    The discrepancy in #2 and#6 is a typo Thanks Martin
     
  10. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,558
    Location:
    Ball Ground, GA
    As a minor point of correction, moving the B+ feed to the AF Amplifier stage to Tap A would reduce the plate (voltage) at the AF Amplifier Stage, and increase it at the plate of the inverter.

    If the inverter section was operating with zero bias on it during your tests, that is most assuredly the source of the oscillation noted. If further reduction in AF stage plate voltage is needed, then using tap A would likely introduce it's own stability issues when the phono preamps are engaged, and is therefore not recommended. If that is the case, then an additional dropping resistor and decoupling cap of appropriate values may be required to supply the AF Amplifier stage when the noose is removed in this application.

    As to whether you should modify your 500C to remove the noose, that depends on your particular situation. Removing it will produce the benefits noted, but only you can determine if the benefits of its removal are worth the effort to successfully do so in your case. I would suggest that you've apparently already made a determination that it is not which is fine since I don't know the particular scenario of the receiver you're working on (whether it's a personal unit or one for resale, and therefore the motivation behind the exercise) -- but also have apparently made that determination without working the relevant issues through to a full conclusion in this particular application, so as to ultimately be able to place any value on the benefits that removal of the noose achieves.

    I plan on doing a full performance evaluation work up of the 500C/800C power amplifier section in my own 800C soon -- which at this point I have not done before -- primarily to examine the HF transient performance of the design. When I do, I will also visit the issues surrounding removal of the noose as well, and be happy to share the specifics of that effort then.

    Dave
     
    Tim D likes this.
  11. vintelectra

    vintelectra Active Member

    Messages:
    467
    The motivation of the exercise is the exercise itself in restoring these to optimum.I sell them when finished to continue with the exercise ( without any profit when considering parts and hours of labor )It is a hobby.
    I will anxiously await for your future comments. But for the time being I don't see purpose in investing more effort in correcting the chaos created in this test. I will wait for your results in the 800. Martin
     

     

    Please register to disable this ad.

Share This Page