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X-101-ST vs 101-B need to decide ASAP

Discussion in 'Fisher' started by ELAW40, Nov 24, 2018.

  1. ELAW40

    ELAW40 AK Subscriber Subscriber

    Messages:
    5
    Hey! Can use some help in this decision. Will use to play vinyl in a thorens 160 and excellent diy speakers. The 101B is gold faced with high quality new tubes. The ST is all original. Both are working st the moment with no discernible issues. This is a remote purchase so I do not have the ability to test the equipment.

    There is $150 difference between the two.
     

     

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  2. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
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    Ball Ground, GA
    Welcome to the AK Fisher forum!

    The X-101B is a "60s" Fisher, with full Tape Monitoring/switching facilities, center speaker/channel output, headphone jack, and drops the 50s plethora of EQ settings for 78 RPM, London, LP, etc. that the X-101 has. The 101B will have a little more power output since it uses 7591 tubes, whereas the X-101 uses the 6BQ5 family of tubes. The X-101 has no practical Tape Monitoring facilities, and also (depending on which version of the X-101 it is) has solidly ganged tone controls, and also much less practical mode switching facilities. The X-101 is a very nice unit, but is clearly a 50s Fisher, offered at a time when many of the standards for stereo reproduction were not yet established -- but it does have 6BQ5 sound if that's important to you. Both units are tube rectified.

    I hope this helps!

    Dave
     
  3. rufleruf

    rufleruf Poor Impulse Control Subscriber

    Messages:
    1,163
    X-101-ST is EL84/6BQ5/7189 based, X-101-B is 7591. Both use 5AR4 rectifiers. As Dave says, the X101-ST has controls that are unnecessary today.

    I listened to an X-101-B for a few weeks after doing some work on it and really came to like it.
     
  4. ELAW40

    ELAW40 AK Subscriber Subscriber

    Messages:
    5
    Thanks, guys! That is good stuff. I will be listening to LPs only so I am inclined to base the decision on the sound. While that is a subjective measure, I understand that the ST has Warmer sound based on the additional tubes. That said the two of you are decidedly voting for the B, and I do not have the ability to do a Pepsi challenge so will defer to your knowledge. I am a total neophyte with tubes. I just know I love the way my Fisher 400 sounds and look forward to adding another piece for “Dad’s room”
     
  5. rufleruf

    rufleruf Poor Impulse Control Subscriber

    Messages:
    1,163
    If you have a 400, then you have the 7591 sound covered more or less, and the ST would be a slightly different warm tube sound for you.

    I personally think the X-101-ST is one of Fishers better looking pieces.
     
  6. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
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    Also - The X-101ST and the X-101B both use the same number of tubes. Only the output tubes are different between the models.

    Dave
     

     

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  7. gadget73

    gadget73 junk junkie Subscriber

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    The 7591 and EL84 tubes tend to have a fairly similar sound. They also often use very similar circuits, which is probably part of it. The 7591 makes about double the power though.
     
  8. ELAW40

    ELAW40 AK Subscriber Subscriber

    Messages:
    5
    Thanks everyone for your input. Greatly appreciated
     
  9. trainbuftony

    trainbuftony Electron Herder Subscriber

    I cant speak to the 100b, but my 101 ST redplates the power tubes in the stock configuration. The problem is due to the cathode bias arrangement which is going to require subatantial work and modification to get right. Everyone seems to like their x101st, but nobody has written a detailed explanation of how to fix the power stage on this specific model. Im working on figuring it out, but im not nearly as qualified to do so as others on this forum.
     
  10. ELAW40

    ELAW40 AK Subscriber Subscriber

    Messages:
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    Thanks. That is helpful. I take it from your post that this is a universal issue with the ST. Is that true?
     
  11. trainbuftony

    trainbuftony Electron Herder Subscriber

    I dont know. Mine exhibited the problem, so i asked Jim McShane about getting some new power tubes which were 7189a instead of the nicely matched 6bq5 GE quad i had in there. I told him about the red plate issue, and his advice to me was this, paraphrased...

    I wont sell you any tubes for it until you implement some sort of adjustable bias, the GE tubes you have are as good as anything out today. The problem lies with the amp and the fact that it runs the tubes at about twice the idle current that they would ideally have.

    My understanding is that he knows what he is talking about, my measurements agree. My method of dealing with it is to build a negative voltage dc supply for the filaments of the front end and the biasing of the power tubes, just like HH scott used. I havent finished designing it but when i do i plan to do a write up on it.
     
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  12. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
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    Train -- Here's something that will help you out as well. The X-101ST did run the output tubes quite hard -- particularly the early version that has ganged rather than concentric tone controls. However the output stage of the X-101ST is nearly identical to those in the TA-600, one of which I happened to do some extensive work on for a fellow AKer. You can find that thread here:

    http://www.audiokarma.org/forums/index.php?threads/ta-600-service-bulletin-early-version.676168/

    With the audio circuits of the TA-600 so heavily lifted from the X-101ST (except around the phase inverter/driver circuit for those versions using a 7199 in that position), you may fine post #47 (circuit) and post # 34 (installation) most interesting in this thread, where I installed an IBAM for this model, because it too was red plating the output tubes with nothing wrong other than the stock design requiring a very tight match on the quiescent current of each output tube. If one doesn't conduct it's full share of the load, it leaves the remainder to be spread among the other tubes, which then causes them to operate in an over-dissipated condition. As the match becomes further off from ideal, the red plating becomes worse in the remaining tubes.

    The heavy current draw is required because like the X-101ST, the TA-600 uses output stage cathode current to light the phono preamp tubes heaters. With a typical plate-to-cathode voltage of nearly 400 volts in these models, and a total cathode current requirement of about 140 mA, it pushes the tubes right to their Design Center rating for dissipation: 140 mA is 35 mA cathode current per tube. Subtracting out 5 mA per tube for screen current, it means that each plate is passing 30 mA. With a 400 volt differential between the cathode and plate, this produces 12 watts of plate dissipation -- which is fine for these tubes, but because they all need to operate at this level, their matching is of utmost importance. In today's audio environment, such tight matching is usually not possible, so some sort of IBAM is required for safe operation today.

    The IBAM circuit I developed allows each tube to be adjusted individually of course. It works very well, but due to the biasing scheme used in general, the controls are somewhat more interactive than when an IBAM is applied to a fixed bias design -- particularly when used with a SS power supply.

    Installing a negative bias supply certainly works as well, but of course requires more modification and an additional transformer which this approach does not require. You will also likely need to look at the resulting screen grid voltage with such a modification as well, to make sure that doesn't get exceeded under dynamic conditions. In any event, the approach I used in the TA-600 provides a very workable method of installing an IBAM to these units -- as will adding the negative bias supply as well -- so the current load can be balanced equally between the four tubes.

    Dave
     
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  13. ELAW40

    ELAW40 AK Subscriber Subscriber

    Messages:
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    As a newbie I am simultaneously thrilled and intimidated. Love learning but damn, I don’t know much. Thanks for the knowledge.
     

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