Setting Bias & AC Bal on X-101-C w/EFB

Discussion in 'Fisher' started by LexDM3, Sep 1, 2017.

  1. LexDM3

    LexDM3 AK Subscriber Subscriber

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    Hi Fisher wizards,

    I just repaired a Fisher X-101-C for audiodon. He has been using this Fisher in his bedroom and it will be handed to one of his daughters somewhere down the line. Either Don or Dave Gillespie put EFB into this 101-C as part of an overall restoration. This unit got a full restoration but it's not Don's other X-101-C that Dave extensively redesigned.

    A few weeks ago, Don told me that one channel dropped out. It turned out that a Jensen 0.047uF 630V coupling cap with copper foil in oil had turned into a resistor. I ordered and installed a replacement cap and the amp seems fine now. (see note below)

    I'm looking for some guidance on adjusting the Bias and AC Balance pots before bringing it back to Don. The Service Manual says nothing about that. I haven't been able to find an owner's manual but manuals for other tube Fishers tell you to bring it to a tech for AC Balance adjustment. I have Dave G's "X-101-B Mods Rev B" schematic.

    The 101-C has 10 ohm cathode resistors. The voltage across them shows that the four output tubes are running at 28mA (3 tubes) and 33mA (1 tube) at a line voltage right around 120VAC. Should those be running a bit higher than that? Dave shows 34mA on his schematic. And what about that one tube running a bit higher? I assume it's the tube itself. I may just leave the Bias control where it is to avoid running that tube too hot.

    On the AC Balance adjust, I noticed that the Left channel pot was almost all the way to one end while the Right channel pot was close to centered. The bad cap was on the Left channel so maybe that cap was problematic all along before it failed completely. I set the Left AC Balance like the right one before testing the amp.

    I have all the tools/toys needed to adjust this including a Keithley 2015 generator and THD analyzer, a scope, dummy speaker loads, etc. I should be able to put a 1KHz 0.5V sine wave from the Keithley into the inputs and read THD across the speaker outputs connected to resistors, right? I did a quick run of that with speakers connected and got about 0.8% THD on each channel. I know it would be more accurate with a resistive load on the speaker outs.

    Thanks for any guidance. I want this X-101-C to sound good and work well for Don and his family.

    --Dan

    Note: This unit was taken down by a failed boutique coupling cap. Don's X-1000 also was taken out by a failed Mundorf Mcap coupling cap. I repaired that as well and it's fine. However, I'm a bit skeptical these days about the reliability of fancy coupling caps. For $33 and $44 respectively, these should last longer than they did. Are Russian PIO caps more reliable? Has anyone had similar experiences? If that turns into a big discussion, let's start a different thread in the "Tubes" forum. Thanks.
     
    Last edited: Sep 1, 2017
  2. audiodon

    audiodon AK Subscriber Subscriber

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    Yeah my appetite for extra fancy caps has diminished.
    I did Larry's phase inverter alignment thing.
     
  3. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Dan -- I just saw this thread after responding to you by email. But that message was primarily dealing with the AC Balance setting which I hope should be of some help to you. As for the output tube bias however, I forget if I installed EFB in that unit or not. At the time, I remember Don going back and forth of few details, and there were enough units that it's all sort of blended in together in my mind now. Anyway, I don't know if it has EFB of not -- which will affect the setting of the output tube bias of course. Can you post an underside pic? Then we could get to the bottom of this real quick........

    Dave
     
  4. LexDM3

    LexDM3 AK Subscriber Subscriber

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    Thanks Dave. Here's the underside of the X-101-C. No EFB in sight unless you integrated it onto a single chip and hid it somewhere. Naw, I don't think so. :thumbsdown:

    There IS a -24V power supply that feeds the heaters of V1+V2 and V3+V4 and also goes to one end of the bias pot. So this unit has a solid DC supply but no EFB if I'm seeing it right.

    The tubes in the amp are Eico branded 7591s. The cathode resistor readings I just took after 20 minutes of warmup and no input signal are:

    V7 = 0.281V V8 = 0.278V V9 = 0.295V V10 = 0.355V

    The grids (pin 6) are all at -21.50V

    Given that, do I pick a bias point that seems to work well for all four output tubes? Does the higher cathode current in V10 indicate a slight mismatch in the tube set? I have another quad of Westinghouse 7591s that you matched for Don but I'm hoping that we do not have to use them now so they can be saved for spares.

    Thanks for any advice you can give me. And Dave, thanks for the email about the AC Balance adjustment procedure. --Dan
    DV X-101-C Non-EFB Wiring Side.jpg
     
    Last edited: Sep 1, 2017
  5. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Nope. No EFB(tm). My work on this one amounts to installing a separate -24 vdc supply to power the 12AX7 heaters (meaning that the output tubes no longer carry that load and can operate waaaaay cooler than the original design allowed for), installing Screen Stability resistors, and some minor power supply adjustments. All the coupling cap and power supply cap work was done by Don. Proper output tube cathode current draw is 32 mA per tube. To the extent that they vary from an averages setting of 32 mA is a direct indication of the match the tubes afford.

    Keep us posted!

    Dave
     

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