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X100b recap for safety, help requested.

Discussion in 'Fisher' started by stereory, May 16, 2017.

  1. stereory

    stereory Member

    Messages:
    57
    I have a X100b that is all stock. Its super clean and it sounds excellent. I would be happy to use it as-is but I am afraid I am pushing it. I assume I am making a poor decision using it.




    What the minimum I need to replace to get this running safely?

    How do I check the bias or otherwise check voltage to assure I am using this safely and not damaging the tubes?


    Thanks.
     
    Last edited: May 17, 2017
  2. Tim D

    Tim D AK Subscriber Subscriber

    Messages:
    568
    Location:
    Melbourne, FL
    I'd say to replace all the electrolytic caps. That would be the C1, C2, C3, and C4. Some of those are combination caps. You can get kits from businesses like Hayseed Hamfest or you can gut the caps and stuff new pencil caps inside or you can put the new caps under the chassis. I just restuffed the caps on my KX-100 which is the kit version of the X-100-B. It wasn't too hard. You might want to check my thread on the KX-100. Basically the same amp. There are other recommended mods for these amps. I've only done a portion of the mods. I still have some tweaking to do and then I'll be mostly done for a while.

    http://audiokarma.org/forums/index....-100-tube-amp-will-need-some-guidance.765247/

    Also make note that outlet voltages have crept up since these were new. This means that the system and tubes are taking a beating with higher than anticipated voltages. There are ways to bring the supply voltage back down so you aren't beating it up so much. Lotsa help here from the real experts (not me). You just need to ask questions.
     
  3. Tim D

    Tim D AK Subscriber Subscriber

    Messages:
    568
    Location:
    Melbourne, FL
    Oh and be careful. These caps can hold a DC charge and they can have 100's of volts on them. You want to make sure they are discharged when working on them.
     
  4. mibag6

    mibag6 AK Subscriber Subscriber

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  5. Tim D

    Tim D AK Subscriber Subscriber

    Messages:
    568
    Location:
    Melbourne, FL
  6. stereory

    stereory Member

    Messages:
    57
    Thanks... the info you provided is a great start.

    As far as checking and setting bias.... what is the procedure?

    Theres that small bias knob next to the tubes... so i assume it was intended to be easy. On mine that dial has a small blob of glue or wax on it so it isnt twisted easily... is that standard?

    I figure if i knew out to set/check the bias I would feel a little better using it.
     
  7. Tim D

    Tim D AK Subscriber Subscriber

    Messages:
    568
    Location:
    Melbourne, FL
    EDIT: Corrected to mention 10 ohm resistor....

    The bias adjustment was probably glued to prevent a casual user from changing it. I believe it was intended to be checked and adjusted by a technician. The way to set it is to first measure the current through the output tubes. I think they used to set it based on control grid voltage, but today I believe the AK experts recommend setting it based on output tube current. To do that, the experts on AK are recommending the addition of a 10 ohm resistor to the cathode on each output tube. The current through each output tube is measured by measuring the voltage across each of the 10 ohm resistors. You then use Ohm's Law to calculate the current. Best case is they all have the same current through them and the current is near recommended values. I don't recall what that recommended value is, but I've seen recommendations in various threads. (My KX-100 doesn't have that pot and that is one of the most obvious differences between the KX-100 and the X-100-B.) So you probably don't want to just give that pot a twist until you can measure that current.
     
    Last edited: May 22, 2017
  8. stereory

    stereory Member

    Messages:
    57
    So then is it just poor idea to use the amp in the meantime?

    Thinking that with higher line voltage and a likely tube changes over the years there is no way the bias is correct.

    I was in no rush to recap since it sounds sweet right now and appears ok but it looks like i have some reading to do with the links you provided.
     
    Last edited: May 22, 2017
  9. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

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    Tim amd Stereory: The Cathode resistor is a 10 ohm 1/4W, NOT A 100ohm as Tim Stated. However the SCREEN resistor is a 100ohm 1/4w resistor.

    Stereory. Put the 10ohm resistors in the cathode circuit (1 per tube), measure the mv and convert to ma on all tubes. Determine the maximum dissipation of the output tube, then 75% of that tube. 70% to 80% is recommended range to keep the tubes in. With old tubes they will be all over the place and very very rarely matched for dissipation. In that case, if they are spread all over, set one of the middle rated tubes to 75% without letting the highest tube go higher than 80% if possible. If the tubes won't allow that, then set the highest for 80%. This will allow you to listen to it to get a baseline sound right before you rebuild it. I would strongly recommend you build the IBAM/IBBA board which would allow you to get the output tubes in line with regard to dissipation.
     
  10. Tim D

    Tim D AK Subscriber Subscriber

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    Location:
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    My bad Larry. I've made an edit of the post to correct. Getting my resistors crossed.
     
  11. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

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    Just as long as your fingers aren't in there when they touch. YEEEOW!
     
  12. stereory

    stereory Member

    Messages:
    57
    Tim or anyone else reading-

    Did you change the sockets of your KX100/X100B to accommodate different tubes? Looks like the 7868 tubes are hard to come by?
     
  13. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

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    The 7868EH are still available but they are having supply problems right now, according to Jim McShane. The stocks of old stock tubes is way down and are priced accordingly high. The 7868EH's work quite well in FISHER's but the only down side is the pin size on them. It's slightly larger than the old stock 7868's and will expand the pin leaves, making a swap to old stock near to impossible. But they still are available but stay away from EBAY for them. Use a reputable dealer, and get a quad of matched tubes (if possible).
     
  14. Tim D

    Tim D AK Subscriber Subscriber

    Messages:
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    Location:
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    I didn't change the sockets. I'm using old stock 7868's. I like to try to keep it original within reason, but soon the prices might force me to go to another tube. I'm taking it one step at a time.
     

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