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IBAM on a X-202-B

Discussion in 'Fisher' started by bobva, Sep 30, 2018.

  1. bobva

    bobva AK Subscriber Subscriber

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  2. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Bob -- The good news is that Fisher already did most of the work for you! Each channel has individual Bias and DC Balance controls, which will let you adjust each tube to a proper setting -- once you finish the job they started. You will need to install individual 1/4W 10Ω cathode resistors at each output tube, and (possibly) provide some test points for easy checking of the setting, but other than that, that's it. When you're in there, you will also want to install individual 1/4W 100Ω Screen Stability resistors at each socket as well -- then you'll be all set. The tubes will be safe, and you can check and adjust each tube to the proper setting. The procedure for setting the tubes properly is somewhat different when using a Bias and DC Balance control than with an IBAM setup for the settings, but it is a better overall setup as you will find out when you go to adjusting the tubes. When you get to that point, there's plenty of good folks here to help you with the procedure.

    Good luck with your X-202B!

    Dave
     
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  3. bobva

    bobva AK Subscriber Subscriber

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    Dave -- Im happy to hear from you.
    A little about me Im a idiot when it comes to the internal workings off these units. So please bare with me. I got the tools, for the work involved & know how to use them. I can rewire My boat. My 50 yr old cars and trucks... This is just not my field of knowledge.

    I just got the X back for the local from the guru. She was in there for a total recap & up date. I was not there for most of the work mainly the bias points & resistors

    Some square blocks came out ( I'll point a pencil at things that I saw reworked in pics ) Then I watched the 1/4W 10Ω cathode resistors were put in on pin 5? of each 7591 tube. Now if they are 1/4W I don't know,, 10 ohm Im 99% sure they are. The test points are now the AC outputs that fisher put on the rear above the speaker outputs.. AC was & is decommissioned, abandoned,, out of service.

    The 1/4W 100Ω Screen Stability resistors. I don't believe those were put in.

    Seems like she is running Hot when using 121 AC currant. Im running her on a Var 110 ac and she's cooler but still.
    It is Hotter than the 500B that I got running 121 AC household current.
     
    Last edited: Oct 1, 2018
  4. bobva

    bobva AK Subscriber Subscriber

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    110 var ac volt age
    V9 .253 dc. 220 temp
    v10 .253 dc. 215 temp
    v11 .260 dc. 213 temp
    v12 .249 dc. 216 temp
    when 121 ac house .. temps were 255-260b abouts.
     
  5. bobva

    bobva AK Subscriber Subscriber

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    In the first pic two 10ohm 5W resistors were removed were replaced with the green ones
     

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    Last edited: Sep 30, 2018
  6. bobva

    bobva AK Subscriber Subscriber

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    1/4W 100Ω Screen Stability resistors ------ were not installed.

    BTW where would someone put them?
     

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  7. bobva

    bobva AK Subscriber Subscriber

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    I've read the instructions in the service manual,,He did also... Adjust to .8 DC wouldn't that make things hotter?? The unit came home with me I have not touched one pot. bias # are what they were when I received it.

    I feel like Its the case of the bad mechanic blaming it on the gas.. Honesty He told me to lower the voltage.

    She sounds great. But things just don't add up.
     
    Last edited: Sep 30, 2018
  8. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Location:
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    So the 10Ω cathode resistors are already installed, and test points were provided as well. Then you're all set to go!

    The biggest reason that the X-202B runs hotter than a Fisher receiver is simply due to the differences in real estate: There is simply more room on a receiver chassis than on the X-202B chassis. Yes, the receiver has more tubes, but they are small tubes, and spread out more. On the X-202B, the power tubes are located down in a channel, while fully exposed at the rear of the receivers (except the earliest 500Bs). Also, and most important, the case is taller for the receivers, whereas for the X-202B, there is little overhead space at all with the cabinet installed.

    Adjusting the tubes to .40 vdc each (.80 vdc total per channel as specified) will definitely make the tubes run hotter, but at .25 vdc, you're running them rather cool and therefore into the territory where cros-over distortion begins. usually, the best overall compromise for the X-202B and the Fisher receivers is to operate the tubes with 30-32 Ma of standing current (equal to a reading of .30-.32 vdc). This will keep the tubes well clear of any cross-over distortion, and lets the tubes operate with a plate dissipation of around 14 watts each, which is just under 74% of the tube's rating, and represents conservative operation. A small fan will do wonders in aiding cooling with the cabinet installed......

    Dave
     
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  9. bobva

    bobva AK Subscriber Subscriber

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    Dave thanks
    You make it sound like I'm good to go..
    So I can bring up the voltage up. OK set 30-32 per test point..
    Yesterday when she was on the silly Scope the cros-over distortion stared coming in at 12oclock, In theory I never play any of my stereos past that point..To loud for my taste..
    The way Im understand the Bias pot CH A brings up the voltage on each 9 & 10 at the same time..but not totally equal.Then DC bal pot kind of balances them 9&10 out like a scale or teter-totter to make then equal.
    Then of course Bside same thing.


    Again Thank you sir I will put her back on line tomorrow. I'll keep a eye on things.
    What do you consider safe operating temp for the tubes & transformers?
    What you saw on pics you think she safe to run on 121 AC?
    The variac Im using is really a pog I mainly use it for speed control out in the garage for grinders ETC .
    I got no fears running my KX-200 500B or my Dyna ST-70 on 121AC

    Thank you
    Bob
     
    Last edited: Oct 1, 2018
  10. bobva

    bobva AK Subscriber Subscriber

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    It's tomorrow now
    121 AC household current
    all points .336 VDC is lowest I can get her. at this time.
     
  11. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,768
    Location:
    Ball Ground, GA
    The best way to determine appropriate AC line voltage is to measure the heater voltage at the output tube furthest from the power transformer (pins 2&7). Best practice is to keep it within +5/-0% of the nominal 6.3 vac that the tubes were designed to operate on. If the heater voltage is higher than this limit, then some sort of line voltage adjustment would be appropriate. Of course, any adjustment will also affect the B+ voltage which is fine as well -- but that is of little concern. Get the heater voltages right, and then the B+ voltage is, what it is, what it is. Then bias the tubes to the ideal 30-32 mA range.

    Your understanding of how the Bias and DC Balance control operates is correct. Understand too however that because the power supplies of the X-202B are not regulated, there will be some interaction between the four controls, so you will need to go over the process a few times to zero all the tubes in correctly.

    As for the screen stability resistors and their installation, you might take a look at my thread of 5 years ago regarding my own X-202B, which you can find here:

    http://www.audiokarma.org/forums/index.php?threads/x-202b-heaven.551876/

    And particularly note the pics of post #11. In it you will see the Screen Stability resistors installed at each output tube socket running between pins 1 and 4. There are a number of ways to install these resistors, with the only real criteria being that their leads be kept short. Basically it involves removing the leads that interconnect all the pin 4 and 8 wiring at each socket, and any other wiring connected to either of these pins at any of the sockets as well, leaving ONLY the pins 4 to 8 jumpers present at each tube socket. The idea is to then create a new screen "buss" circuit that all the removed wiring connects to, as well as one end of each new resistor -- the other end being connected to pin 4 at each tube. In this way, the screen grid of each tube is powered through its own Screen Stability resistor. In the receivers, many of the folks just use an overhead buss lead to do the job. But in the X-202, space is limited, so I developed the approach of using the unused pin #1 of each tube socket as the new screen buss terminal. To do a neat job of it requires some work, but I always like my work to look like I was hardly ever there. As a result, I reuse the original leads interconnecting the sockets -- but now interconnecting them at pin 1, and create the buss that way.

    If you're using the new Tung Sol tubes, they tend to run hot. Once you ensure your AC voltage is correct, if you still can't lower the current draw enough, then a simple component change will take care of that.

    I hope this helps!

    Dave
     
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  12. bobva

    bobva AK Subscriber Subscriber

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    Now .320 across all points.. :thumbsup: :music::music:
    Tubes and transformers are cold...:thumbsup: I did put a fan on her need to get a smaller CFM computer style soon.

    Yes Dave you have a huge help BIG time. :thumbsup::thumbsup:
    Thanks for the link.:thumbsup:

    Im using my last set of Westinghouse that I bought years ago they are all basicly NOS..
     
  13. NoTransistors

    NoTransistors Dual Turntables Super-Restorer Subscriber

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    Cool tweak. Happy bias balancing!
    My X-202-B runs hot enough to roast a chicken [and a few small potatoes]. I put it aside in favor of an H.H. Scott 299D. Sounds better [in my opinion], and runs so very much cooler.
    BTW, these 7591 tubes do not last very long, even if bias is trimmed-down a bit. When they start to wear out, the amp seems to lose its magic. I am also on my last quad of NOS tubes. Already wondering about those new Tung Sol.
     
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  14. bobva

    bobva AK Subscriber Subscriber

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    Thank you
    I did put X-202-B back on the variac 116 AC got her got her .310 all across the points running a LOT cooler no fan needed anymore. Putting in those bias points was real easy. At least it looked it to me. I just don't know one of those tiny parts from anther or let alone how to buy them online... I got ways if needed.
    The new Tung Sol tubes are in my in my 500B & KX-200 they are a good tube. Just don't buy them from internet peddlers. They are not really matched. Every one here is saying Jim McShane is the go to guy. Next set will come from him.
     
  15. NoTransistors

    NoTransistors Dual Turntables Super-Restorer Subscriber

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    I recall that a matched quad is about $120 on ebay. The sellers are being dishonest? Jim has a wonderful reputation. How are his prices on these things? I do not begrudge him making money. Just want to have an idea.
     
  16. bobva

    bobva AK Subscriber Subscriber

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    The tube world is funny the way I understand. You got A-list retailers that get the better tubes from the factory ( Jim ). Then you got the B-list ( tube depot). More than likely a C-list D E F G ( Ebayers ).
    I bought mine from tube depot Tung Sol box with hand written numbers..
    The guy that fixed re caped my has TS tubes all of the place with lots of printed info on box about the tube inside..
    Im not calling any seller dishonest.. If thats what they get thats what they sell you.
    I have no Idea what Jim sells his for. Cant be way much more than tube depot I think I gave 150$
     
    Last edited: Oct 1, 2018

     

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  17. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Last set I bought from Jim went around $130-135 IIRC. But he matches his on Real World voltages (plate and screen) to within a red hair on a gnat's ass. I've never got a quad of ANY output tube (7591's, 7868's 6v6's, 6L6's, EL-84 type) from him that were out more than 1ma across the board. My oldest 7591's from him are EH's (7591's) and they are running in the Sansui 1000A with no problems. I haven't had to re-adjust them in almost 8 years now. And the Tung Sol's are running all the time in the X-101B or the 800c.
     
  18. NoTransistors

    NoTransistors Dual Turntables Super-Restorer Subscriber

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    Just as I suspected.
    Jim is honest. A rare attribute.
     
  19. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Jim only has a few 7591 TungSols and has Call or Email for Details. Possibly supplies are limited from the Factory, who knows. The prices he's showing for the EH 7591's haven't changed since I got my last quad 4 years ago. 7868 prices are same as 1 to 3 years ago for the last 2 quads I got for 400's.

    I buy small signal tubes from different vendors, but JIM is the only guy I go to for OUTPUT TUBES and Rectifiers.
     

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