X100B receives 400 output transformer swap?

Discussion in 'Fisher' started by rufleruf, Apr 26, 2017.

  1. rufleruf

    rufleruf AK Subscriber Subscriber

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    Pardon my inexperience, but how do I translate the column on page 506 presented above, that has a 10K OPT recommendation in cathode bias, into values to check for or circuit modifications in the X100B? I'm asking for the method not the answer. I'd like to be able to evaluate a change like the one I'm proposing without feeling like I'm asking a bunch of bone-head questions.

    My inclination is to look at the X100B schematic and compare voltages there to those recommended in the manual, and make changes to the X100B to meet the requirements if anything is not within the recommended range.
     
  2. gadget73

    gadget73 junk junkie Subscriber

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    pretty much how I'd go about it. Don't know if thats the best way or not, but making it match datasheet conditions usually gives workable results.

    You may have to get a little creative with the cathode resistor value since you really can't alter it much. Also keep in mind that all voltages are measured relative to the cathode, so while the grid and cathode voltages to ground might not jive with what the datasheet says, they might work out when measured from the cathode. The grid should be somewhat positive to ground because of how they did the bias setup. Looks "wrong" compared to a typical cathode bias setup but it works.
     
  3. rufleruf

    rufleruf AK Subscriber Subscriber

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    I found a few minutes on Sunday and compared the X 100 B schematic to the 7868 manual.

    For cathode bias with 10K output transformer, the datasheet says the 7868 wants 450V at the plate and 400V at Grid 2. The stock X 100 B has 435V at the plate and 365V at grid 2. So far, so good - or at least, I will measure these later, but I am going to guess that these values are going to be close enough to get it going and test it. May even end up being closer to the data sheet since the wall voltage is higher these days.

    The third item is a 170 ohm cathode resistor common to both cathodes. Pins 3 of all 4 output tubes are connected and route into the cathode bias network. How do I tell if this network provides the 170 ohm value? The resistor I am tempted to call the cathode resistor is R71, a 560 ohm 5W item.

    I don't even know where to begin with the Zero-signal, Max-signal specs.

    Help?
     
  4. gadget73

    gadget73 junk junkie Subscriber

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    A single 12AX7 cathode when wired in 12v mode is approximately 84 ohms. (12.6v / 0.15a). four in series is 336 ohms, that in parallel with the 560 ohm fixed resistor and the bias network of 3.3K + 5K + 820 ohms is 199 ohms.

    So effectively you've got a 199 ohm cathode resistor. Thats offset by the bias network though. Since it feeds some positive voltage into the grid, it makes up for the fact that the resistor is probably a smidge too large strictly speaking.

    You can bump the screen voltage by altering the value of that 2.7K resistor in the power supply too.


    The zero signal / max signal things really you don't need to worry all that much about. It just explains how much current it will draw at idle and at full power. At output levels between zero and eleven, current draw will also be somewhere between idle and max. Its got a bit more to do with power supply planning than anything. You need a PS trafo that can deliver at least that much current if you expect to get max output.
     
  5. rufleruf

    rufleruf AK Subscriber Subscriber

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    So if I read this correctly, I'm in the ballpark with the cathode resistor, but I may need to change the 2.7K resistor in the power supply to get the bias adjust target of 40 volts at pin 3 of one of the 7868's.
     
  6. gadget73

    gadget73 junk junkie Subscriber

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    yeah, doesn't sound completely unreasonable.
     
  7. rufleruf

    rufleruf AK Subscriber Subscriber

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    I will make it so!
     
  8. larryderouin

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

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    No Matt it's......Make it so, Number 1, and you have to do it with a British accent!
     
  9. gadget73

    gadget73 junk junkie Subscriber

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    Tea. Earl Grey. Hot.
     
  10. rufleruf

    rufleruf AK Subscriber Subscriber

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    I'll keep a stiff upper lip while I solder.
     
  11. larryderouin

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

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    You don't have to be crazy to be here, but it always helps!
     
  12. rufleruf

    rufleruf AK Subscriber Subscriber

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    With some inspiration from Tim here: I decided to have a look at my failed output transformer. I measured continuity first and focused on the blue lead which tested open circuit. About ten minutes poking and cutting away at the paper cover and I found pretty much the same issue he had. A break in the wire where it went down in to the transformer. Tough to see, so even tougher to solder. Once I had it soldered I checked continuity again and it was reasonably close to the 400 transformer I had to compare it to. Next I checked the blue striped wire again and it's like 35,000 ohms! Way way way higher than the side I just fixed and the 400 OPT I was comparing it to. I guess I have to dig into it again...

    [​IMG]

    [​IMG]
     
  13. rufleruf

    rufleruf AK Subscriber Subscriber

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    No such luck - the blue and yellow lead is about 60K ohms and disappears into the layers without sign of damage. Oh well. Waiting on arrival of replacement.
     
  14. fred soop

    fred soop Super Member

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    For future reference, cut the outer insulation at the center, not near where it disappears into the core. That way, tape for the final closure will have good surface on both sides of the cut. The example above would need to have the tape stuffed down into the core gap to provide good protection.
     
  15. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Matt -- One thing you could do -- or consider working up to as you gain confidence in your modification skills -- is to add the separate little DC power supply and EFB(tm) circuits similar to the way I showed in my thread on the X-101C. With a simple change in a couple of resistor values to that circuit, the EFB parameters would then be adjusted for those I developed for the 400 receivers using the 10K primary transformers. Not only would the tubes then be operating correctly for that transformer impedance, but you'd have a serious performance boost in the process as well. For that matter, since you've got the 400's transformers, you could go ahead and install the entire modified 400 power amplifier circuit as well. Understand, I'm hardly trying to push this on you if you're not comfortable with it or its simply more modification than you care to do. Rather, I'm simply suggesting it as an ultimate answer to your output transformer swap.

    Dave
     
  16. rufleruf

    rufleruf AK Subscriber Subscriber

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    Dave,

    I was originally thinking of going this route - and probably will with the next one. I bought an X 100 C OPT from an AKer that showed up yesterday, so I'm going to just put this guy to stock and call it a day.

    I recently got a pile of integrated Fishers of this sort and will probably do at least one as you recommend. A preview of the thread is: I have a nice KX200 with no OPTs, pairs of 400 and 500B OPTs and an X202B complete and in terrible shape. What would you-all do? For that matter I have so me Dynaco ST70 OPTs, maybe the KX 200 becomes an EL34 based franken-amp?
     
  17. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    One suggestion: Those X-202B OPTs would be a drop in fit for the KX-200.

    Dave
     
  18. rufleruf

    rufleruf AK Subscriber Subscriber

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    Yeah, I know, that's the logical thing to do, but I have stock KX200 and X202B examples in the collection. Would be interesting to experiment a bit no?
     
  19. gadget73

    gadget73 junk junkie Subscriber

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    I'm personally a lot more inclined to franken-amp something that is already fugly. If the amp with no transformers is nice, it would get matching transformers and put back how it belongs. The ratty one that now has no transformers gets the resto-mod since there is nothing of value to lose.
     
  20. rufleruf

    rufleruf AK Subscriber Subscriber

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    That's a good point - there are other factors, but I think the KX200 could look pretty good - X202B... not so much. I'll start a thread about it once I work through the other few project threads I have here - starting with this one!
     

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