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Sansui 1000A biasing scheme

Discussion in 'Exclusively Sansui' started by gkargreen, Oct 9, 2018.

  1. gkargreen

    gkargreen Active Member

    Messages:
    481
    Location:
    Fairfax, VA
    I have added 10 ohm cathode resistors so that I can adjust the cathode current via the IBAMs that are a part of the g1 circuit and wondering what kind of value I should be seeing (voltage) across the 10 ohm resistors. Thanks!
     

     

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  2. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
    21,889
    Location:
    Glen Burnie Md.
    With 470-V to 480V on the plates you should see 32 to 33ma or so. That will give you around 15W to 15-1/4W per tube. Within the 80th percentile.

    Do the same thing for this as a Fisher, except the Sansui's plate Voltage is higher so the bias ma needs to be slightly lower to get the same wattage on the 7591's.
     
  3. gkargreen

    gkargreen Active Member

    Messages:
    481
    Location:
    Fairfax, VA
    Thanks again, Larry! I was thinking about the same, right now I am seeing around 18 ma on each of the tubes as measured across the newly installed cathode current resistors, so I guess I need to adjust the IBAMs on this set. BTW, this thing sounds amazing now that I have restored it, although until I got ALL the caps replaced (except for the 4-section can in the front) I was having balance problems. This receiver is probably one of the clearest-sounding tube pieces I have ever had on my bench!
     
  4. gkargreen

    gkargreen Active Member

    Messages:
    481
    Location:
    Fairfax, VA
    Hi Larry, I have another question for you now that the Sansui is up and running well, or sorta. With the current @ 30 ma (plate is 460v with installed CL-80), I let this run for a couple of hours and the output transformer closest to the big power resistors and power tranny is running pretty hot, a good bit hotter than the other one. I noted some waxy residue around that output tranny when I was cleaning it, and I am beginning t think the tranny is compromised, that at some point a power tube went and over-heated the output tranny and melted out some of the wax. What do you think, do I have a bad tranny that is over-heating again? It sounds great, but I am concerned about this development. Thanks!
     
  5. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
    21,889
    Location:
    Glen Burnie Md.
    No wax in the Trannies. The Wax is from the old Can caps leaking in the past. Mine had done the same thing. The center tranny will always run a little hotter than the end trannies. Couple of things you can do to cool them down tho (get some readings on the trannies when hot, should be below 150*F or so.) Take some HVAC Aluminum DUCTING Seam Tape and cover the end bells facing the tubes on all of the trannies. Remove the separator plate between the trannies and the can caps and drill holes in it or cut it down so air flows unimpeded. 6" Desk fan about 12" away blowing on the back will help. A couple of 2" AC computer fans on the back mesh blowing in btwn the trannies will vent out the top. LAST Combined is to put in TUNGSOL Re-Issues vs. EH7591's (if you are using Russian Tubes), and to replace the power resistors while mounting the new ones in an other location. I do believe I posted my fix for that in another of the 1000A threads you started. Mine while running with the cover on now runs about 140* on the trannies, and 125* with the cover off.
     
  6. gkargreen

    gkargreen Active Member

    Messages:
    481
    Location:
    Fairfax, VA
    Thanks again, Larry, I will check out the temps on those trannies tomorrow!
     

     

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