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LK-72 Question.......

Discussion in 'Tube Audio' started by larryderouin, Dec 8, 2018 at 8:46 AM.

  1. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
    21,887
    Location:
    Glen Burnie Md.
    Working on a LK-72 and have a question about the 1st resistor off the Rectifier tube. I've figured out it's likely an early lk-72 vs. a 72B based on the 18 ohm 1W resistors on the S.S. Rectified side. However almost nothing matches on the tube rectified side. That side is more like a 72B. So is the 1st resistor off the rectifier is supposed to be a 10W 33ohm or a 20ohm in the 72b. Out of circuit it's showing 20ohm. No markings on it at all. The big tower resistors off the 1st resistor are 15K (as Marked) and 8k (as marked). The 1.2K has been replaced by me. It's CORNFUSING AS HELL! Serial # on it is 177599. Do I use the LK-72 or LK-72B scats.

    Pic of offending resistor with the pen on it. It's blistered and ceramic bubbled. I'm thinking a 15w or 20W might be more appropriate.

    100_5442.JPG

    Any and all help is appreciated.

    Larry
     

     

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  2. crispycircuit

    crispycircuit AK Subscriber Subscriber

    Messages:
    1,447
    Location:
    near Buffalo NY
    Those are high quality power resistors, If it reads wrong then replace it. I'd stick with what's in the original amp (ohms rating). As you see various amps have different values.... And Nice amp there!
     
  3. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,765
    Location:
    Ball Ground, GA
    Larry -- One of the qualities of those types of older wire-wound resistors is that they typically either read the value they were manufactured to, or they are defective as in being completely open. Therefore, if it measures 20Ω, then that's what I'd stick with, but upping the power rating of the resistor would definitely be a good thing. Does the resistor look original, or is it a replacement? Scott typically did not use those types of WW resistors in their units, so you might also consider that the current resistor is itself a replacement, and maybe only one of close value at that. That may be where all the confusion is coming in.

    Dave
     
  4. primosounds

    primosounds Powered with pure tube sounds. Subscriber

    Messages:
    2,793
    Location:
    Terra, 21st century CE
    I agree with Dave in that IME, Scott, used mostly "sand or cement" type below the chassis and ceramic enclosed for the ones up top. Still the voltage drop difference between 20 and 33 ohm wont be very much and in a pinch subbing one for the other would not hurt anything. If you would rather have a lower B+ using the 33 would help that. Lately, i have been removing that resistor and installing a choke up top in the space left after removing the can cap.
     
  5. Dave451

    Dave451 AK Subscriber Subscriber

    Messages:
    1,172
    Location:
    Philly area
    Larry,
    My LK-72 has the original 'sand/cement' resistors installed: 33 ohm 10W for the first off the rectifier and 1.2K 10W for the screen supply resistor. Attached shot shows them still in place on my re-worked LK-72 (and checking good, as Dave G mentions--good or nothing for these types).
    Dave
     

    Attached Files:

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