LK-72 Question.......

Discussion in 'Tube Audio' started by larryderouin, Dec 8, 2018.

  1. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    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 The 1.2K has been replaced by me.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

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    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

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    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

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    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

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    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:

  6. trainbuftony

    trainbuftony Electron Herder Subscriber

    On my lk72 restoration, that resistor was blown nearly in half and arcing to the chassis. I think I used a 30 ohm to replace it because thats what I had. If I have another, I'll put it in the box with the knobs.
     

     

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  7. Dave451

    Dave451 AK Subscriber Subscriber

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    Man, train, there must have been some heavy current going through that resistor!
     
  8. trainbuftony

    trainbuftony Electron Herder Subscriber

    A main power supply cap that is a dead short will draw a fair bit of current...
     
  9. Dave451

    Dave451 AK Subscriber Subscriber

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    Yes, indeed. Glad the resistor and rectifier (and fuse, no doubt) protected the PT.
     
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  10. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    OK. Trainbuftony has graciously donated a few power resistors to the knob order so I can see exactly what the 1st resistor should be with regard to DCV. I've got the chassis about totally recapped, waiting on the Hayseed Cans', got the new tubes from McShane last night, so I'll be ready to rock on it probably by Monday. Mouser order made for new 1 and 2w resistors for the CAN sections ordered, plus a 4A rectumfrier. I'm still hunting thru my stash of stuff for parts I know I have but not sure where they are, (in process of moving (slowly) to Girlfriends place as youngest son is renting my house now). He just tossed everything on the bench into boxes with no regard to what it was, and I get to sort it out. (KIDS!!!!!!!!!!!). Rescueing the incomplete 400 I got almost 2 years ago today, and will bring that to the condo and finish it up (bias circuit and IBBA board). When I get the LK-72 up and running, I'll compare it against my running X-101-B and keep the winner.
     
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  11. trainbuftony

    trainbuftony Electron Herder Subscriber

    I ended up selling mine to a club member. We listened to it at his house yesterday for a few hours. Youll love it, they really are great. I have a 299D which is the same amp, which is why i sold the 72. My friend doesnt turn it off. Its been sitting running for over a year, quiet as a church mouse.
     
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  12. lilwing

    lilwing CAGE FREE AUDIO ZOO Subscriber

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  13. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    TrainbufTony's stuff arrived Thursday, as did the Hayseed Hamfest Main Caps, and a quad of T-S 7591 Re-Issues, and 2 6EA8's (replace the 6GH8s). Got most all of the small caps underneath done (except those on order (front panel pots)) and 2 of the 4 Main's in. Waiting on Rectumfrier bridge, and the resistors for the Main Cans'. I'm gonna try the 20ohm replacement and if ok leave it. Otherwise I'll adjust the B+ off the resistor to spec.

    Lilwing. Thanks for the links. Already had them but reinforcement is always welcome.
     
  14. Dave451

    Dave451 AK Subscriber Subscriber

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    Sounds like you're ready to cook, Larry. One question, why 6EA8's substituting for 6GH8's? Do you like them better in the circuit, or have them on hand?
    thanks
     
  15. Oerets

    Oerets AK Subscriber Subscriber

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    You guys are killing me.

    Now I will have to get mine out and listen to it today.
     
  16. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Had them in hand. Jim McS. didn't have any 6GH8A's to match. I scarfed them years ago out of an RCA console I think
     

     

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  17. Dave451

    Dave451 AK Subscriber Subscriber

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    I've had good luck with GE 6GH8A's. Can also use 6U8's and I've had good luck with the Jan Philips.
     
  18. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Thanks Dave. Good to know about the 6U8's. I have GE 6EA8's so I'll see what they sound like. I'll see if I can find a pair of GE's in 6GH8 and 6U8's. I like most everything so far in small signal tubes and RF/IF tubes from GE.
     
  19. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Had to order 2 500K Alpha pots from AES for the DC BAL. The originals were a bit flakey, so out they come. Need to find a bottom plate for it. Other than that it's ready to go. I'll post some more pics this weekend.
     
  20. gadget73

    gadget73 junk junkie Subscriber

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    Pretty sure I've got some 6GH8's if you need them. Couldn't begin to guess what kind or if I've got two alike though.
     

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