1. Time for some upgrades in server hardware and software to enhance security and take AK to the next level. Please contribute what you can to sales@audiokarma.org at PayPal.com - Thanks from the AK Team
    Dismiss Notice

Identifying Output transformer secondary wires

Discussion in 'Tube Audio' started by multichamp12, Jul 8, 2018.

  1. multichamp12

    multichamp12 AK Subscriber Subscriber

    Messages:
    524
    Hey everyone. So I recently picked up a pair of Stromberg AP-50 amplifiers, which have a bunch of different OT secondary taps:

    0ohm - Black
    8ohm - Yellow
    25v - (Green or Orange?)
    16ohm - (Green or Orange?)
    32ohm - Black/Yellow
    70v - Black/Red
    100v - Black/White

    When I got the amp, the 16ohm solder terminal had nothing going to it, so it was disconnected at some point (also where the NFB is suppose to be) and the 25v tap had the NFB going to it. So I am trying to determine if someone moved the 16ohm tap and the NFB wire to the 25v point, or it was wired incorrectly and the 16ohm tap is indeed the one disconnected.

    The 2 color wires that I am unsure of are Green and Orange, so one is the 16ohm tap, and one is the 25v tap, but they are so close when measuring the resistance with a multimeter, that I dont know which is which. According to the sams manual, the 25v should be .4 ohms, and the 16ohm should be .43ohms.

    Is there another way I can determine which tap is which?
     

     

    Please register to disable this ad.

  2. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    36,764
    Location:
    Southern NJ
    Feed AC voltage across the primary and measure on the secondary. The higher the voltage output, the higher the output impedance. I'd probably run enough across the primary to get about 1v on the 8 ohm tap and go from there. Isolation transformer and a variac make a fine source.
     
  3. multichamp12

    multichamp12 AK Subscriber Subscriber

    Messages:
    524
    Okay thanks! I have heard of some people using a fiction generator, is that possible?
     
  4. multichamp12

    multichamp12 AK Subscriber Subscriber

    Messages:
    524
    Im at my bench now and used a function generator across the primary. When measuring the secondary, the AC voltage on the orange lead is slightly higher than the green. So the Orange is the 16ohm, and the green is the 25v. So whoever wired the NFB did it incorrectly. Thanks!
     
  5. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    36,764
    Location:
    Southern NJ
    The 25v tap should be right between the 8 and 16, so it probably wasn't off by that much. With a 50w amp it should be 12.5 ohms. If it should be on the 16 and was on the 25v it got a little less feedback than it should have had originally.
     
  6. multichamp12

    multichamp12 AK Subscriber Subscriber

    Messages:
    524
    Yea so not that much of a difference, but glad to figure it out and put it back to the original state. Thanks gadget.
     

     

    Please register to disable this ad.

  7. s-petersen

    s-petersen Scott Subscriber

    Messages:
    4,572
    Location:
    Middletown,New York
    I know a few people that are fiction generators:rflmao:
     
    bob smog and nj pheonix like this.
  8. multichamp12

    multichamp12 AK Subscriber Subscriber

    Messages:
    524
    Haha me too!
     
  9. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    36,764
    Location:
    Southern NJ
    Best be careful talking politics now :)
     
  10. cannext

    cannext Active Member

    Messages:
    401
    Its not fiction it are only alternative facts .

    Sorry couldn't resist .
    Regards F.
     
    s-petersen likes this.

Share This Page