Fisher 500C Bias Question

Discussion in 'Audio Hardware' started by Wrench247, Jan 25, 2023.

  1. Wrench247

    Wrench247 Member Thread Starter

    Location:
    Colorado
    [​IMG]

    Hello everyone. When I was little my step dad was really into hifi audio. We used to build tuned quarter wave pipe speakers and modify amps. When he passed away, I inherited his Fisher 500C. Recently, it stopped powering on and I had a customer of my auto repair shop offer to look at it. He decided he didn't want to mess with it and returned it with some wires desoldered.

    I'm pretty good at reading wiring diagrams and schematics for cars, but I am just not familiar with all of the old vacuum tube terms and designs. I've confirmed the power switch is bad (only one leg shows continuity all the time and the other is open all the time, no matter the switch position).

    The other thing he desoldered was the bias adjustment. From my understanding this is not in the original schematic of the 500. I know my step dad had the amp restored and modified, it must have been 20 or so years ago. Can anyone help me out with how to wire the bias back up? Thanks in advance!
     
  2. Wrench247

    Wrench247 Member Thread Starter

    Location:
    Colorado
    Bear with me as my picture link didn't work...
     
  3. Wrench247

    Wrench247 Member Thread Starter

    Location:
    Colorado
  4. Thorensman

    Thorensman Forum Resident

    Doubtful guy who originally modded
    It is still around.
    In UK There are people like Graham
    Tricker who is a genius at vacuum
    Tube technology.
    In USA look for someone similiar
    It's well worth the effort.
    Good luck.
     
  5. DigMyGroove

    DigMyGroove Forum Resident

    Have it brought to an experienced audio repair tech so any work is done right. I have an 800C that I first had worked on that took care of 80%, but ultimately he was a tad out if his depth. I wound up going to a more experienced tech that I had been too inpatient to wait for the first time. Many months later when it made it through his waitlist I finally got everything the Fisher could deliver.

    Take the time and do it right and you will be richly rewarded. Expect around $500 for the work. The Tung Sol 7591A power tubes sound great should you need to replace those, no need to buy expensive vintage tubes.
     
  6. TarnishedEars

    TarnishedEars Forum Resident

    Location:
    The Seattle area
    I would have sworn that the 500C used self-bias rather than a fixed bias.
     

Share This Page

molar-endocrine