Auto-biasing?

Discussion in 'Fisher' started by cason, Sep 5, 2018.

  1. cason

    cason AK Subscriber Subscriber

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    This is probably a very dumb question, but I’m going to ask anyway. Is there an auto-biasing modification for the 500c?
     

     

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  2. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    In a word, NO. The 500c works on a fixed value of negative bias voltage, however the trend over the last ten years or more, has been to adjust them to a set power setting (ma's) about 70-80% of maximum dissipation of the output tubes. The Fixed voltage is still there but it's not usually used except in preliminary testing and troubleshooting. There are a whole lot of threads on 800c restorations which is a 500c with an added AM Radio band. 400 restoration threads carry a lot of stuff in them that is directly relateable to the 500/800c's.
     
  3. cason

    cason AK Subscriber Subscriber

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    Larry,
    Thanks for the reply and explanation. I had been perusing a number of the threads/posts on IBAM mods and just wondered if anyone developed an auto version of that.
    Bob
     
  4. gadget73

    gadget73 junk junkie Subscriber

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    I'm thinking you mean a different automatic bias than Larry means. The old method that was considered auto bias is also called cathode bias. It has a big resistor from cathode to ground and current draw through the tube creates the bias voltage. The more modern method involves electronics that monitor current flow through the tube and adjust the negative grid supply to maintain current at some set level.

    The electronic version in theory could be fitted up, but frankly I suck at modern electronics so I'm not quite sure how to do it. I don't believe anyone has developed one specifically for the Fisher, but I expect any of the systems that exist to do this could be fitted up, though it likely wants some tweaking to get the values correct for the Fisher.
     
  5. cason

    cason AK Subscriber Subscriber

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    Gadget,
    Thanks fo the reply. You understood what I was asking. As I alluded at the outset, my asking exposes my ignorance of these things.
    Bob
     
  6. Tim D

    Tim D AK Subscriber Subscriber

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    I get that you really are looking for auto-bias, but there is also Dave's handy-dandy Enhanced Fixed Bias (EFB) which is yet another interesting bias scheme. It is not an auto-bias per se, but Dave is like a local AK professor here and I take his innovations seriously. It is easy to search the net for information as his idea is well known and appears often in searches. I am about to receive a Fisher X-100-B that needs repaired and restored and I am thinking to do the full refurb, all of the normal enhancements plus I think I'll try the EFB too. I recently completed a Fisher KX-100 (99% the same as X-100-B), but stopped short of the EFB.
     

     

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