Happy with new 5F1 mods

Discussion in 'Amp Central Station' started by alexwilds, Jun 13, 2019.

  1. alexwilds

    alexwilds TDPRI Member

    Age:
    64
    Posts:
    80
    Joined:
    Jan 27, 2018
    Location:
    Japan
    I just finished a 5F1 built from my scrap pile. It has four significant mods:

    1. choke replacing the resistor between B+1 and B+2
    2. old brown base Tungsol 5881 with 300K cathode resistor rather than stock 470K
    3. full bridge rectifier to get a bit more wallop from the tiny PT
    4. 10" eminence Lil Buddy Patriot speaker (but I may go with a Jensen alnico I got lying around).

    Results are good volume and lots of headroom, up to about 10 on the dial. Above that it breaks up nicely.

    I am now playing around with NFB resistor. Stock 22K is too quiet, and no NFB, while the tone and volume are good, limits range. Probably 56K will be the sweet spot. I will put a semi-closed back on it to boost the bass a bit, and likely a 100pf bright cap across volume pot to keep the highs sharp.

    While no longer truly a 'Champ', indeed, sounds a bit like a Vox, it has lots of singled-ended class A chime. I recommend these mods.

    And now I convert the 5E3 to 5F10 with choke, fixed bias, NFB, bigger PT, paralleled 12AX7, and 5881s. Wish me luck as I dive into the aroma of solder smoke.
     
    dan40 likes this.
  2. hamerfan

    hamerfan TDPRI Member

    Age:
    57
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    Dec 8, 2018
    Location:
    Germany, Bavaria
    I Like the Tungsol 5881 in my 5F1 very much. Caveat for the heater current snd the OT impedance.
    I did fool around with the NFB too. I liked the 1 meg for cleans (nearly Off) and 11k for taming mud on distorted sounds.
     
  3. alexwilds

    alexwilds TDPRI Member

    Age:
    64
    Posts:
    80
    Joined:
    Jan 27, 2018
    Location:
    Japan
    I am digging through my pile of old resistors, looking for something in the 100 - 150K for the NFB. Right now, however, I have it disconnected and it sound so fine. Still, I want the range and balance NFB gives.
     
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