5f1 NFB switch implementation

Discussion in 'Shock Brother's DIY Amps' started by choosebronze, May 21, 2019.

  1. choosebronze

    choosebronze TDPRI Member

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    Hey guys. I've studied Rob Robinette's mod section re NFB switching and I think I get it. I have a 5f1 I built a couple years ago and I'd like it to be grittier. I built it from the BYOC kit (original layout attached). If I wanted to switch between the stock value and some other pre-set value, is this how I'd place the switch? (Or an on/off/on if I wanted a no-NFB option). I see tons of guys talking about the mod but no one shows it in place in a 5f1 layout and I'm always hesitant. Any suggestions on a good value for the X resistor? Is no-NFB usable in the 5f1, or is it too much gain?

    Thanks!

    5f1 nfb switch.png champlifierlayoutorig.png
     
  2. Wally

    Wally Telefied Ad Free Member

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    Schematics tell the tale better than do layouts, imho.
    https://schematicheaven.net/fenderamps/champ_5f1_schem.pdf
    This would be a good time to start understanding schematics...if you have not already. To help, go to Duncan Tube Data for tube pinouts.

    In your layout diagram above, find the 22k ohm resistor that is in line between the tip lug of the speaker jack and the 1.5k ohm cathode resistor of the driver. That is your NFB loop circuit. You need to lift the connection of that 22k ohm resistor from the speaker jack. I prefer to implement an adjustment potentiometer in series with that resistor.
    I personally have never needed to reduce the NFB in a 5F1, but I do implement an adjustable NFB circuit in larger amps.
     
  3. D'tar

    D'tar Tele-Afflicted

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    I agree with wally re:using an adjustable pot. I also have never wanted more grit from a 5f1 with a fully bypassed v1a cathode. Do you have a bypass cap there. Some kits/layouts have omitted this key component. I might rather a switched bypass cap than the NFB switch on 5f1.

    If you really want a fixed value, temporarily connect a, lets say 50kl, potentiometer in series with the stock circuit. Play around with the settings until you find what you like best. Remove the pot and measure the resistance at the setting and there's your resistor value you want.

    your switch config would do fine.
     
  4. choosebronze

    choosebronze TDPRI Member

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    Thanks guys. I do understand the schematic. I'm just always hesitant, and wanted to make sure I'd implemented the switch properly to switch what I was hoping to switch. And yes this amp does have a v1a bypass cap.

    But I was just making the dumb mistake of thinking about NFB backwards. A 50k pot in series with the 22k resistor makes the minimum resistance still 22k. Of course it will. I've got to remember that higher resistance means less signal coming through. I don't know why that's so hard. Been tinkering a long time and I still get tripped up on resistance. I'll throw a pot in and see how it sounds.
     
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  5. robrob

    robrob Poster Extraordinaire Ad Free Member

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    If you do a switch make it a 3-way switch with center off for no NFB at all.
     
  6. jtcnj

    jtcnj Tele-Holic

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    I did the NFB pot as suggested in an Uncle Doug video. I used a 25k pot.
    Dont recall A or B, and either a 4.7k or 2 - 4.7k in series with the pot. So the floor / lowest value attainable is either 4.7k or @ 9.4K.
    It is a VERY usable mod.

    my v1a has the 25µf bypass cap.

    I find I use primarily it to add more NFB signal when running the volume at a max of 3:00 or a hair more with HB to tame the low end a little or a lot, it tightens things up. Just self destructs after that, but I can go higher with SC or P90's.
    The difference / range in tone is also very noticeable.

    My pot is wired so turning it up / clockwise lowers the overall resistance value / lets more NFB signal back to the driver triode cathode.
     
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  7. dan40

    dan40 Tele-Afflicted Silver Supporter

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    Be sure to wire the pot in series as D'tar mentioned. If you accidentally wired it in parallel to the 22k, it would lower the overall resistance and cause more nfb voltage to get through.
     
    choosebronze likes this.
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