5f1 Negative feedback help

Discussion in 'Shock Brother's DIY Amps' started by IITTII, Sep 24, 2020.

  1. IITTII

    IITTII TDPRI Member

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    So i have a 5f1 clone and the biggest problem i have with it is headroom. I tought about doing a NFB mod by adding a potentiometer.

    My Question is can i achive 100% negative feedback by removing the 22k and placing a jumper wire insted of it. Will that damage the amp? does it need to have some resistance. I saw uncledougs video about NFB and he said that you dont want too much negative feedback (why is that?). He did it in a way by adding a 10k Resistor in series with 50k pot so that when you go all the way to the left you still have 10k of resistance. I saw some guys doing it only by adding 100k potentiometer so that all the way to the left is 0ohms(100% NFB) and all the way to the right 100kohms (0% NFB).

    Will 0ohms in the place of 22k damage the amp, will i get more headroom, what values to use.

    P.S.
    It has a 8" eminence speaker and i know replacing it with a 12" will get me more headroom but i still want this option.
     
  2. janglemore

    janglemore TDPRI Member

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    I remember that UD video and I believe he said you don’t want a dead short. So some resistance is required if you have an NFB connection.
     
  3. tweedy_woodpecker

    tweedy_woodpecker Tele-Meister

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    The NFB is not about headroom.
    You feed a certain amount of signal (which is determined by the 22k divided by the 1k5 resistor in a Champ) from the speaker back into the circuit at a point where the feedback and original signal are 180degrees out of phase. So in fact more NFB signal (a smaller resistor) will dampen the signal more.

    The idea and benefit of NFB is not headroom but more bandwith (The signal frequency range).

    Can you define what you exactly mean by more headroom? That the amp stays clean at a larger volume?
    The obvious and probably only choice without larger modifications (more efficient speaker) has been ruled out.

    If you are looking for clean headroom an 8 inch speaker 5f1 is probably the wrong amp...
     
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  4. D'tar

    D'tar Friend of Leo's

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    Direct connecting the negative feedback without a resistor is not anything you will want I promise. Recent thread here...

    https://www.tdpri.com/threads/need-help-with-5f1-2a-please.1049796/

    I wouldn't even experiment below 5k there. You will reach a point of diminishing returns on alternate values. I believe the dumble went to 27k,IIRC. You may like to switch values or even remove the V1a bypass cap. And yes a much more expensive route for more perceived clean volume would be a more efficient speaker. Using the low input and or turning down the guitar volume with the amp turned up may be the ticket as well.
     
    Last edited: Sep 24, 2020
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  5. jsnwhite619

    jsnwhite619 Friend of Leo's

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    The lower the resistor value, the more signal it feeds back into the NFB, and it gets cleaner but quieter and starts losing bandwidth, most noticeable the treble begins to get muffled. Higher values will feed less signal in and will be louder & brighter, and it will get dirtier when you crank it. It's worth experimenting with or getting a pot to adjust. There usually is a level above stock value that will open up more volume and brightness before it starts getting too wild. If you're using a 4ohm OT and you are currently stock 22k nfb, I'd try up to 48k; if you're using 8 ohm, up to 56k. Beyond that will start getting harsh usually. Or install a pot.
     
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  6. andrewRneumann

    andrewRneumann Tele-Holic

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    Can you expound on your problem? Does the amp overdrive too soon? If so, when does it start overdriving on the volume pot? What kind of pickups are you using? Any effects in the signal chain?

    It’s possible that your 5F1 is like everyone else’s just you drive hotter signals into it. If you want to drive an amp louder and keep it clean, I think it usually comes down to increasing the voltages on your power tube.

    Or it’s possible you have something out of whack. If you know how to safely measure voltages while the amp is running, send us the voltages (ground referenced to chassis) for all tube socket pins (heater voltages in AC), preamp and power amp. If you are uncomfortable or unsure how to do this safely, don’t do anything inside the amp. Sounds like you are familiar with the inside of an amp but I have to say it anyway.
     
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  7. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    Agree a hot pickup may be driving the circuit too hard. Possibly a 12a_7 change would help.

    Agree with a more efficient speaker for added headroom.

    Agree with checking and possibly nudging the B+ voltages. A rectifier swap may be beneficial.

    The output transformer can also play a roll here. I have found no specs for the joyo OT. It appears to have 4 and 8ohm taps. What is the primary impedance? Does it have more than one primary tap to choose from?

    Possibly reducing bypass and/or coupling cap values will clean things up a little.

    Good luck on the quest.
     
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