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5F1 mod to make it take pedals better? Switchable cathode bypass cap?

Discussion in 'Shock Brother's DIY Amps' started by itsGiusto, Apr 16, 2018.

  1. itsGiusto

    itsGiusto TDPRI Member

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    I've been over the moon with my 5F1 since I've built it a couple of months ago. It has a totally killer sound, but it's also very bassy and muddy, and I feel like I can't use some of my favorite fuzz pedals on it. Maybe it's just not an amp that can take pedals, but I figured I'd ask to see if I could do something about it. I typically run it at 10 (out of 12) and use my guitar volume to clean it up. Maybe if I could make a switchable mod to make it brighter, it could take pedals a little better, and have a little more of a Marshally sound?

    There are a few mods I am aware of, and I'm wondering what might be most effective. There's the bright-cap mod for the volume pot, but since I usually run the volume so high, I don't know if that'd be very effective - as far as I understand it, it doesn't add brightness in, it just prevents highs from being lost when the pot is dialed back.

    Another mod I could do might be to put the cathode bypass cap on a switch - currently I have a 22uF there. If I made that switchable out of the circuit, or to a higher value, would that be effective at reducing low-end/adding highs?

    I'm being choosy about which mod I do partly because I feel like everything is so cramped inside the amp as is. I'm not even sure where I can drill a hole in the chassis to put a switch. If there are suggestions for where a switch could go, I'm all ears.
     

  2. JD0x0

    JD0x0 Poster Extraordinaire

    Age:
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    Feb 22, 2009
    New York
    4.7uF cathode bypass caps, and smaller coupling caps will trim up the bass and take that mud out.

    If you calculate the bass cutoff in fenders, most are way below the threshold of human hearing. (IIRC the 22uF bypass cap gives a 4hz cutoff point) It's just not necessary and can make the amp go into blocking distortion as well (which isn't pleasant sounding) You can trim the low end enough that it takes out the mud but doesn't thin out the amp. It tends to help to remove more bass in earlier stages of the amp, rather than later stages, IME, as well.
     

  3. jimgchord

    jimgchord Tele-Meister

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    Try reducing the value of the nfb resistor.
     

  4. Tapatio

    Tapatio Tele-Meister

    126
    Jun 13, 2016
    New Jersey, USA
    This chassis has so little room on top... there's plenty of space on the bottom between the tubes, but that's hard to reach.
     

  5. itsGiusto

    itsGiusto TDPRI Member

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    Cool. Currently I have the standard .022uf coupling caps. How low to try? .0047uf? .01uf?

    Also, will my switches have to be rated for some particular voltage?

    Also, if you happen to know any videos that show the effect of any of these mods on sound, I'd love to hear. If not, that's alright.
     

  6. kleydejong

    kleydejong Tele-Holic

    591
    Aug 30, 2010
    Orange City, IA
    Smaller value coupling caps will trim bass frequencies. .022uf is a fairly moderate value though, I'd try other solutions first - like the cathode bypass caps.

    The only time I think much about the power handling of a switch is on the speaker output impedance switch. Using a weanie SPDT switch like these - https://www.amplifiedparts.com/products/switch-carling-mini-toggle-spdt-2-position - has handled every preamp mod I've ever dreamed up just fine.
     

  7. elpico

    elpico Tele-Holic

    674
    Sep 14, 2011
    Vancouver BC
    An ac30 uses a .0005uf coupling cap so there's quite a range. I think you'd find that too thin in your amp, but nobody can say for sure what will work with your gear and your ears. Try a .005uf and a .001uf, see what you think.
     

  8. itsGiusto

    itsGiusto TDPRI Member

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  9. itsGiusto

    itsGiusto TDPRI Member

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    More negative feedback makes it less bassy? I can never remember. Also, what's a good value NFB resistor to use? I'd like to maybe have something switchable with an ON-OFF-ON switch, so I can get stock (22k), no NFB, and one other value.
     

  10. jimgchord

    jimgchord Tele-Meister

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  11. jimgchord

    jimgchord Tele-Meister

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    Not necessarily less bassy, but cleaner, which may or may not make it a bit more receptive to gain pedals. It seems to work better for me. I use the blackface value which is 2.7 k
     

  12. jsnwhite619

    jsnwhite619 Tele-Afflicted Ad Free Member

    Age:
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    Sep 10, 2013
    Georgia
    I didn't read through ALL the responses, but if you want to use it for pedals I have an easy suggestion to try. A 12ay7 really kills the gain in those amps. But, if you are using a pedal board with a fuzz and overdrive, you might can use those to boost the front end exactly where you want and use your other pedals as well.
     
    robrob likes this.

  13. Deepblankspace

    Deepblankspace TDPRI Member

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    I recently built a 5f1 from mojo, i had them switch out the "original" output tranny to one with a 8ohm tap, less original but good. First time i ran in was into a 2x12 cab. Volume on 4, boss gt 1 multi effects, the champ took the pedal very well, tried out all the preamps pn the pedal through the 5f1, sounded great.

    Bigger speakers, roll back the amp volume, cleaner pallete for pedals?
     

  14. jtcnj

    jtcnj Tele-Meister

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    Not my ideas but i did a variable NFB 25k pot in series with either 4.7k or 10k, (dont remember) and a 3 way on off on switch for 25µf / none / 0.68µf (i think) bypass cap.
    I think they come from Uncle Dough and or Dave Hunter.

    no bypass really drops the gain and high NFB pulls out a lot of lower end mush; much more well mannered.

    It is not champ like at all, but does take pedals well.
    I prefer the rudeness modes myself.
     
    robrob and itsGiusto like this.

  15. itsGiusto

    itsGiusto TDPRI Member

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    If I'm going to try having the preamp tube cathode bypass caps be switchable, is it worth it to use two separate on-off-on spdt switches, one for each cap, or should I put both caps on the same on-off-on dpdt switch? I'm planning on having them be switching between 25uf, 4.7uf, and off. Can you get any good sounds by having them be switched to separate values, or should I just use the dpdt and have them be the switchable, but always equal?

    Or is it better to just put the first cathode bypass cap on a switch and forget about the second one, leave it at 25uf?
     

  16. jsnwhite619

    jsnwhite619 Tele-Afflicted Ad Free Member

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    Did you try a 12ay7 yet? Before drilling any holes?
     

  17. itsGiusto

    itsGiusto TDPRI Member

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    Thanks, but I'm interested in getting some different sounds and voicings of the amp, even separate from the pedal-issue, and I'd like to keep the original sound intact as well because I think it sounds great as-is for some purposes. I feel like if I want to get lower gain, I'll just turn down the volume on the amp.
     
    Last edited: Apr 23, 2018

  18. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    For the greatest flexibility I recommend two separate cathode resistor switches.

    You definitely want to be able to run 25uF in V1A and no bypass in V1B for the stock configuration.

    A 1uF for V1A would also be a nice option for a 3-way switch (25uF-none-1uF)

    A .68uF or 1uF for V1B would be adequate for an extra gain boost.
     
    itsGiusto likes this.

  19. itsGiusto

    itsGiusto TDPRI Member

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    Cool, thanks! I just realized that what I was previously thinking of as the V1b bypass cap was actually the power tube bypass cap! Just out of curiosity, does anyone ever mess with that value, the power tube bypass cap? Does that have an impact on tone?
     

  20. itsGiusto

    itsGiusto TDPRI Member

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    When you say "4.7uF cathode bypass caps", are you referring to the adding a 4.7uf cap to the second preamp stage? Or lowering the value of the power tube cap from 25uf to 4.7uf?
     
    Last edited: Apr 30, 2018

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