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5f11 bias resistor

Discussion in 'Shock Brother's DIY Amps' started by Linkjr, Aug 16, 2017.

  1. jhundt

    jhundt Poster Extraordinaire

    Age:
    64
    Mar 23, 2003
    Netherlands
    hey your 5f11 just showed up on that "Random Media" feature. I clicked on it and admired the beauty for a few minutes. You did a good job!
     

  2. JuneauMike

    JuneauMike Friend of Leo's

    May 5, 2015
    Alaska
    I did read #31 and #32 and I'm reviewing your build thread too, which is illuminating but not precisely on point since we have different PTs.

    I'm using the mojotone 757 PT, with a 350-50-0-350v 75mA secondaries. The 50v is the bias tap.

    But I am wiring my adjustable bias similar to yours. The 50v tap goes to a 10k resistor, to the diode, then to a 10k wiper pot with a 33k resistor grounded to the pot. I'm going to see where that puts me before dialing in the bias. But this phase of the build is worrying.
     
    Last edited: Jan 3, 2018

  3. Linkjr

    Linkjr TDPRI Member

    Age:
    48
    23
    Aug 16, 2017
    Uk
    Juneau Mike good luck with your build, i adjusted my bias to recommended settings here and red plating stopped which i was happy about but trem was not great so i biased cooler and trem sounded much better but not real strong/ deep as i have heard jsnwhites sounded, i have however heard vintage 5f11 with the same level of trem i am getting.

    I wonder if there is another component that can be swapped out to improve trem but could not find any further info on this part of the circuit
     

  4. jsnwhite619

    jsnwhite619 Tele-Afflicted Ad Free Member

    Sep 10, 2013
    Georgia
    I learned that the speaker plays a big role in how much trem comes through. More bass response gives a heavier tremolo "throb" -- it's not really the high end you notice. You feel the bass when it cuts in & out. Before I finished my cabinet, I hooked it to a Veteran 10 in my Pro Jr cabinet just to play it. The trem worked, but there wasn't much to it. Later on, I hooked it up to a friend's ET65 and I thought I was going to get vertigo! haha It was incredible -- lots more bass presence. My video with it has a Cannabis Rex, so again, plenty of warmth there to enhance the tremolo.

    I think someone listed it out on my build thread, but you can change the capacitors in the trem circuit to slow it down, I know, but I can't remember if it increased the depth.

    I've been really lucky in my builds that all of my problems have all been user error ;) Once i fix all of my mistakes, everything usually works like it should!
     

  5. zook

    zook Friend of Leo's

    Aug 6, 2003
    Cochise, AZ
    I have found that this works really well as adjustable bias. The pot replaces the resistor to ground.
    Vibrolux adjustable bias.jpg
     

  6. JuneauMike

    JuneauMike Friend of Leo's

    May 5, 2015
    Alaska

    Can you explain what's going on there? Looks like you are grounding the bias circuit to the depth pot? Won't the bias change as you adjust the depth?
     

  7. jsnwhite619

    jsnwhite619 Tele-Afflicted Ad Free Member

    Sep 10, 2013
    Georgia
    Here is what I used with 16k on the back of the pot, and 100k coming from rectifier (no bias tap)
     

    Attached Files:


  8. jsnwhite619

    jsnwhite619 Tele-Afflicted Ad Free Member

    Sep 10, 2013
    Georgia
    If I remember correctly, this amp has "bias wiggle" tremolo. The tremolo actually works by affecting the tube bias & output. That's why a lot of people have issues getting the tremolo right because a lot of people end up with it biased at 50-55% for the trem to sound good and deep enough. For whatever reason, I seemed to have gotten lucky with mine set at 70%, getting good tremolo and overdrive.
     
    King Fan likes this.

  9. JuneauMike

    JuneauMike Friend of Leo's

    May 5, 2015
    Alaska
    Yeah, its modulating the power tubes but the bias should remain at the point it is set all through the volume range. With this wiring the depth pot would scale the bias, right? Or am i missing it.
     

  10. zook

    zook Friend of Leo's

    Aug 6, 2003
    Cochise, AZ
    The pot replaces the 56K resistor. That Changes the voltage that bleeds off to ground. Not by much, but enough to set the bias.

    It looks like this:

    Vibrolux adjustable bias.jpg

    Here;s how the depth control is normally connected, the same place.

    Vibrolux bias.jpg
     
    Last edited: Jan 15, 2018

  11. zook

    zook Friend of Leo's

    Aug 6, 2003
    Cochise, AZ
    This is similar to my design. The difference is you use a pot and a resistor in series. I only use a pot.
     
    jsnwhite619 likes this.

  12. King Fan

    King Fan Friend of Leo's Ad Free Member

    Jan 1, 2013
    Salt Lake City
    Agree. FWIW my 6g2 — similar circuit and same trem I believe — sounds great with tons of trem at 70% bias.
     
    jsnwhite619 likes this.

  13. jsnwhite619

    jsnwhite619 Tele-Afflicted Ad Free Member

    Sep 10, 2013
    Georgia
    I
    Looks like we both got lucky! Yeah, most everything I read going into it had people saying that 55% or so is what it took to sound right. I think mine would be pushing 400v at that bias level, which is way higher than I would want.

    Btw, I didn't do it on the Vibrolux yet, but on a Harvard (same minus tremolo) I built for a friend, I changed the NFB resistor to 68k and it added some nice crunch and overdrive. So, especially for anyone ended up having to bias on the cold side, that might be something to keep in mind.
     

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