Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com
Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com
Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com

NFB switch help

Discussion in 'Glowing Bottle Tube Amp Forum' started by El Famoso, Sep 24, 2017.

  1. El Famoso

    El Famoso TDPRI Member

    56
    Sep 16, 2016
    Nashville
    I recently picked up one of those little 5w tube amps from Monoprice. On sale those things are ridiculously cheap. Got it to screw around with, maybe steal parts for a project or use it to practice modding. Like lots of others I think it sounds pretty good stock, so I think I'll leave it alone for now. Plays nice and quiet, which is great since I have a 2 year old and a 5 year old who go to bed early.

    I was hoping to get it to break up a little earlier at lower volumes, and thought putting in a negative feedback switch might help. I've done some work on electronics/soldering, etc. so I know how to use the tools. I know how to drain caps and not get nailed by electricity (electrician by trade...) My knowledge base is laughably small compared to a lot of the others on here, though. So I turn to you for help with my rookie question. Will putting in an on/off for the NFB help get me break up earlier?

    I think I just clip one end of the NFB resistor and interrupt with a standard spst switch. Is that right? I assume it doesn't matter which side. Any help is appreciated.

    I have a pic of the inside. I am guessing that the only resistor on that little pcb is the one?

    NFB.JPG

    Thanks.
     

  2. Silverface

    Silverface Poster Extraordinaire Platinum Supporter

    Age:
    65
    Mar 2, 2003
    Lawndale CA
    Which model is it - they make several. Need to look at the schematic to know and possibly determine if the one in the picture is it.

    It has no identification on it - is the circuit board marked with the value?
     
    Last edited: Sep 24, 2017

  3. El Famoso

    El Famoso TDPRI Member

    56
    Sep 16, 2016
    Nashville
    Circuit board says 220R 2W5. Marked R8 for the schematic. That is a 220 ohm resistor, 2 watt? Not sure what the 5 is. 5% tolerance?

    The model is the one with the control panel on the front, not on the top. 611705 i guess is the model #.

    Unless i am misunderstanding this whole neg feedback thing, doesn't it have to come off the output side of the power transformer? There is a jumper (2conductor) back to the main circuit board, so I guess it could be after that. In that case, perhaps I could interrupt the jumper wire with a switch? That would be easier than messing with the pcb...
     

  4. El Famoso

    El Famoso TDPRI Member

    56
    Sep 16, 2016
    Nashville
    Unless i am misunderstanding this whole neg feedback thing, doesn't it have to come off the output side of the power transformer?



    sorry, output side of the output transformer, not power transformer. You know what i mean :)
     

  5. UKStratopastor

    UKStratopastor TDPRI Member

    22
    Sep 16, 2017
    Stockton-on-Tees, UK
    To answer your question, yes; use a switch to interrupt the NFB path; doesn't matter which side of the NFB resistor. But that's assuming the amp uses NFB at all - some don't, for example my Electar Tube 10. I wouldn't proceed without tracing the schematic (gulp) or finding one (better).

    If there is no NFB, it'll probably be possible to get a bit more gain out of the preamp with a couple of resistor swaps.
     

  6. JD0x0

    JD0x0 Friend of Leo's

    Age:
    27
    Feb 22, 2009
    New York
    220R 2W5 does not sound like a NFB resistor. No need for a cheap Chinese amp to use a 2 watt resistor there. I think that's the cathode bias resistor for the power tube.
     
    Doctorx33 and robrob like this.

  7. El Famoso

    El Famoso TDPRI Member

    56
    Sep 16, 2016
    Nashville
    Found a schematic online. Pulled what I was pretty sure was the appropriate wire off the jumper and the sound muddied up nicely. Switch is on order :).

    Thanks for all your "feedback" on this;)
     

  8. LudwigvonBirk

    LudwigvonBirk Tele-Meister

    Age:
    116
    150
    Aug 26, 2017
    Madison
    Consider an nfb/presence pot rather than a switch (which by dentition only gives you a very few fixed options). A potentiometer doesn't take much if any more space on the front or back panel, practically zero dif wrt to parts or work, and it significantly opens up your tone-shaping option. Worked for me! :)
     
    WineCaster likes this.

  9. El Famoso

    El Famoso TDPRI Member

    56
    Sep 16, 2016
    Nashville
    What value pot would you recommend? I like the idea of some variability.
     

  10. WineCaster

    WineCaster Tele-Meister

    396
    Jul 12, 2012
    Oshkosh, Wisconsin
    You’ll have to wait for someone else to chime in, or do some searching as to what value pot to use. I think a resistor across the lug is also part of it... but I wanted to echo the use of a pot over a switch, much more variety and you can really hit a sweet spot.
     

  11. UKStratopastor

    UKStratopastor TDPRI Member

    22
    Sep 16, 2017
    Stockton-on-Tees, UK
    Post the schematic, and we'll give it a shot.
     

  12. D'tar

    D'tar Tele-Afflicted

    Jan 11, 2013
    WNY
    10kr and a 50k or 100k pot
     

  13. El Famoso

    El Famoso TDPRI Member

    56
    Sep 16, 2016
    Nashville
    do you need a resistor? If I leave the factory one in and just add a pot wouldn't that work?
     

  14. D'tar

    D'tar Tele-Afflicted

    Jan 11, 2013
    WNY
    Without looking at the schem. You may like more or less feedback. You dont want 100%, so yes, I would use a resistor in series with the pot. If the value was less than say 10k then justadd the pot.
     

  15. El Famoso

    El Famoso TDPRI Member

    56
    Sep 16, 2016
    Nashville
    I'll see what i have kicking around in the spare parts bin. Might play around a bit and see. Probably the lowest value pot i have is a 250k, but I might have a 100k. I'll see what sounds I get. I think I'll do the wiring and just put a crimp connector w/ a sleeve end so I can swap things around. Do I need to worry about the wiring picking up noise? Should I twist this with the wire to ground to make sure I'm not picking up any interference in the audio?
     

  16. UKStratopastor

    UKStratopastor TDPRI Member

    22
    Sep 16, 2017
    Stockton-on-Tees, UK
    I doubt you'll pick up any noise worth worrying about, although part of the NFB's job is to reduce the amp's own noise; so as you wind up the NFB resistance you might hear a little noise fading in.
     

  17. tap4154

    tap4154 Poster Extraordinaire Silver Supporter

    Apr 14, 2009
    Southern California
    Have a switch on my Pro Jr NFB and it does add some noise when switched off, but really not a significant amount.
     

  18. LudwigvonBirk

    LudwigvonBirk Tele-Meister

    Age:
    116
    150
    Aug 26, 2017
    Madison
    (Sorry for writing this in a thread about a proposed on-off switch!!!!)

    Having had (and modded a couple) amps with a Presence control, the game-changing feature for me has been the ability to turn the Presence down enough to get a fat-enough pick-swallowing tone for fatback jazz playing at lowish volumes. Way brighter cranked Presence is of minimal interest to me, but some people may love it and I certainly don't discount anybody who might want it. If it is a pot, then hi, low, and taper are rather signifant considerations.

    ...

    Very different amp but the Wattage being similar, when we think about the (added) Presence pot and the resistor value, I'd like to hear thoughts on a control with a "big sweep". Good description about the added mod in a BJR and ***very*** worthwhile BJR mod http://billmaudio.com/wp/?page_id=72

    ...
    I have played through a (Silverface? Iirc) Bassman with a Presence control that was fantastic, had it for about 2 weeks and have never forgotten it. It had a big sweep and could notch into the pick-swallowing fat slot, so great, and dialing in a brigher cool strat sound was also easy. What an amp...

    The Presence on the lower/mid line Mesa, all that I have tried (have had two under the roof, still have 1), feels like it can hit higher nice sounding values, but going to zero on the knob is still too-bright, for pick-swallowing jazz tones, for me. *Noting, these things are so crazily configurable the sound I would like to find is probably doable and I simply haven't figured out yet.
     

IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.