5F1 NFB Pot and Speaker Selector Switch Mod Confusion.

Discussion in 'Shock Brother's DIY Amps' started by UK Mark, Oct 18, 2020.

  1. UK Mark

    UK Mark TDPRI Member

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    Hi

    Just finished building my second amp. Yet another 5F1 this time using a Fender Champion 600 amp that was going cheap on eBay. Got idea from Uncle Doug on Youtube and have followed his method . Point to point build. Success worked fine.


    I then decided to add a NFB pot and Carling switch ( On On ) for the option of using the the 4ohm speaker for the amp or 8ohm speaker from an external cabinet.

    Completed the Mod for both please see attached picture. However amp still sounds as before. NFB does not seem to have much effect!!!!!

    Just wondering if I have wired it up correctly.


    From the picture provided


    NFB pot (50K ) wiper goes to pin 8 of 12ax7 (orange wire)
    Outer pot tab goes to 10k resistor then on to top solder lug on switch connecting to 4 0hm (Yellow wire)

    Bottom switch lug is connected to 8ohm ( green wire ).


    I tried to take some readings from wiper to 4ohm connection ( read 1.5k ohm )
    from wiper to speaker jack ( read 1.5k ohm ). Turning the pot had no effect I was hoping it would show variable resistance from 10k to 60k. Confused! Maybe its something simple but cant see it. Lack of knowledge!!

    Any help appreciated.

    Mark
    NFB 5F1.jpg
     
  2. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    Do you have a schematic? That would make it much easier for us to help you.
     
  3. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Well put on your imagination hat. Let's think about where that 4ohm tap comes from. Did I say *tap*? Yes that 4ohm tap is the same wire as the 8ohm tap just a few coils away. The end of the wire is the negative side of that coil. Then you put in a resistor and a variable resistor. What roller coaster are these electrons traveling on? Oh, and then there is the 1k5 cathode resistor between the pot and ground. And then a speaker coil resistance may be plugged in.
    See where we are going with this???... all over the place. hah.
     
  4. Mongo Park

    Mongo Park Tele-Holic

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    The one schematic I have looked at had a cap involved with the pot, I don’t know why, but it might jog somebody’s memory.
     
  5. andrewRneumann

    andrewRneumann Tele-Holic

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    Hi Mark,

    @Lowerleftcoast is right. There is a parallel circuit at play with a lot less resistance, so you are not measuring just the 10K+50K. To check your pot and resistor, temporarily desolder the wire connected to the wiper and then measure ohms between the wiper lug and the top lug of that switch. Hope this helps.
    098980BF-F652-41A5-B49F-298441705A01.jpeg
     
  6. UK Mark

    UK Mark TDPRI Member

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    Hi Lowerleftcoast your certainly right where was I going with certainly all over the place for sure testing this and testing that thanks for your input and help.

    Mark
     
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  7. UK Mark

    UK Mark TDPRI Member

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    Hi Andy did as you said but to my surprise still no change just OL on the meter. So unsoldered all lugs to the pot then tested to see if pot had any measurement!!!! None!!!

    Replaced pot and resoldered new pot except wiper. Bingo. Combined resistance of pot showing correct figures. Soldered wiper,
    I can now here a definate differance in sound when i use the NFB pot.


    So I now have a usable NFB pot witha a 4ohm and 8ohm switch.

    Thanks to everybody for all the replies and to you and Lowerleftcoast for all your help. Much appreciated.


    However one thing still confuses me in Uncle Dougs Youtube video " conversion of a PCB Amp Circuit to Point to Point " at 38.00 mins onwards shows him measuring with the wiper on the pot soldered to the wire that connects to pin 8 of 12ax7. Please take a look if you have time.

    Any comments would be appreciated.

    Mark
     
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  8. andrewRneumann

    andrewRneumann Tele-Holic

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    Great question! Since it looks like he has the cathode bias resistor wired to ground, the only thing I can think of would be that he has temporarily lifted the ground from OT secondary. It’s a head scratcher for me too.

    EDIT: It looks like the OT secondary is grounded by the output jack, so I’m officially stumped too.

    Now that you have it working properly, what kind of readings do you get when you measure resistance across the pot?
     
    Last edited: Oct 19, 2020
  9. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Hah! You stopped Uncle Doug's recording too soon. Pick it up again at 40:07.
     
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  10. andrewRneumann

    andrewRneumann Tele-Holic

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    Doh! Good catch!
     
  11. UK Mark

    UK Mark TDPRI Member

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    Well the resistor is 10k and the pot is 50k so at maximum im getting 64.8K. What is nice with the dial at halfway im hitting that
    magic 22k Fender value so its easy to find. Fron then on get a good gritty overground sound but it never becomes too much.

    Will keep this pot for a while maybe increase it to 100k for experimental purposes in future.
     
  12. UK Mark

    UK Mark TDPRI Member

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    Hey Good spot. Things starting to make sense now.
     
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