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Bassman head Negative Feedback switch

Discussion in 'Shock Brother's DIY Amps' started by JohnnyCrash, Oct 28, 2020.

  1. JohnnyCrash

    JohnnyCrash Doctor of Teleocity Gold Supporter

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    I’m thinking about putting a 3-way Negative Feedback switch on a Bassman build (based mostly on the 6G6-B, with a fixed resistor instead of a Presence control).

    One of the switch’s modes will be the stock 56k resistor.

    What other values should I use?

    I’m also usually not thrilled with a “raw” zero-negative feedback setting, but I’m open to options.
     
  2. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    A 50K or 100K pot, play with it for a bit, and then find the couple resistor values you like?
     
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  3. Wally

    Wally Telefied Ad Free Member

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    Imho, a permanent potentiometer to adjust that NFB resistance from the 56K to higher resistances is preferable to switching to static resistances. The pot gives the ability to fine tune the NFB rather than have only two or three options. I prefer that versatility...OMMV.
     
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  4. D'tar

    D'tar Friend of Leo's

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    I was just thinking....

    How about a center detent pot that the detent is stock and allows you to add or subtract the NFB value from there?
     
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  5. Wally

    Wally Telefied Ad Free Member

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    one could use a 27k resistor in series with say a 50k pot. The 27K resistor would take one back to the 5F6A type of NFB level, which is more cancellation than in the 6G6B amp....more than in the BF amps, in fact. Then one would have the sweep of that pot to go from 27K to 77K. This would give you the stock 56K resistance at some point or a rawer, richer sonic when applying more resistance up to that 77K. The added 21k resistance compared to stock would not be a huge difference, but the signal could be richer if wanted.
     
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  6. RollingBender

    RollingBender Tele-Afflicted

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    I was just messing with something similar yesterday on an AB763. I discovered that I had hooked up something wrong in the negative feedback circuit. I thought I had been playing with a rather heavy amount of NFB but it turns out I was playing completely without. I fixed the problem only to discover I didn’t care for either heavy NFB or raw in this amp.

    Connected a decade box in place of the NFB resistor and found 2 values I really like and find useful. One is the stock value (go figure) and the other is very light... almost the same as raw tone-wise but enough to kill a little buzz.

    The lesson I learned from this... even a cheaply made decade box off Amazon is a very useful tool in amp tweaking.
     
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  7. D'tar

    D'tar Friend of Leo's

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    Yes, surely we have to have a limiting resistor in series. And if you can find one, a 50kl center detent in series with your 27k r, just as you state Wally, would provide jtm level at detent, 5f6a at zero and everytging else in between and beyond.
     
  8. Wally

    Wally Telefied Ad Free Member

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    D’tar, what is different from the 5F6A in the big JTM Marshall’s....I see the same values in the Marshalls as in the 5F6A....27k/5K.
     
  9. D'tar

    D'tar Friend of Leo's

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    Ot/speaker impedance 2 vs 16 ohm if I am not losing my mind!?!?
     
    Last edited: Oct 31, 2020
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  10. archetype

    archetype Fiend of Leo's

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    Some day I'll make both resistor and capacitor decade boxes with shielded leads to use in amp chassis.
     
  11. Wally

    Wally Telefied Ad Free Member

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    I had not considered that OT secondary impedance but rather noted that the basic loop had the same values. As for the OT, I see that Marshall varied their thought on which tap to use for the NFB loop connection.......sometimes 16 ohms and sometimes 8 ohms.
     
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