Thinking about NFB on a Bassman Micro

Discussion in 'Shock Brother's DIY Amps' started by Hyakuya, Jul 23, 2021.

  1. Hyakuya

    Hyakuya TDPRI Member

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    Hello you lovely people....

    I'm thinking through my upcoming Bassman Micro build and wondering about adding a 3 way negative feedback switch to give me 5F6A / none / JTM45 levels of NFB.

    Using Robs Mods page:
    https://robrobinette.com/5F6A_Modifications.htm#3-Way_Negative_Feedback_Switch

    The ratio for 5F6A:JTM45 is 2.83 and Robs design for the Bassman Micro has a 15k resistor on the NFB loop to an 8 Ohm OT.

    From this, am I right to think that the right resistor for JTM45 NFB is 5.3k (15k / 2.83)
     
  2. 2L man

    2L man Tele-Holic

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    I think there is no "right" NFB for guitar amp. NFB resistors are perhaps the components what I have changed more than any other so those values can be considered OK to begin the tuning ;) I have seen NFB calculator somewhere in internet but I just use first what schematic calls.

    Micro amp output voltage is lower and it should effect NFB but perhaps it stay proportionally the same as with high output circuit? Perhaps someone can give you more info?
     
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  3. Hyakuya

    Hyakuya TDPRI Member

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    I'm sure the 15k 5F6A NFB resistor in Rob's design is well thought out so I'll leave that as is.

    The JTM45 equivalent must obviously be a lower resistance so I'm thinking of wiring in a 10k pot for that. I can set the pot to 5k (ish) and be able to adjust it if I want to.
     
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  4. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    Yes.

    On robrob's site we can read that the Bassman takes feedback from the 2 Ohm output, the JTM45 takes it from the 16 Ohm output. As the power output is the same for each output impedance, P = V^2/R results in a signal sqrt(16/2) = 2.83 times higher for the 16 Ohm output than the 2 Ohm output.
    As the amplification (/sensitivity) of the 6L6, 5881 and kt66 seems to be the same, we don't have to correct for that.

    I'm sure robrob has done similar calculation to get the same level of feedback in the original and the micro version by adjusting the feedback resistor.
     
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