bandmaster impedance switch

Discussion in 'Amp Tech Center' started by cuddledragon, Dec 13, 2019.

  1. cuddledragon

    cuddledragon NEW MEMBER!

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    hey guys, got an 66 bandmaster and replacing the OT with one that has 3 taps for 4/8/16 ohm.

    At first I was thinking of using the external speaker jack for an 8 ohm tap and the main speaker jack for 4 ohm. But looking at the jacks its seems the negative feedback is connected to positive and the main jack is shunted to ground when unplugged which makes it complicated.

    So then I thought I would use the beefy polarity toggle switch which is not in use after the 3 prong chord was put in. I could just toggle between 4 and 8 ohm but Im wondering if it will introduce more noise having the OT wires run between the power tubes. What do you guys think?
     
    Last edited: Dec 14, 2019
  2. zook

    zook Friend of Leo's

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    Put the NFB wire on the highest impedance you plan to use. If 4 and 8, attach it to the 8 ohm tap. If you use a rotary impedance switch for all three output taps, put it on the 16,
     
  3. schmee

    schmee Poster Extraordinaire Silver Supporter

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    The NFB always goes on the tap matching the factory provided impedance. The NFB components on the board are valued for that. So for your BM that would be 4 ohm tap.
    Just wire like this pic and it will be fine. (except this pic is for a normally 8 ohm output amp) Or if you want 3 output impedances, get a 3 way switch.
    [​IMG]
     
  4. zook

    zook Friend of Leo's

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    That would only be in the case of using a 4 ohm speaker load, when you have a multi tap OT it should go to the highest one in use.

    Where did you get this info? If I'm wrong I'd like to know.
     
  5. schmee

    schmee Poster Extraordinaire Silver Supporter

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    Years ago in a long discussion on the Weber boards regarding OT primaries, NFB and multitaps.. The more tech oriented than me agreed on that. It makes sense to me I guess. If you look at the AB763 schems, the 4 ohm amps have different resistor values than the 8 ohm amps in the NFB circuit etc. I suppose if you match those resisistor values to whatever tap you connect to you're good to go.
     
    Last edited: Dec 14, 2019
  6. D'tar

    D'tar Friend of Leo's

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  7. zook

    zook Friend of Leo's

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    Huh, well what do you know.. Thanks for that.
     
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