New OT in my 5F1

Discussion in 'Shock Brother's DIY Amps' started by IggyT, Jan 14, 2020.

  1. IggyT

    IggyT Tele-Afflicted

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    Hello good people:
    I am overhauling my 5F1 by installing a larger 12 inch speaker . I had an alnico eminence 8 ohm lying around. The cabinet I had built was already "deluxe" size so I only had to make a new baffle.
    I then ordered a new OT because the weber I had was spec'd for 3.2 ohms.
    I got the Hammond 1760C (image attached) and there are choices....

    My questions are these:
    1) I plan to use the 8000 ohm primary . The 5 K seems too low
    2) the secondary side : I plan on sending the 3.2 ohm line back into the 22k feedback loop and then sending the 8 ohm line to the speaker output (Ill just wrap off the 16 ohm line)
    Does this seem like the correct way ??
    thanks!
    Iggy
     

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  2. King Fan

    King Fan Friend of Leo's

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    Yes on the 8K primary. The NFB question is unclear to me, I'm afraid.
     
  3. drneilmb

    drneilmb TDPRI Member

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    I don't think that this is what you want. The signal that was going through the NFB resistor is developed across the secondary as it works into a speaker load. If you aren't using a speaker on the 3.2 ohm winding, then the only load on that winding will be the negative feedback path which will probably not provide the load that you need to make the NFB signal level that you had before.

    I'd suggest taking the NFB signal off of the 8 ohm winding that you are using and fiddling the value of the NFB resistor if you find that it doesn't perform as you expect.

    I'm only speaking hypothetically, so mostly I'd suggest that you try things and see what works. :)

    -Neil
     
  4. FenderLover

    FenderLover Poster Extraordinaire

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    You are correct, sir. The stock NFB uses the 4 ohm (3.2) tap with the 22K resistor, and you can use any appropriate tap for your speaker load. There are many amps that use 8 ohm loads with the NFB connected to the 16 ohm tap, so you are not required to use the same tap as the speaker load or else you would be connected to the wrong tap every time you use a different load...(You can use any NFB tap as long as you correct the NFB resistor, but that's not what you asked)
    However...
    The Classic Tone 40-18110 5F1 output transformer shows 17K for primary impedance. Big surprise, a difference in opinion. You can try your 8 ohm load on the 4 ohm tap (16K reflected) to see if you like it. Or double the 5K primary the same way to 10K and see what you like. If your B+ is on the low side (350-ish) then 8K is better math; If your B+ is a little higher (380-ish) the 10K is better math. YMMV.
     
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  5. IggyT

    IggyT Tele-Afflicted

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    Thanks: things are coming into focus: If I connect the 2ndry 8 ohm tap to the speaker/load and then the 2ndry 3.2/4 ohm tap to the NFB resistor (currently 22K based on the original spec)... I assumed that I could keep the 22k value. if not...how do I calculate/correct that component value??
     
  6. schmee

    schmee Poster Extraordinaire Silver Supporter

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    For the sound of the original use the OT tap that is the same as original impedance.
     
  7. IggyT

    IggyT Tele-Afflicted

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    even though I have installed an 8 ohm speaker ??
     
  8. FenderLover

    FenderLover Poster Extraordinaire

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    Yup.

    There's a factor of SQRT(2). If you double the NFB tap (4 to 8) you reduce the NFB resistor (22K) by 1/SQRT(2) = 15K.
     
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  9. schmee

    schmee Poster Extraordinaire Silver Supporter

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    It's not a big deal, but has more to do with what resistors etc are in the NFB circuit. If you were 3.2 ohms and the circuit was designed with NFB you like for that ohms, then solder the NFB wire to the 3.2 or 4 ohm tap to be closest to original. But NFB is a variable thing, people even adjust how much NFB, so no big deal.
     
  10. IggyT

    IggyT Tele-Afflicted

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    So I am trying to turn this info into a simple connect the dots (wires) step and it looks like this: I use the 8 ohm tap to the jack AND into the NFB circuit WITH a 15 K resistor
     
  11. FenderLover

    FenderLover Poster Extraordinaire

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    That'll work. I would probably try different values just for fun and maybe even learn something. It's your amp, you could discover something by accident and be famous!
     
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  12. Wally

    Wally Telefied Ad Free Member

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    Tangent....since drneilmb brought the subject up. There are some rare OTs that do have separate secondary windings for each tap, but they are..again...rare. Most of these multi tap OTs have taps coming off of different points on one secondary winding to accomplish the separate impedance.
     
    Last edited: Jan 15, 2020
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  13. dan40

    dan40 Tele-Afflicted

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    As the fellas mentioned above, many amps tap the nfb voltage from one tap while running the speaker from one of the others. Marshall has done this for years...nfb connected to 8 ohm but still have the option of using a 4, 8 or 16 ohm cab at the same time.

    To keep it simple, do as FenderLover said above and connect nfb to the 4 ohm winding and use the 8 ohm winding for your speaker. This way you can leave the stock value nfb resistor in place. You could be adventurous though and add a pot for variable nfb. Very fun on a Champ!
     
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  14. IggyT

    IggyT Tele-Afflicted

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    Thanks to all for the input !! I will get to the work probably tonight and report back :)
     
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