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Thread: 1200S load resistor

  1. #1
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    1200S load resistor

    Hi folks,
    I have a 1200S here with a BBQ'd load resistor. Can't decipher the band code, but this closest matching schematic suggests a 680ohm 1W across the 16ohm secondary. In this amp it's a 2W CC resistor..
    Sunn Dual Rectifier Amp-1.pdf

    I can't imagine a fault condition, other than no-load, that would cook that resistor.
    The owner plays loud through a custom cabinet, which he says is working fine.
    Lets say the cab, cable, and jacks all check out, what else?

    I thought of flyback voltage, but that would appear on the primary side...
    Any thoughts?

  2. #2
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    Quote Originally Posted by hylaphone View Post
    I have a 1200S here with a BBQ'd load resistor. Can't decipher the band code, but this closest matching schematic suggests a 680ohm 1W across the 16ohm secondary. In this amp it's a 2W CC resistor..
    Sunn Dual Rectifier Amp-1.pdf

    I can't imagine a fault condition, other than no-load, that would cook that resistor.
    The owner plays loud through a custom cabinet
    If it can put out 120W then at full power it can put 120*16/680=2.8W into a 680ohm resistor connected to the 16ohm tap. The resistor needs to be >3W if you play loud.
    Chuck H and J M Fahey like this.

  3. #3
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    Quote Originally Posted by Dave H View Post
    If it can put out 120W then at full power it can put 120*16/680=2.8W into a 680ohm resistor connected to the 16ohm tap. The resistor needs to be >3W if you play loud.
    I should've known Ohm's Law was the culprit
    Thanks, Dave.

  4. #4
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    The Sunn 1200S schematics at the Hoffman amp forum show that 680 ohm as a 7 watt resistor.

    Greg

    Sunn Schematics - Tube amp Schematics
    J M Fahey and The Dude like this.

  5. #5
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    Quote Originally Posted by soundmasterg View Post
    The Sunn 1200S schematics at the Hoffman amp forum show that 680 ohm as a 7 watt resistor.

    Greg

    Sunn Schematics - Tube amp Schematics
    Indeed, now I wonder if the 1W shown in the schematic I posted is a typo..
    But like I said, this amp had a 2W CC there..

    The ground scheme is goofy, curious to see how noisy this is when I get it fired back up.

  6. #6
    Old Timer J M Fahey's Avatar
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    And that amp driven balls to the wall can easily give you some 200W squarewave.

    Now the resistor doesn´t know or care about waveforms, it will happily heat up.
    In that case, the resistor will be asked to dissipate almost 5W, so 7W is minimum.

    Original schematic *might* have asked 10W and one digit was lost along the way.

    Used as a PA, no big deal, voice is (hopefully) used clean, but as soon as you plug a Guitar (or even a Bass) there and rip off, all chips are down.
    Chuck H likes this.
    Juan Manuel Fahey

  7. #7
    Supporting Member Chuck H's Avatar
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    I can't even count the number of times I've replaced resistors that were under rated for what players do with the amps. Though I'm with Juan on this, Perhaps a 10W spec typo'd to 1W? Still, there are a few cases here and there where a truely vintage amp driven balls out is up to the task but for a couple of resistor ratings.
    "The man is an incompetent waste of human flesh. He should donate his organs now to someone who might actually make good use of them." The Dude re: maybe I shouldn't say, but his name rhymes with Trump

    "...less ear-friendly but handy for jazz." Leo_Gnardo

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