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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?

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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.

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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.

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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

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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.

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  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.

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    Juan Manuel Fahey

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    Bent 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.

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    "I'm just going to perform a bit more scientific investigation, turn it up to 11 and rip of the knob." überfuzz

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