Metal Oxide vs Carbon Film for screen grid resistor

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by HarryB, May 5, 2008.

  1. HarryB

    HarryB Member

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    Hi. I'm working on an Ampeg SVT Classic. One of the screen grid resistors has failed (open). It looks like a metal oxide resistor. Will a carbon film resistor substitute? I'm not sure how these compare as to the way they fail. Normally I'd just order the same part but the guy I'm doing the repair for is leaving in a few days. Thanks for your help.
     
  2. Wakarusa

    Wakarusa Member

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    A carbon film will substitute. However, metal oxide flameproof are preferred because when the screen grid resistors let go they tend to let go sort of spectacularly.
     
  3. HarryB

    HarryB Member

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  4. mooreamps

    mooreamps Senior Member

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    There is also another consideration. If the SGR burned up, it could be the result of too much screen current from a tube going bad. If you make this repair, and energize the amplifier, I would recommend measuring the voltage drop across the SGR, and calculate the current flowing through the resistor. If the current, well as a rule of thumb, is greater than 10 percent of plate current, you might have a bad tube.

    -g
     
  5. VikingAmps

    VikingAmps Member

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    That's unusual. The plate resistors more often than not pop first on an SVT. You should always check tubes when you lose either.
     

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