Ampeg SB-12 blowing fuses. Suspect the OT is toast.

Discussion in 'Amp Tech Center' started by jmczaja, Aug 10, 2020.

  1. jmczaja

    jmczaja TDPRI Member

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    I have a 1969 Ampeg SB-12 that was given to me blowing fuses. Replaced all of the filter caps, powered it up with a current limiter and the bulb lit bright after hitting the standby switch.

    I removed all of the tubes including the rectifier tube, current limiter bulb did not light when coming off of standby.

    Inserted rectifier tube (known good tube), without any other tubes and the bulb lit up.

    Disconnected the newly installed filter caps and bias supply from the circuit, bulb still lights up! Damn..

    Disconnected B+ to the CT of the OT.. Bulb does not light! :(

    I measured the resistance on the primary side of the OT and I'm only measuring 30 ohms from the CT to chassis, 170 ohms from pin 3 to chassis on the first 6l6 and infinite resistance from pin 3 to chassis on the other.

    Sounds like OT is done to me.. Is there anything else i can possibly be missing?

    Thanks!
     
  2. jhundt

    jhundt Doctor of Teleocity

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  3. Quixotic

    Quixotic TDPRI Member

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    Nah, sounds like that's just your OT. You've done what you should do, start at the power cable and work your way back. From what you're saying, that seems like the only thing left that's suspect.
     
  4. jmczaja

    jmczaja TDPRI Member

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  5. jmczaja

    jmczaja TDPRI Member

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    Just a quick update.. The OT is indeed no good.

    I removed the OT from the chassis and measured little to no resistance between the primary CT and secondary leads.

    Most likely the previous owner tried playing the amp without a load connected. I noticed that the speaker connector was not making contact with one of the pins on the speaker jack.
     
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