Need help with mid 60's Gibson amp

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by samdjr74, Apr 15, 2008.

  1. samdjr74

    samdjr74 Supporting Member

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    Hi All,

    I posted this on the amp garage forum but I wanted to ask here as well. I just got a mid 60's Gibson GA-45 RVY Saturn amp. I want to replace the original 2 prog plug with a 3 prong plug. The issue is the amp has a polarity switch built into the main power switch, it's a rotarty switch, Off, Standby, On and On. To install the 3 prong switch I need to remove the poliarity function of the rotary switch. What's the best way to wire this up?

    Thanks,
    Sam
     
  2. VikingAmps

    VikingAmps Member

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    You really don't have to all you really need to do is ground the 3 prong cord to chassis and remove the cap to ground from the switch.
     
  3. TopBooster

    TopBooster Member

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    Redman's right. Looking at the schematic, you'd want to remove C40, which would essentially disconnect pole C of the rotary switch from the circuit. I would remove C39 as well. (Either that, or upgrade it to a proper suppression capacitor if it isn't one already...)

    Your amp is one of the rare models that have the standby switch located midway through the power supply chain, meaning that even with your switch on Standby, your output tubes are still receiving B+. If you plan to use Standby a lot with this amp, I recommend that you rewire the standby switch to where it should be in this case - right after the rectifiers. Otherwise, I'd just let it be...

    EDIT: If you decide to move the standby switch wiring, you may want to verify that the existing rotary switch can handle 460V, or buy one that can... This may be the very reason why Gibson put the switch later in the chain.
     
  4. gldtp99

    gldtp99 Member

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    I've redone several mid 60's Gibson Atlas and Titan heads that use the same power/standby switch set-up---i did as Redman suggests and it has never been any problem---- you just have two "on" positions--- in practice, it really doesn't seem to matter which one is used.
    These amps do indeed hit the tubes with B+ when cold, even on standby---- despite this, they seem to give long tube life--- the Silvertone 1484/1485 amps also have a similar "hot tube start" standby circuit and they don't wear out tubes quickly, either.
    One '63 Gibson Atlas i did has been running for over 2 yrs as a gigging bass amp--- on the same set of JJ 6L6GC's without a hiccuup................gldtp99
     
  5. samdjr74

    samdjr74 Supporting Member

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    Hi All,

    Thanks for the info. I'll work on it this weekend. The only reason why I brought this up was I have a 66 Atlas IV and I installed a 3 prong in it. When I turned it to standby everything seemed fine but as soon as I hit On, the amp sparked and killed my PI tube, not a pretty site or sound. I just don't want to make the same mistake twice.

    Thanks,
    Sam
     
  6. samdjr74

    samdjr74 Supporting Member

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    Hi All,

    Just wanted to say thanks for the help and advice. I removed the one cap that went from the switch to ground and all went well.

    Thanks,
    Sam
     

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