Guitar rig noise

Discussion in 'Guitars' started by Jeremy Dean, Oct 28, 2016.

  1. kmetal

    kmetal Kyle P. Gushue Well-Known Member

    Jul 21, 2009
    Boston, Massachusetts
    Home Page:
    Awsome! Great clarification. lol I had a lot of fun w those Mesa amps.

    So in an ideal world would you want to have all three items conditioner, ISO transformer, and battery backup? Is that unecesary resundence?

    I'm assuming a case where 'typical' power is supplied from the breaker, i.e. Nothing special as far as circuits go.
  2. rmburrow

    rmburrow Active Member

    Dec 26, 2012
    western PA

    dvdhawk: Is your late '60's Fender amp with the two prong non-polarized plug a tube type unit with a power transformer? If the amplifier uses 6 volt tubes such as EL34, 6L6, etc., a power transformer is probably used to obtain heater and B plus voltages for the tubes. I wrote a comment a while back in response to someone getting shocked off a LA-2 unit.

    Some equipment manufacturers connected low value capacitors (typically 0.001 to 0.01 uF, 600 volt paper or mylar) across each side of AC to ground (chassis), after the equipment fuse and at or before the power transformer primary. (These capacitors were for RF or transient suppression.) Sometimes, another capacitor was connected across the primary itself. The assumption at the time (even with the two prong non-polarized AC plug) that the equipment chassis was effectively grounded. With a power transformer, there was no reason to connect one side of AC to the chassis, unlike typical AC/DC gear built at the time. However, the transient suppressor capacitors from AC to chassis did "bleed" an extremely small amount of AC to the chassis, and someone would feel a shock, especially with a mike at the mouth. (The reactance of each .01 uF capacitor at 60 Hz is around 265 k ohms; the current through each capacitor with 117 volts applied is around 0.4 mA.) The obvious repair is to install a grounded power cord (3 conductor) to the amp, and make certain the receptacle ground is good. (If you are a "purist" and don't want to modify the amp, connect the chassis to an effective ground like a grounded METAL cold water pipe; there may be a ground post on the back of the amplifier for that purpose.)

    The other scenario is a defective (leaky) power transformer short from a winding to the core. The core is essentially connected through the transformer case to chassis. This scenario could elevate the ungrounded chassis to a dangerous voltage. Again, the obvious need for a grounded power cord. With a grounded power cord, a shorted power transformer to chassis should immediately blow the equipment fuse, or your eyes and/or nose will lead you to the problem.
    kmetal likes this.
  3. dvdhawk

    dvdhawk Well-Known Member

    Dec 18, 2008
    Western Pennsylvania, USA
    Yessir, that's correct. The only amp I still own like that is a '69 Super Reverb, and that's right - 1x GZ34 Rectifier tube, 2x 6L6GC Power tubes, and 6x assorted 7025, 12AX7, & 12AT7 pre-amp & reverb / vibrato driver tubes.

    So this is basically what we're looking at, as you've described. I'm not adverse to putting a ground cord on it, but I am adverse to what they're asking for a genuine Fender cable. Ideally, I'd want to change out the receptacle on the back of the amp to a grounded version at the same time.

    The days of me breaking my back lugging around an amp that size are behind me, and the days of traveling with a road-crew are waaaay behind me. So the '69 Super and I will probably both be comfortably planted here for some time, until someone is looking for that signature vintage Fender tone.
    pcrecord and kmetal like this.

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