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unstable amp volume..


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  • unstable amp volume..

    I've had my '64 vox ac50 head for maybe a month or two now and there's something that really grinds my gears..

    the amps volume is very unstable for about 20 minutes after I turn it on.. then it is fine.

    But for the first 15-20 minutes the volume goes up and down every now and then.. it's not a huge change in volume but enough for me to have to turn it up or down and it gets pretty annoying.

    could it be just a matter of tube replacement? or could it be something worse?
    that's just, like, your opinion, man.

  • #2
    Could be alot of things. I would remove the preamp tubes one at a time and carefully clean the pins with some sandpaper to remove the tarnish. Be cool that you don't damage the tube.

    If that doesn't work id try cleaning the power tubes if they're EL-84's. Keep tubes in order and do them one at a time.

    You might try spraying some contact cleaner on a jack plug and working it in & out of the input jack/s.
    I endorse Laura & Valve Queen tubes, you should too.Marshall, Gibson, Fender, Charvel/Jackson."I'm happy to pay my tech $40 bucks to take the Zap for me."One kick-butt deal with johnpace2 Dave Friedman of RACKSYSTEMS is a genius.After the Firebird X, i'm quite convinced Henry Juszkiewicz is crazy.Member of the JMP Hall of Fame.


    • #3
      If that doesn't work also spray contact cleaner to all potentiometers. If the volume still keeps fluctuating after that examine all solder joints with a magnifying glass and resolder if neccessary.
      Honestly...this is not music... it is hatred and self loathing expressed with loudness.


      • #4
        As noted, it can be a lot of things. Volume fluctuation is often tube related, but with an amp that old it can be a ton of components that are drifting out of spec.

        My first question is are the electrolytic caps (power filter caps and the caps used to biased the cathodes of the tubes) original? Or very old? Electrolytic caps are a compromise, they allow a lot of capacitance in a relative small package but you compromise with wide tolerance and short life. Replacing them every 20 years or so is just one of those routine maintenance issues with amps, like replacing the transmission fluid in a car.

        Then we get into the other caps and resistors. The stock carbon-comp resistors have been baking and drifting in value for 40+ years. Many around tube plates, screens, etc. burn out over time. Caps from that time period aren't as robust as modern caps and many start to leak DC voltage or drift and fall out of tolerance.

        Within I buy or work on a vintage amp, I go through and make sure the electrolytic caps are fresh, and then blueprint the whole circuit to make sure all the components work and measure within their manufacture tolerances.