Ampeg Mercury: What does this circuit do?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by JJman, Jan 16, 2008.


  1. JJman

    JJman Member

    Messages:
    990
    Joined:
    Mar 12, 2006
    Location:
    New Jersey
    http://www.schematicheaven.com/ampegamps/m12_6v6.pdf

    Working on a friend’s Ampeg. On the plates of the PI are a 470k and 510k resistor joined together then fed to a .02 cap then on to a cap/resistor to ground. What does this do? Is it some sort of “balancing” thing?

    This is an “Accordion amp” with a separate channel for an “instrument.” No, it will not be used for accordion so I’d like to eliminate this part of the circuit if it’s not geared towards guitar.

    I’m guessing I should leave the resistor/cap-to-ground portion since it’s also attached to one of the PI’s grids?

    BTW, so far, 5 of 5 tested Astron couplers are leaky.
     
  2. doctord02

    doctord02 Member

    Messages:
    1,066
    Joined:
    Mar 11, 2004
    Location:
    San Mateo, CA
    Leave it alone. Your PI wont work without it, or at least not properly as it was designed. I wouldnt worry about the accordion channel either, just dont use it. Removing any of that circutry will change the operating voltages, and will certianly devalue the amp.
     
  3. Nolatone Ampworks

    Nolatone Ampworks Silver Supporting Member

    Messages:
    2,735
    Joined:
    Jun 4, 2006
    Location:
    Flowery Branch, GA
  4. JJman

    JJman Member

    Messages:
    990
    Joined:
    Mar 12, 2006
    Location:
    New Jersey
    Well I disconnected that .02 cap at the side towards ground to check for leakage. It passed nicely. :AOK I scoped the grids on the 6v6s with it lifted and only one side was operating. So this path is certainly integral to driving the “other” side of the PI, as mentioned.

    I added screen resistors and increased the cathode resistor to 400 ohm from 250 ohm to get the idle down to ~11watts/tube. Still pretty hot but much better than the 18 watts/tube before I changed out the resistor and the former Sovtek rectifier. The schematic on the amp implies 10watts idle/tube.

    All electrolytic are changed and I’m gonna play with the feedback resistor then wrap it up. It actually sounds pretty good. Jensen P12R. And how about that “Stereo” input on Accordion input #1 :dude
     
  5. jjasilli

    jjasilli Member

    Messages:
    106
    Joined:
    May 22, 2006
    Yes those resistors are of different values to balance out the phase inverter sections. There's an explanation for this on the Unofficial Hot Rod Deluxe website (which you can Google). E.g., The Fender Deluxe AB763 has an 82K and a 100K in those locations.

    The accordion input should be pretty hot or for guitar or bass and overdrive nicely: no input resistors, and large grid load should make for a lot of gain. Yet it seems to be voiced pretty deep, practically a bass channel.
     

Share This Page