Mesa Subway Rocket - Bias Mod Advice

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by UncleLarry, Feb 26, 2012.

  1. UncleLarry

    UncleLarry Member

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    I have a '95 Mesa Subway Rocket that I just love that has the same problem that many EL84 amps have - it runs the tubes way hot and they're only good for about 3 or 4 months of regular use. About the only tubes that last very long are Sovtec EL-84Ms, but they don't' sound quite as nice as the JJs that I've been using.

    The schematic for the SR is here: http://www.freeinfosociety.com/electronics/schemview.php?id=295

    The only mod that's been done by me is that I replaced the 470 ohm screen resistors with 1K 5-watt wire-wounds, which cooled off the tubes a bit and gave the amp a bit more compression, which I like.

    After the screen grid resistor mod, the plate voltage (pin 7) is about 392 (was close to 400!) and the screen voltage (pin 9) is about 315 (was 320). The bias (pin 2) measures at about -11.5 volts. Up less than half a volt from stock. A drop of about 5-6 volts from stock on the HV side. Not much, but apparently enough to make a difference to prevent mild redplating on certain types of tubes.

    I'd like to install a bias pot with a pot failure backup resistor in place of the existing bias circuit. This is where the amp experts on this forum come in.

    I've got some experience working on high voltage amp circuits, but it doesn't go real deep, just enough to track down and replace bad components, do minor repairs, mods and such. Most of what I've learned is by trial and error and places like this forum. So, if the experts can tell me what/where to change things on the schematic, I can make it so.

    Thanks in advance for any help!
     
    Last edited: Feb 26, 2012
  2. WaltC

    WaltC Member

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    "The bias (pin 2) measures at about 11.5 volts."

    you mean - (minus) 11.5 volts, right?

    I suspect that much of what you like about this amp is closely related to how hot Mesa runs the power tubes, and any substantive change to that will adversely affect the sound and may not suit you nearly as well.
     
  3. UncleLarry

    UncleLarry Member

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    Yes, -11.5 volts :bonk

    I realize that there's the likelihood of changing the tone of the amp, but I would also like the option of balancing that with extended tube life. It's my "low volume" amp that I primarily use for church gigs and home practice, so the master volume rarely gets opened up, and the power section isn't driven hard. However, with the bias set to broil, the tubes still wear out much faster than I would like. So, I still like help on what components (pot value, etc.) to change out to make the bias adjustable.
     
    Last edited: Feb 27, 2012
  4. UncleLarry

    UncleLarry Member

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  5. WaltC

    WaltC Member

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    try replacing the 33K resistor (lift one end) with a 22K and a 10K pot and measure how that affects the bias voltage (make sure it goes more negative to run cooler) and see what you think.

    remember you run the risk of personal injury, death and amp destruction any time you mess around inside a tube amp.
     
  6. UncleLarry

    UncleLarry Member

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    So, just to be clear, less resistance = increase in negative voltage and cooler bias.

    Example: with above mod, if I roll back the pot to 1K, the bias will be colder.

    Correct?
     
  7. WaltC

    WaltC Member

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    with that bias circuit in the Mesa, that's the safest way. You could always try different discrete resistors instead, but that a little more tedious and time consuming.
     
  8. UncleLarry

    UncleLarry Member

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