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Thread: "The Twin" Fender Amp bias adjust

  1. #1
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    "The Twin" Fender Amp bias adjust

    I have a bias adjustment for .04 volts and tube balance on back of amp for o volts. The front of amp has a high power 100watt and low power 25watt switch. When I adjust bias for low power then check bias at high power I get different readings that are much higher. Should bias be adjusted for power output that I am using or what is the proper way to adjust bias. I use the amp with the front switch in the low power position. Thanks

  2. #2
    Supporting Member Jazz P Bass's Avatar
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    The Twin

    Which Twin amp do you have?
    I have schematic for The Pro.
    The wattage switch applies different voltages to B+ (output tube plate voltage for one)
    Higher wattage setting, higher B+ voltage.
    So yeah your bias reading will be higher for the higher B+.
    Set the bias for your highest wattage setting.
    Tremelo MUST be OFF.
    The Pro amp calls for 60ma's per side.
    Last edited by Jazz P Bass; 11-01-2009 at 04:23 PM.

  3. #3
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    I have a fender amp called "The Twin" it has red knobs and reverb. Although I use the amp at low power 25watts by using front low power switch. I should adjust bias at 100 watt level? If I do this the bias at 25 watts will not be 040 volts as called for in manual but less. Thanks

  4. #4
    Supporting Member Jazz P Bass's Avatar
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    100 watt bias

    Set it at the 100 watt setting.
    Trust me & Fender on this.

  5. #5
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    ok, Thank you for your response and help.

  6. #6
    Supporting Member Jazz P Bass's Avatar
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    Bias setting

    Where are you getting this .040 volt setting from.
    That looks like a "longer tube life" thing.
    Sounds kinda low for a 6L6.
    I would shoot for .050 to .060 myself.

  7. #7
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    The back of the amp has stamped :set bias to .040 and 0 volts for tube balance near test points on back of amp. Also manual states the same. It does not give info if to adjust with low 25watts or 100watts output.

  8. #8
    Supporting Member Jazz P Bass's Avatar
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    The Twin biasing

    O/k. I found The Twin schematic. Also The Twin Pro.
    Different animals as far as what the wattage switch does.
    On your amp when you go to the 25watt setting, the switch grounds the bias circuit through a 68K resistor. Along with lowering the B+.
    I would expect this to give a lower reading at the rear probe points.
    The Twin Pro specifically states set at 100 watt wattage setting.
    I hope this helps.
    Also, bias is "only" what the tubes idle at.
    Too high, red plate.
    Too low, cross over distortion.
    How does the amp sound at the factory setting?
    If it sounds good, ie: no cross over distortion (you will know if it's there. sounds really bad!) play it.
    Good luck.
    Attached Thumbnails Attached Thumbnails twin-schematic-parts.pdf  

  9. #9
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    Hi Guys

    The supply voltage in these amps is 500V for full power. A 6L6GC is rated at 30W plate dissipation so _could_ be operated as high as 60mA - 60mV on the DC volt meter. However, the tubes are upside-down and living in a sound field so a more conservative setting is highly warranted.

    Bias is always set at the high-power setting for any multi-power or variable output amp.

    Have fun
    Kevin O'Connor

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