Fender UL Twin Going into OD Too Soon.

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by DeaconBlues, Sep 10, 2017.

  1. DeaconBlues

    DeaconBlues Member

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    I just recapped a Fender 135 watt TR. The amp came to me with a very high noise level at idle. It had recently been retubed with all JJ tubes and they all test to be good. So, I recapped the amp and replaced the plate resistors. The amp idles very quietly now and sounds great at lower volumes. But, now if you dig in on a chord the amp easily goes into OD. It's actually a very pleasant sounding OD, but my customer plays country and wants it clean.
    What could possibly cause this amp designed for ultra cleans to push into overdrive so easily?
     
  2. Blue Strat

    Blue Strat Member

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    Measure the values of all the resistors you replaced, trust me! Also, measure all voltages that are marked on the schematic and compare. Pay particularly close attention to the PI circuit. If one side of the PI tube isn't operating you'll get very early distortion. This is troubleshooting.
     
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  3. DeaconBlues

    DeaconBlues Member

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    Thanks, Mike. The PI was suspect and on my list to troubleshoot. I'll also measure the resistor values.
     
  4. Blue Strat

    Blue Strat Member

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    Measure EVERY voltage that's on the schematic too.
     
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  5. DeaconBlues

    DeaconBlues Member

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  6. zenas

    zenas Member

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    Might be worth a shot to swap a few tubes in, I'd start with the PI. Might get lucky unless you're like me, I never get lucky.
     
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  7. pdf64

    pdf64 Member

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    Measure the amp's power output. If it's clean up around 100 watts, it may just be that the regular eminence oem speakers are pretty weak, so try it through a different cab.
     
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  8. DeaconBlues

    DeaconBlues Member

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    pdf64, yes the speakers were my first thought. They are JBL Fenders, btw. I tried the amp with a cabinet loaded with Jensen Neos.
     
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  9. DeaconBlues

    DeaconBlues Member

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    "Measure the values of all the resistors you replaced, trust me!"

    Bingo! Grid resistors were 470 M Ohm! Should have been 470 K Ohm. Works much better now.






    1
     
  10. J M Fahey

    J M Fahey Member

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    Glad you solved it :) , but the first (and fastest) thing you should have done is to feed it 100mV 1kHz tone into it and scope it, trying various volume and tone settings to check whether you could get clean 20V RMS into 4 ohms at the output.
    And if not, go backwards until you found the clipping stage.


    :eek::eek::eek: where did you get them from?
    Not a common or easy to get value nd very unlikely to find in s regular Tech parts bin :confused:
     
  11. DeaconBlues

    DeaconBlues Member

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    I misordered/fat fingered the wrong value. Entirely my fault, nothing to do with the distributor.
     
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  12. Sweetfinger

    Sweetfinger Supporting Member

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    Yowzah. My meter's top range is 20 Meg.
     
  13. Tone Meister

    Tone Meister Supporting Member

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    I'm late to the party so Mike K beat me to it. Did that once with a screen grid resistor on a Fender tube socket - 470K on one socket instead of 470R.
     
  14. DeaconBlues

    DeaconBlues Member

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    Thanks to Mike K. and all of you for your help.
     

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