UF4007 VS 1N4007 diode? JMP 76

Discussion in 'The Workbench' started by Dominic Lampron, Feb 26, 2021.

  1. Dominic Lampron

    Dominic Lampron Member

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    Hi! I have to change 2 diodes into my Marshall 50W JMP 1976 MKII 1987 model. 3 are 1N4007 and 2 seem to be original (10D8). 2 in series read 0.548V, 0.565V, the reverse one in line with the bias feed resistor is 0.568V but the 2 others in series read nothing (0V), so the main fuse is blowing.

    1-What are your opinion on UF4007 VS 1N4007 for my JMP?

    2-True to say that the UF will reduce the background buzz and sweeter the highs of the amp?

    3-As the PCB will be lift, better to replace all of the 5 diodes?

    Thanks!
     
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  2. thetragichero

    thetragichero Well-Known Member

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    i would replace all the b+ diodes and keep the one in the bias circuit. might as well go for the uf ones, price difference should be minimal and may reduce switching noise
     
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  3. South Park

    South Park Well-Known Member

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    Are you sure the diodes are bad . Does the fuse blow when the power tubes are pulled. If you replace you want a high amp rating diode
     
  4. Dominic Lampron

    Dominic Lampron Member

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    Yes, all tubes were pulled out. My mistake when reading voltage on heater pin 7 accidentally touch pin 6 with meter probe, had a spark, then blow fuse. Replace fuse by a 2A and when i fire it up, the fuse re-blow... So, i’ve check all the diodes and 2 in serie on one side of the PT had no reading... So, that’s why i’ve asked about the UF or 1N...
     
  5. neikeel

    neikeel Well-Known Member

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    As above, I agree leave the bias diode
    Replace all the rectifier diodes, 1N4007 will be fine and will fit your board holes.
    If it were a turret board I would use UF5408 (that is largely because I have a box of 1N4007 and a box of UF5408s:applause:) YMMV
     
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  6. Dominic Lampron

    Dominic Lampron Member

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    Thanks! Why leave the bias diodes? UF5408 (3A i think) won’t fit on my pcb?
     
  7. wakjob

    wakjob Well-Known Member

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    You'll have to drill the holes out a little to get the legs through the board holes.
    I did it.

    I tried UF's in every amp rating in my 2204...
    The higher you go, the more tubey rectified (softer/bouncier/saggy) the amp behaves.

    Without thinking, I shotgun all the diodes including the one in the bias circuit with UF4007's. They just do something that's not subtle, but not drastic to the amps sound and feel.

    P.S. if you're popping fuses, double check the direction of the diodes
     
    Last edited: Feb 27, 2021
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  8. neikeel

    neikeel Well-Known Member

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    "Thanks! Why leave the bias diodes? UF5408 (3A i think) won’t fit on my pcb?"

    Read again! IF IT WERE A TURRET BOARD!

    - no need to desolder the bias diode

    - but you will be heating the pads of the old diodes, two have failed so replace all 4 that will be seeinhg heat again
     
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  9. Dominic Lampron

    Dominic Lampron Member

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    Job's done, i've replace all 4 by 1N4007, only thing available here in the store. I've leave the original diode in the bias circuit. Everything seem to be OK now, so i've check the voltage and see that the heaters are at 3.4VAC on tubes, seem low... Amp is tap on the 110V, -28,61V on pin 5, biased at 38mA with a plate voltage at 387VDC. How can the heaters are so low?
     
  10. neikeel

    neikeel Well-Known Member

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    Is it 3.4 + 3.4 = 6.8 or actually 1.7 + 1.7?
     
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  11. Dominic Lampron

    Dominic Lampron Member

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    well... It's 6.75VAC (3.4+3.4)
     
  12. Dominic Lampron

    Dominic Lampron Member

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    Without any tubes, voltages on the 110V tap are: B+ at 414VDC and B- at -36,77VDC and at 120V tap: B+ at 382.7VDC and B- at -34VDC. When all tubes (Mullard XF2 OO getter for PT) are into the amp, tap on 120V, plate voltage drop below 340VDC to have correct bias setting (around 44mA). That's why i've tap at 110V to raise the B+ at around 386VDC and bias point at 38mA...
     

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