Replacing Tubes On The Silver Jubilee 2525 1x12 Combo

Discussion in 'The Workbench' started by Skimbleshanks, Mar 2, 2018.

  1. Skimbleshanks

    Skimbleshanks New Member

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    I have a Marshall MR 2525c which started making weird noises. Can't really explain the nature of the noise. It has a tremolo kind of feel, not crackling and not really a hum either. Tried different cables and guitars. It doesn't change even when I unplug the guitar cable. Turned all the knobs both ways. Nothing changes except for the volume and tone of the noise which probably is normal. Besides the noise the clean, crunch, lead tones sound fine to me. Anyways, I asked my local music store where I got it what to do. They don't service Amps and I don't want to send it in if I can avoid it. They suggested I try to replace the tubes. I have my doubts that a tube went bad but I will give it a try. Now I'm inexperienced with exchanging tubes and I don't want to accidentally change the nature of the jubilee sound.

    I've searched the Internet about replacing tubes but it got me confused. From what I have understood is that the Amps need to be biased for the new tubes to work correctly. But apparently this is not the case if the amp is cathode biased? Some people suggested the amp is cathode biased but I haven't found any documents that confirms it.

    Does anyone have suggestions how I should try to solve this problem? First replace the pre amp tubes and if the problem still persists go for the power amp tubes? Or any other idea what could be the problem?
     
  2. houseofrock

    houseofrock Well-Known Member

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    I'm sure someone with more technical knowledge will chime in as to whether your amp is cathode biased or not.
    You can start with changing out preamp tubes, that will not effect bias in any way. It is always a good practice to have a spare set of tubes handy,
    preamp tubes and power tubes. With preamp tubes it is not necessary to change them all at once. Change V1 with a known working tube and see if that makes a difference with the noise. If yes you have found your culprit. If not replace V1 with the original tube and swap out V2 with a known working tube and so on down the line. Your noise happens whether or not your guitar is plugged in? You can replace power tubes to see if that is your problem, change them as a matched set. For the purpose of eliminating your noise checking bias is not an issue. Sitting at idle there not enough juice running through it to harm your amp. The problem would arise playing your amp with volume. Bias would then become an issue. However before running your amp for any length of time I would strongly suggest having the bias checked. I would turn the amp off while working on it.
    Disclaimer: please stay out of the internals unless you are familiar with working on tube amps. Voltages will kill you. Have a nice day.
     
    Last edited: Mar 2, 2018
  3. curnla

    curnla New Member

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    No need to set bias. It’s cathode biased.
     
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  4. Skimbleshanks

    Skimbleshanks New Member

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    Thank you for the help. It was indeed a bad preamp valve. I replaced the first valve and that did the trick. I'm glad I could fix it so easily without having to spend much Money.
     

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