Why is my Marshall low on output?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by jezzzz2003, Jun 6, 2006.

  1. jezzzz2003

    jezzzz2003 Member

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    Hi Everbody,

    I have recently noticed that my 50w plexi sounds low on volume,
    I replaced the output tubes 2 months ago and play loud about 3 times a week,
    I find a Vox AC-30 is about the same volume if not louder than my amp, even my friends Valvestate 100 drowns me out.
    Could these tubes need changing already or could this be due to something else?:(
     
  2. John Phillips

    John Phillips Member

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    A 50W Marshall through a 4x12" (or even a 2x12" with 100dB speakers) should be a little louder than an AC30, and definitely louder than a 100W Valvestate.

    Is this the same amp that had a near-meltdown of one of the power tubes?

    If so, I'd guess either the tube is dead, or there's some other problem in the power stage which means it's only really running on one tube. There's an easy way to find out - pull one tube and see if you get any sound out (it won't do any harm). If you do, try pulling the other one - if either of the tubes produces no sound, it's most likely dead. It's also possible that this could be caused by a problem in the phase inverter rather than the power stage - try putting the 'dead' tube in the other socket and see if it now works. Try changing the preamp tubes (one at a time) too.

    Yes, tubes can fail at any time for no obvious reason even if they're nearly new.
     
  3. jezzzz2003

    jezzzz2003 Member

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    Will I still get sound whilst pulling preamp tubes?
    If so, if one is bad will the output lift noticably?
    That is to say if my output valves test good beforehand
     
  4. Dave C

    Dave C Gold Supporting Member

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    Replace preamp tubes one at a time with a new or known to be good tube . The power section can run on one tube to check each side but you'll have to have all the preamp sockets filled.
    Dave C
     
  5. jezzzz2003

    jezzzz2003 Member

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    Well Im just done troubleshooting,
    Both power tubes are fine and I went through all preamp tubes with a new spare I had lying around.
    Could I have a broken choke or something?
    Mind you thie amp has never really been that loud
    I have though about buying the 100w version.
    Still does not seem normal.
     
  6. Dave C

    Dave C Gold Supporting Member

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    I'd check bias , try some different power tubes, check output tranny for shorted windings......because something is wrong somewhere.......a 50w Marshall should be noticeably louder than an AC30 and way louder than a Valvestate for sure. I would expect to see plate voltage low to mid 400's and set bias to low to mid 30's ma. Have a tech check and set this if you're comfortable doing so .Be safe and good luck.
    Dave C
     
  7. John Phillips

    John Phillips Member

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    What cab and speakers are you running it through?

    Have you got the impedance matched correctly?

    How do you set the controls - including tone controls?

    It is possible that there's a (non-tube) fault with the amp too, but those things make a big difference to the output volume, and hopefully I'm not insulting your intelligence here... :)
     
  8. kbphx

    kbphx Member

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    Is this thread legit? Most people willing to spend Plexi money already know how a Plexi is supposed to sound (as compared to AC30, etc.). My apologies if you're serious, but it seems odd. FYI, non-master Marshalls should be, in a word, LOUD.
     
  9. darkbluemurder

    darkbluemurder Member

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    If the amp seems low on power even with known good properly biased tubes and correct operating voltages on the tubes it is very likely a bad output transformer. I recommend taking the amp to an expert to have this checked out.
     
  10. jezzzz2003

    jezzzz2003 Member

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    Well I have just had my trannie replaced and it sounds great,
    We put an upgrade in, Sounds alot better than before.
    And guess what... Its loud. :eek:
    Thanx guys.
     

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