Marshall 9200

Discussion in 'The Workbench' started by vudu-child, Feb 24, 2011.

  1. vudu-child

    vudu-child New Member

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    Hi everyone!

    I'm experiencing a strange problem with my Marshall 9200 power amp -
    the volume seems to be very low on both channels - I need to turn it up to about 3/4 of the way to get acceptable band practice levels, where as using the power amp of my peavey ultra 60 head I get incredibly loud at about 9 o'clock of the master volume dial (same preamp settings).

    I have recently retubed the right side of the amp with 6L6WXT+'s but it didn't change much except for some tone improvement compared to the older (probably stock) 5881 tubes.

    Do you have any suggestions what the problem might be? Blown components?
    Bad driver tubes?
     
  2. vudu-child

    vudu-child New Member

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    I've measured the plate voltages on both the power and the driver tubes and they seem to be reasonably high for normal operation: 500V power 370V driver - unless i'm mistaken, this seems alright?

    Maybe it's the volume knob? I often noticed that the biggest volume increase was somewhere between 8 and 10 on the gain(volume) dial.
     
  3. vudu-child

    vudu-child New Member

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  4. thrawn86

    thrawn86 Well-Known Member

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    Did it come stock with the 5881's?
     
  5. vudu-child

    vudu-child New Member

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    yea, i'm pretty sure it did.
     
  6. thrawn86

    thrawn86 Well-Known Member

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    I don't recall if they are straight-across replacements for 5881's. Maybe another member will chime in.

    Did you try the old valves just to see if they act the same? I assume you rebiased when you swapped them as well. It's possible that the new valves are just no good.
     
  7. vudu-child

    vudu-child New Member

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    I thought the 5881's were basically identical to the 6L6 - just different size - that's what I've read anyway. I did rebias them and everything - still doubt it's the valves since the other side (still on stock valves) exhibits the same behaviour. That's why I thought it could be the driver tubes (12ax7 and 12at7) but after trying them in a different amp (peavey ultra head), that doesn't seem to be the case either?

    Could it possibly be a blown resistor or something? I know that the resistance of a blown resistor usually goes way up, which could potentially explain the lower volume if, for example the said resistor would serially be in the signal chain - it would then decrease the amplitude of the signal by a bigger margin.

    Could calling/e-mailing Marshall be of any help? I haven't heard the best about their customer service till now and considering the amp is waay old, I have my doubts.
     
  8. thrawn86

    thrawn86 Well-Known Member

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    If it's happening amp-wide, yeah, it could possibly be a preamp tube. If you've got any spares you can roll them through the preamp to see if there's any change to the amp's sound. Just note what position makes a difference and you can find a bunk tube that way.

    I'm in no way a tech, so I will recommend one of our resident techs speak about the feasability of your problem being resistor related. I typically rule out the easy fixes first if possible.
     
  9. vudu-child

    vudu-child New Member

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    I doubt it's the preamp since I can run it no problem into the return of my peavey ultra fx loop.

    Thanks for the suggestions and for replying so fast though. :)
     
  10. vudu-child

    vudu-child New Member

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    Could maybe someone who owns a 9200 share their experience with it?
    How loud is it even supposed to be? I'm somewhere around bedroom levels with gain at
    9 o'clock? Maybe I'm wrong and it's not even a problem at all.

    Could it be the problem with the phase inverter?
     
  11. gjohnson

    gjohnson Well-Known Member

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    What kind of pre amp?100W of Marshall power should make your ears bleed !
     
  12. vudu-child

    vudu-child New Member

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    It's an Anvil Preamp from andyszeugs.de (3-channel homemade preamp).
    But, as stated, it runs fine through the return of the fx loop of my peavey head (loud and everything), so I wouldn't imagine the preamp to be the problem.
     
  13. vudu-child

    vudu-child New Member

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    If I crank the volume all the way, it does get very loud - I'm just not sure it's 100W loud - is there any way I could measure this?
     
  14. MM54

    MM54 New Member

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    1) If you have an Oscilloscope you can hook it up, get it up to max clean power, read peak-peak volts, square it, and divide by your load.

    2) 100W cranked should be deafening you (literally), so somethings wrong methinks.
     
  15. eljeffebrown

    eljeffebrown New Member

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    I'm glad some one is bringing this up because when I had mine I thought I was out of my mind. It was doing the exact same thing. I had the 9200 Dual Mono block 5881 equipped. I had it changed out to accept E34L's cause I wanted a tighter bottom end out of it. that amp was never very loud. I was pre amping it with a Pod Pro 2.0 and had to turn that up almost 3/4 to compete with the drums with the amp dimed, the pres at noon, and the voice switches engaged. I never really figured it out. I was running it in stereo to two 1960 A cabs it should have been killing people!

    It's really the only reason I never went back to it again. :(:confused::hmm:
     
  16. vudu-child

    vudu-child New Member

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    I'll try and measure the output as soon as I get a chance to crank it, although I'm pretty sure it won't be anywhere near 100W.

    @eljeffebrown
    Have you tried changing the driver tubes in the amp (12at7 and 12ax7), because
    according to this website:
    Low Power or Loss of Volume
    The phase inverter is one of the first things to go in a tube amp and the symptoms
    are very similar to what I'm having.

    Just wanna know if changing the phase inverter would help maybe?
    Because I've tried the 12at7 from the marshall in my peavey (not phase inverter position) and did not notice a big change - except lower gain obviously.
     
  17. RICHIE GRIMES

    RICHIE GRIMES New Member

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    There is one phase inverter ** per channel ** on the 9200, so you have to be experiencing the failure of both at the same time if both channels are low in volume.

    Here's some background on the 9200 that I have.

    I am just about to retube my 9200. I've been using the Mesa short bottle STR-425 replacements for the 5881s. I also have Mesa 12AX7 and 12AT7 tubes in there. Obviously, any 5881s that would be used here would need to be the short bottle tubes like the Mesa or Groove Tubes if you want to put the cover back on the amp.

    I can crank the 9200 to max in any room, depending on

    A.) the volume setting of the preamp and/or

    B.) the input / output settings of any effects that may be in the loop or wired between the pre and power amps.

    If my preamp is at 12:00, then the 9200 cannot be turned beyond 9:00 without people outside hearing it a little.
    If I have anything in the effects loop and/or between the pre and power amps, the input/output levels of those units factor into how strong the signal is that reaches the 9200.


    Also, If my JMP1 preamp effects loop level setting is set to max and effect outputs are set low, this will be a factor in how strong the signal is that reaches the power amp.

    I recently tossed out a speaker cable because THAT was bad and causing the volume to be lower. I don't know if you used the same cable when you tried the other power amp, but that's a possibility just as patch cables or any cable could be.

    Are your ohms settings matching the cab?

    With the 9200 being a dual monoblock amp, I don't know of much that would affect both sides equally besides a power supply issue or maybe if the amp suffered damage to both sides due to a power surge or something like that.

    When overall volume loss is an issue and it's not related to anything above, then the next move is to check/ re-check/ replace tubes and have a tech adjust the bias for me. When all of that is ruled out, I'll let the tech tell me what's wrong.

    I bought my 9200 brand new in the early 90s and it's been rock solid, only needing the maintenance that any tube amp does.
     
    Last edited: Mar 12, 2011
  18. vudu-child

    vudu-child New Member

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    I did not change the preamp setting when testing it with both amps -
    and yet I need to turn the 9200 way up to get similar output levels.

    I also had nothing but the preamp in the loop without any effects or anything similar.
    The cables etc. are all the same as well. The ohm setting is fine too.

    I'm aware that both channels have their own phase inverters, but since both are probably still stock (pretty sure they are), there is a possibility that both failed, especially considering the fact
    that I bought this thing on ebay and have no idea how much it was used and in which mode (mono/stereo).

    The power supply should be all right as well since I get decent plate voltages and
    bias currents. The only thing I can't really tell whether it works correctly is the output transformer - is there any way to check this?

    Otherwise thanks for the suggestions.
     
  19. RICHIE GRIMES

    RICHIE GRIMES New Member

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    I don't anything about checking output transformer output, but both of those would have had to fail too. That's a bummer if that's the case. Good luck. Let us know what you find out.
     

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