Marshall Valvestate 8200/8280 Volume Loss On Od2 With Full Gain When Pick Hits

Discussion in 'The Workbench' started by Dan-Ibanez, Jan 26, 2019.

  1. Dan-Ibanez

    Dan-Ibanez New Member

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    I have both a Valvestate 8200 (into a 1960) and 8280 (2x12). Both of the amps have the same problem - When I have OD2 on, and when the gain is on MAX, there is a volume LOSS right when I hit the strings with the pick. If I put the gain down to 75%, there is no volume loss at all. At first I thought it was the 8280 that had a problem, so I tried to replicate it in the 8200... and sure enough it does it as well. There are no volume loss issues on OD1 or the clean channel.

    Both units are "new to me" (bought last fall, but have not played them loudly until today) and I have no idea on valve replacements on either.

    If I recall, the valve is just used on the OD channel. Could it be the culprit?

    Thanks,
    Dan
     
  2. ampmadscientist

    ampmadscientist Well-Known Member

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    The volume loss is compression.
    If the filter capacitors are old enough they won't hold much of a charge anymore.

    What you are doing is placing demand on the power supply and the reserve voltage isn't there.
    You are getting sag when you push the amp harder.

    This could be because the filter caps are too old.
    The zener diode voltage regulators also go bad.
    Or it could be that the amp was designed that way... but less likely.

    So I would try first to get the filter caps freshened up, and to check and see if the power supply voltage is coming up to full voltage.
    (because zener diodes could go bad and the voltage will drop out)

    You need the schematics hopefully this will show what the voltages should be.
    But we also know that
    if you have a 9 volt zener diode, there should be 9 volts DC across it when you measure it.
    If it is a 15 volt diode, there should be 15 volts DC across it...and so on.

    Now we are talking about electrolytic power supply filter caps, and not talking about the coupling caps between audio stages.
    Usually we don't need to replace audio coupling caps, but the power supply filter caps do go bad with age.
     
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  3. Dan-Ibanez

    Dan-Ibanez New Member

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    Thanks. I think it’s worth to have a pro check it out.
     
  4. Dan-Ibanez

    Dan-Ibanez New Member

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    So here's a video.

    From the start to 0:35 I have the gain on max, selector switch on Lead2, and volume on 4. There’s a very short drop in output right after the strings are strum. From 0:43 to the end I have dialed the gain back to 70% and you hear output from the amp right away.



    I'm being told this is either normal or unfixable. Tough pill to swallow since the one I owned 20 years ago didn't have this issue.

    Can anyone recommend an amp repair tech in North Carolina, South Carolina, or Georgia?

    Or a way to fix this myself?
     
  5. wakjob

    wakjob Well-Known Member

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    While AMS is correct, the power supply caps should be updated, these amps have a few issues that have come with age. And that's what your video sounds like.

    I have two. The 8100 and the 8080.

    I've been through both amps and have found that everything around and in the FX Loop circuit gets very dodgy with time.

    For some reason the effects Master potentiometer goes wonky. If it doesn't come around with a good cleaning, then it's time to replace it.

    But first, just grab some q-tips and isopropyl alcohol and give all the cliff jacks a good cleaning. If you feel confident, you could spray clean all the pots yourself.

    Doesn't hurt to put a small patch cable connecting the
    FX Return to Send, just to see what is does maybe eliminate one problem at a time.
     
  6. Matt_Krush

    Matt_Krush Well-Known Member

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    As a former 8200 owner myself, a volume dip is not normal in any channel at any gain level.
    The 8200 is now what 22-25 years old? I think they were only made between 1994 and 1996...

    If it was happening at all distortion levels and in both channels, I'd agree that it sounds like supply voltage sag..
    The valve really acts as more of a buffer than actually the 'distortion tone' maker of the amp. You can certainly try a replacement (for what? $10 its worth a shot).

    Definitely need a schematic to isolate the areas that are are overdrive 2 only, since the problem only exists there.
     
  7. Edokter

    Edokter Member

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    Nothing is unfixable.

    First, I would recommend replacing the tube anyway, especially is it is 22 years old.

    Second, the tube is not "just a buffer". It is the final gain stage after the clipping stage. The first triode provides asymetric gain, the second triode acts a a cathode follower to drive the passive tone stack. This arrangment is an exact copy of the V3 stage of Marshall tube amps. After that, it goes out the FX loop.

    Now, the fact it only happens in OD2 full gain leads me to believe the problem is a bad switching opamp. IC3 is a M5201S dual switching opamp (no longer made, replacement NJM2121D) responsible for switching between OD1 and OD2. It sounds like one of the opamp inputs is busted. Have the opamp replaced to see if that fixes the problem.
     
  8. ampmadscientist

    ampmadscientist Well-Known Member

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    "I'm being told this is either normal or unfixable."

    Yeah, that why I started fixing audio equipment myself.....Because I couldn't get the local hacks to fix anything the right way.
    I eventually became a pro over decades of time.
    Un-fixable? Well I don't buy that at all. Well, it's probably fixable if you find the right person who understands it.
    It's probably (for starters) the old electrolytic caps. Unfortunately, some of the parts this amp is built from are no longer available and a knowledgeable person will need to research substitutes.
    But I'm fairly certain it could be fixed, just maybe economically impracticable.

    This is why you need to start working on your own amps. Because most "technicians" are hacks who really don't know much of anything. Most of these hacks don't have any test equipment either, etc...

    Here are some professionals, but I fear they might charge too much to make it worthwhile.
    (these are people who have test equipment and knowledge)
    You may just want to consider buying a fresh amp instead of paying too much to have the old one fixed.

    Clue: make sure the technician knows how to play electric guitar. This is a big advantage compared to novices.

    • Heyday Musical Instruments & Repair
      108 N Lexington Ave
      Asheville, NC 28801
      Ph: (828) 254-0402
    • Reel Time Sight & Sound
      726 Fairfield St
      Burlington, NC 27215-6419
      Ph: (336) 213-7543
    • Bull City Sound & Electronics
      1001 Broad St
      Durham, NC 27705
      Ph: (919) 286-1991
    • Edwards Music Company
      524 N McPherson Church Rd
      Fayetteville, NC 28303
      Ph: (910) 868-1185
    • DJL Vintage Audio
      2007 Binford Street
      Greensboro, NC 27403
      Ph: (336) 274-6767
    • Tone Technologies
      1844 Pembroke Rd Ste 6
      Greensboro, NC 27408
      Ph: (336) 378-8049
    • COSC
      4779c HWY 117 N
      Pikeville, NC 27863
      Ph: (252) 521-4162
    • Rhodes Electronic Services
      11133 NC Hwy 32 S
      Plymouth, NC 27962
      Ph: (252) 927-4604
    • Mad Science Works
      1101 Capital Blvd.
      Raleigh, NC 27603
      Ph: (919) 341-4278
    • Duncan Music
      6329 Unity St Suite D
      Thomasville, NC 27360
      Ph: (336) 475-2200
    • Mark Moore Audio Solutions
      3302 Bragg Dr
      Wilmington, NC 28409
      Ph: (910) 409-0103
    • Jackson's Music Store Inc
      1409 S Stratford Rd Ste D
      Winston-Salem, NC 27103
      Ph: (336) 760-9635
    • Pecknel Music Co Inc
      1312 N Pleasantburg Dr
      Greenville, SC 29607
      Ph: (864) 244-7881
    • Integrated Systems & Design
      3265 Waccamaw Blvd.
      Myrtle Beach, SC 29579
      Ph: (843)-448-5030
    • Jerry Tillman Music
      596 N Anderson Rd
      Rock Hill, SC 29730
      Ph: (803) 366-3232
    • Bakos Amp Works
      1091 Euclid Ave Ste C
      Atlanta, GA 30307
      Ph: (404) 607-8426
    • Berkshire Guitars
      428 5th Street
      Augusta, GA 30901
      Ph: (706) 823-5800
    • Pro Audio Video Plus LLC
      3566 Club Dr.
      Lawrenceville, GA 30044
      Ph: (678) 380-8840
    • Southwind Sound and Recording
      12 Commerce Court
      Rome, GA 30161
      Ph: (706) 291-6491
    • Oade Brothers Audio
      316 S Broad Street
      Thomasville, GA 31792
      Ph: (229) 224-7319
    • Logical Sound and Industrial Electronics
      2620 D Mountain Industrial Blvd
      Tucker, GA 30084
      Ph: (770) 934-4887
     

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