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

    Joined:
    Oct 22, 2018
    Messages:
    10
    Likes Received:
    4
    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

    Joined:
    Jun 26, 2014
    Messages:
    10,300
    Likes Received:
    4,539
    Location:
    In the rain.
    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.
     
    Dan-Ibanez likes this.
  3. Dan-Ibanez

    Dan-Ibanez New Member

    Joined:
    Oct 22, 2018
    Messages:
    10
    Likes Received:
    4
    Thanks. I think it’s worth to have a pro check it out.
     
  4. Dan-Ibanez

    Dan-Ibanez New Member

    Joined:
    Oct 22, 2018
    Messages:
    10
    Likes Received:
    4
    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

    Joined:
    Dec 29, 2011
    Messages:
    4,678
    Likes Received:
    2,836
    Location:
    C-137
    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

    Joined:
    Dec 14, 2015
    Messages:
    935
    Likes Received:
    581
    Location:
    Saginaw MI
    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.
     

Share This Page