Black JCM 25/50

Discussion in 'Marshall Amps' started by Snedge, Jun 3, 2020.

  1. Snedge

    Snedge New Member

    Joined:
    Apr 14, 2011
    Messages:
    6
    Likes Received:
    9
    Location:
    usa pa
    Hello. This is my first actual post since my introduction post. Thanks for a great, and educational forum. I found you guys doing research for ....this

    I purchased a late 80's JCM 25/50 head (Black Jubilee).
    Well, I had some trouble with it right away. First time I plugged in it sounded pretty damn good (Marshall tube amp... Duh). Straight in, no pedals. About 2 minutes in, it began to cut out. It acted like a bad cable, or corroded input. It fizzled in and out. Tried a new cable, speaker cable, and different cabinet. I unplugged it, and checked for loose tubes, bad connections, and all the stuff a novice amp guy can check. So much for a good deal. :(
    I delivered it to the smartest amp guy I knew.
    He went all through it. 137.00, and a month or so later, I picked it up. I played through it a few times with no problems. I used it for about an hour on night, then... Boom! Same symptoms. I parked it. Eventually I took it back to him. He said he had to play it for a day or two before it acted up for him.
    Long story short... It's working so great all of my Fenders are getting dusty and jealous.
    I could tell you what we found, but I thought some of the gurus might want to guess..
     
  2. Kelia

    Kelia Well-Known Member

    Joined:
    Feb 26, 2018
    Messages:
    980
    Likes Received:
    950
    Welcome to the forum ,:welcome:..........lots of amp gurus here that will chime in shortly !
     
    Mitchell Pearrow likes this.
  3. scozz

    scozz Well-Known Member

    Joined:
    Jul 20, 2008
    Messages:
    3,451
    Likes Received:
    7,829
    Well I'm not one of the gurus here, they'll be here soon enough. I'd like to take a moment to welcome you to The Marshall Forum, I see you've been a member for 9 years. This place is loaded with a bunch of great guys with loads of information and a great willingness to help out when someone has a problem.

    How long have you owned your Jubilee?
     
    Kelia likes this.
  4. Gutch220

    Gutch220 Well-Known Member

    Joined:
    Feb 19, 2018
    Messages:
    1,088
    Likes Received:
    895
    Location:
    NJ
    I'm going to guess a faulty/really dirty potentiometer.

    Guess #2, a spider crawled in it and died

    assuming the other guy checked the tubes already.
     
    Last edited: Jun 3, 2020
  5. marshallmellowed

    marshallmellowed Well-Known Member

    Joined:
    Jun 26, 2009
    Messages:
    3,771
    Likes Received:
    4,598
  6. Jethro Rocker

    Jethro Rocker Well-Known Member Premium Member

    Joined:
    Mar 11, 2011
    Messages:
    8,164
    Likes Received:
    9,608
    Location:
    Saskatoon, Canada
    Could be tubes. There are so many options though that you might get 100 different responses.
    Would be nice to know what was done for the 137 dollars. May help narrow ot down.
    Cold solder joint, bad ground, the above ideas....
    Have had a bad volume pot do that too.
     
  7. Gutch220

    Gutch220 Well-Known Member

    Joined:
    Feb 19, 2018
    Messages:
    1,088
    Likes Received:
    895
    Location:
    NJ
    I had an old JCM900 that fizzed-out and would randomly drop in volume. I thoroughly cleaned-out the pots with deoxit D5 then relubricated them with deoxit D100 and it never happened again. The key is to relube after the cleaning.
     
  8. Snedge

    Snedge New Member

    Joined:
    Apr 14, 2011
    Messages:
    6
    Likes Received:
    9
    Location:
    usa pa
    You win.
    No prize.. But winning is cool right?
    I had never even plugged into the fx loop yet, but that was the issue.
    There was also some switch/relay that he flipped upside down so dust couldn't settle. ?? He replaced tubes and biased it the first time he had it. I still have the originals.
    I guess I've had it 3 or 4 years.
     
    Neptical and Kinkless Tetrode like this.
  9. Jethro Rocker

    Jethro Rocker Well-Known Member Premium Member

    Joined:
    Mar 11, 2011
    Messages:
    8,164
    Likes Received:
    9,608
    Location:
    Saskatoon, Canada
    Easy peasy. Kind of lame to pay that kinda dough for that.
    Your original,preamp tubes may well be fine so yes keep them. It is rare to need to change 3 of those at once. I cant recall what my 1987 had originally for pre tubes but I think they were decent. Have alook at yours!
     
  10. Snedge

    Snedge New Member

    Joined:
    Apr 14, 2011
    Messages:
    6
    Likes Received:
    9
    Location:
    usa pa
    I guess I'm lame. Thanks
     
  11. Jethro Rocker

    Jethro Rocker Well-Known Member Premium Member

    Joined:
    Mar 11, 2011
    Messages:
    8,164
    Likes Received:
    9,608
    Location:
    Saskatoon, Canada
    Sorry man, don't mean YOU are lame for doing that. I mean it is lame of a tech to charge and not actually fix the problem. Intermittents are the hardest to find but still.
    BUt do have a look at the type. They may be very good tubes.
     
    J Saw likes this.
  12. Snedge

    Snedge New Member

    Joined:
    Apr 14, 2011
    Messages:
    6
    Likes Received:
    9
    Location:
    usa pa
    It's all good Jethro, I'm not offended.
    In his defense it worked well for him, and myself for awhile, and there was no charge the second time around. On top of that, he has a bunch of cool guitars and stuff that I can play anytime I want.
     
    Jethro Rocker likes this.
  13. Jethro Rocker

    Jethro Rocker Well-Known Member Premium Member

    Joined:
    Mar 11, 2011
    Messages:
    8,164
    Likes Received:
    9,608
    Location:
    Saskatoon, Canada
    Well yeah I guess when it is intermittent it is hard to check if amp is working. I should be a bit more forgiving. It would depemd what he did do and check the first time too.
    Have a peek at those pre tubes, I am very curious.
     
  14. coolidge56

    coolidge56 Well-Known Member

    Joined:
    Jan 30, 2019
    Messages:
    806
    Likes Received:
    1,736
    Location:
    WA state hinterlands
    Common issue on JCM900's those Marshall jacks are actually double poll switches operated by inserting/removing a cord. The signal travels through those switches even when not in use. The effects loop jacks sit there for years unused, with the switch contacts closed, corrode, and you get these issues.
     
  15. Snedge

    Snedge New Member

    Joined:
    Apr 14, 2011
    Messages:
    6
    Likes Received:
    9
    Location:
    usa pa
    1591311417451-1512511794.jpg I use the loop now. What a difference. I run a Boosta Grande (BBE), damn. Makes me sound good. Lol

    I love this amp.
     

Share This Page