Results 1 to 8 of 8

Thread: Marshall JTM 30 Bias Adjustment

  1. #1
    Member
    Join Date
    Oct 2006
    Location
    San Diego, CA
    Posts
    33

    Marshall JTM 30 Bias Adjustment

    I have a JTM 30 on the bench and there is no adjustable bias. The resistor, R132, which I assume is the one I need to change is burried. Any experience on this model? Any help would be greatly appreciated.

    kramps

  2. #2
    tim
    tim is offline
    Senior Member
    Join Date
    Sep 2006
    Location
    Boscawen, NH
    Posts
    241
    I would replace the power tubes on this one (if that's what you're doing) and check bias first before you change anything. It may be fine where it is. If it needs to be tweeked, it may be advantageous to stick a 250k pot (linear) in the place of that 100k resistor if there's room and make it an adjustable bias...it's been a while since I've had one of those on my bench. I just got done fixing a JTM60 but that has a pot in the bias circuit already.

  3. #3
    Senior Member oc disorder's Avatar
    Join Date
    Jul 2006
    Posts
    889

    Check Zeners 2 x 15v + 9.1v

    Hi
    Have seen a couple here one had burnt power transformer other
    o/c output transformer.
    I think for memory I upped the 9.1 zener in both to 11v as it appeared
    to be pushing the tubes too hard.
    Schematic here:- http://www.drtube.com/marshall.htm#JTM30.
    The trannies were hard to get hold of so I would err on the side of caution.
    Just thought I'd add my tuppence worth....
    Attached Thumbnails Attached Thumbnails jtm-30.jpg  

  4. #4
    Junior Member
    Join Date
    Aug 2010
    Posts
    2
    I have one that was running at 58 ma. and changed the 9v. diode to a 15v. It lowered the plate current to 45ma. Still a little hot, but much better. Changing R132 and R133 didn't do anything on this amp.

  5. #5
    Senior Member
    Join Date
    Feb 2007
    Posts
    882
    edit --( found a zener)
    Last edited by Valvehead; 04-25-2011 at 04:10 AM.

  6. #6
    Senior Member
    Join Date
    Feb 2007
    Posts
    882
    forgot to mention the bias is showing 60 and 70 ma at the Ot ! Hope the 15v zener knocks that down....holy moly..
    Last edited by Valvehead; 04-25-2011 at 04:10 AM.

  7. #7
    Supporting Member Jazz P Bass's Avatar
    Join Date
    Dec 2008
    Location
    Wernersville, PA
    Posts
    7,937
    Quote Originally Posted by Valvehead View Post
    forgot to mention the bias is showing 60 and 70 ma at the Ot ! Hope the 15v zener knocks that down....holy moly..
    Smokin!!

  8. #8
    Junior Member
    Join Date
    Aug 2010
    Posts
    2
    Quote Originally Posted by Valvehead View Post
    edit --( found a zener)
    NTE 145a is the diode I used. They have many different voltages. Should be real handy for getting your Marshall cooled down.

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. Marshall TSL 1000 high bias voltage - help
    By sonic_tooth in forum Maintenance, Troubleshooting & Repair
    Replies: 7
    Last Post: 10-13-2006, 05:48 PM
  2. Marshall jcm2000 dsl 100 Bias low
    By pbdog in forum Maintenance, Troubleshooting & Repair
    Replies: 15
    Last Post: 09-28-2006, 01:36 AM
  3. help with bias ?
    By john t in forum Tweed Builders
    Replies: 15
    Last Post: 09-07-2006, 11:58 PM
  4. Bias measurement
    By Paul P in forum Maintenance, Troubleshooting & Repair
    Replies: 9
    Last Post: 06-10-2006, 01:11 AM
  5. 5f4 bias
    By scole in forum Tweed Builders
    Replies: 0
    Last Post: 06-02-2006, 07:23 PM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •