Results 1 to 8 of 8

Thread: JCM900 100W Dual Reverb - Low Volume Issue

  1. #1
    Member
    Join Date
    Sep 2017
    Posts
    33
    Thumbs Up/Down
    Received: 0/0
    Given: 0/0
    Rep Power
    0

    JCM900 100W Dual Reverb - Low Volume Issue

    Hi all,

    I've been working on my Marshall JCM900 100W Dual Reverb (EL34s) and have not been able to solve this problem. The amp is essentially operating at half of its normal volume. The tone sounds great, but I compared it to another and there is clearly something going on.

    I've gone through the necessary steps to try to fix this: replaced all power tubes, replaced power tube sockets with Beltons, checked voltages to power tubes, bias voltage, power resistors. So far everything looks healthy. Some voltage on one of the power supply caps was slightly lower than expected (around 320v vs 380v), but not sure this is that significant.

    Could anyone recommend other issues to look for to remedy this? It's been super frustrating!

    As a side note, I have a similar problem with my Silvertone 1483 head, operating at half volume with great tone! (figure they may be similar problems)

    Thank you in advance,
    -B

    0 Not allowed! Not allowed!

  2. #2
    Member
    Join Date
    Feb 2010
    Posts
    53
    Thumbs Up/Down
    Received: 21/0
    Given: 1/0
    Rep Power
    10
    I've had a half power switch at JCM900 30thAnniversary gone bad. When closed ( full power ) there was still resistance instead of short.
    So one pair of EL34's didn't deliver enough.
    Compare the cathode voltages from the EL34's.

    0 Not allowed! Not allowed!

  3. #3
    Member
    Join Date
    Sep 2017
    Posts
    33
    Thumbs Up/Down
    Received: 0/0
    Given: 0/0
    Rep Power
    0
    Quote Originally Posted by strato56 View Post
    I've had a half power switch at JCM900 30thAnniversary gone bad. When closed ( full power ) there was still resistance instead of short.
    So one pair of EL34's didn't deliver enough.
    Compare the cathode voltages from the EL34's.
    Thanks for the reply! I believe that I tested the half power switch and it appeared to be functioning as expected. I can double check this evening when I get to the amp again. All cathode voltages on the EL34s appear to be consistent/healthy as well but I will double check.

    0 Not allowed! Not allowed!
    Last edited by bw1985; 10-02-2019 at 06:05 PM.

  4. #4
    Member
    Join Date
    Feb 2010
    Posts
    53
    Thumbs Up/Down
    Received: 21/0
    Given: 1/0
    Rep Power
    10
    My mistake : JCM900 Reverb only have a Pentode/Triode switch.
    30th Anniversary have Pentode/triode and Halfpower switch.

    0 Not allowed! Not allowed!

  5. #5
    Supporting Member
    Join Date
    Mar 2018
    Location
    Germany
    Posts
    2,266
    Thumbs Up/Down
    Received: 1,723/1
    Given: 977/2
    Rep Power
    5
    All cathode voltages on the EL34s appear to be consistent/healthy as well but I will double check (6-7Vac)
    The cathodes of the 4 EL34s in the JCM900 power amp are grounded via fuses. So cathode voltages are zero - except if someone installed additional cathode resistors. If so you should see a DC voltage.
    The hi-low power doesn't turn off 2 of the EL34s but switches between triode and pentode mode for all 4 of them.

    (6-7Vac)
    Did you confuse cathode with heater voltage?



    Please post appropriate schematic as there seem to be different versions.

    0 Not allowed! Not allowed!
    Last edited by Helmholtz; 10-02-2019 at 06:28 PM.
    - Own Opinions Only -

  6. #6
    Member
    Join Date
    Sep 2017
    Posts
    33
    Thumbs Up/Down
    Received: 0/0
    Given: 0/0
    Rep Power
    0
    Quote Originally Posted by Helmholtz View Post
    The cathodes of the 4 EL34s in the JCM900 power amp are grounded via fuses. So cathode voltages are zero - except if someone installed additional cathode resistors. If so you should see a DC voltage.
    The hi-low power doesn't turn off 2 of the EL34s but switches between triode and pentode mode for all 4 of them.



    Did you confuse cathode with heater voltage?



    Please post appropriate schematic as there seem to be different versions.
    Yes, I confused the heater voltage with cathode embarrassingly so I edited it.

    I will need to check the PCB revisions again, but the model is the 4100 with 4xEL34s.

    0 Not allowed! Not allowed!

  7. #7
    Member
    Join Date
    Sep 2017
    Posts
    33
    Thumbs Up/Down
    Received: 0/0
    Given: 0/0
    Rep Power
    0
    Here are the schematics of the PCB revs in my amp:

    Pre-Amp
    https://drtube.com/schematics/marsha...-iss7-0349.pdf

    Power Amp
    https://drtube.com/schematics/marshall/cd0192-iss10.gif

    0 Not allowed! Not allowed!

  8. #8
    Don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    11,750
    Thumbs Up/Down
    Received: 1,611/23
    Given: 4,120/11
    Rep Power
    22
    Faulty filter caps where you have high AC ripple can give odd readings depending on the meter. As you have reported low DC on the filter caps, it could be that there is high AC ripple. In that case the B+ may be falling even more when delivering power.
    Try measuring for AC on the filter caps, or even better, scope the B+ when amp is delivering full power.

    0 Not allowed! Not allowed!
    Just because they don't have tubes doesn't mean they don't have feelings! - glebert

Thread Information

Users Browsing this Thread

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

Similar Threads

  1. Marshall JCM900 100W Dual-Reverb Full Restoration Questions
    By GunalORSEL in forum Maintenance, Troubleshooting & Repair
    Replies: 24
    Last Post: 07-27-2016, 10:11 AM
  2. jcm900 high gain dual reverb very low volume
    By cooldude666 in forum Maintenance, Troubleshooting & Repair
    Replies: 15
    Last Post: 07-08-2013, 05:11 PM
  3. Marshall jcm900 4501 higain dual reverb volume loss problem! Any help/ideas?
    By HEADSMASHER in forum Maintenance, Troubleshooting & Repair
    Replies: 11
    Last Post: 11-26-2011, 07:00 PM
  4. Silly Doubt JCM900 Dual Reverb 2x12 Combo 100W (5881)
    By Lee_ranaldo in forum Maintenance, Troubleshooting & Repair
    Replies: 5
    Last Post: 06-04-2010, 12:23 AM
  5. JCM900 50w dual reverb low on power
    By Alex R in forum Maintenance, Troubleshooting & Repair
    Replies: 24
    Last Post: 07-19-2007, 04:12 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
  •