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Thread: Help need jtm60 amp guru>>>>>>>>>>>>>

  1. #1
    Junior Member
    Join Date
    Jul 2009

    Question Help need jtm60 amp guru>>>>>>>>>>>>>

    Help marshall jtm 60!!!!!!
    I have had this amp that i got used for about 2-3 years decided to change tubes and did so. man told me how to bias it with a multimeter???? done what i was told by checking across the only pot on the power board that shorted something tubes went extra hot i killed the stand by or power i cant rember fired it up again poped fuse.... got my self a rash of spares and started my novice tech work... turned down the pot and re tubed her had a slight buzz for several hrs but worked rather well. maby played 20hrs oh i should mention i have had high pitch feedback that is uncharacteristic of this rig...then all of a sudden it dies. well not exactly i can hear maby 2% volume all clean/dirty reverb ect seems in order it just has no power.main transformer seems ok i swaped power tubes with others and have the same condition.. caps,rectifier,heater????? any sugestions graciously accepted.Thanks in advance..101
    agree that i probly goofed with the help of some bad advice and am verry intrested in learning how to repair this myself with help from willing people who know amps i do exelent solider work and know fair amount about other eletronics no parts in there i cant replace for a few dollars..i used to know this great teck but some how lost him. my labor is fee to me plus i enjoy the education. any parts to check or formatable test to run on them will be greatly appreciated.thanks in advance for any help

    thanks every one ill take other suggestions also i hate to part with this one, love the tone. this is a head and i have a 4-10 fender open back cab witch i culled a 4-12 85w celistion cab for i enjoy this amp when it is working properly. I just want to learn how to keep it on the road myself ive known some verry good and reasonable tecks but do not know where they are now plus with the exception of one i knew would take a long time. thanks again 101

    ok guys n gals effect jumpering had no efect on the condition of this rig the bias pot checks ok but the tubes do not respond when it is turned they look as if in a low power state they are glowing but not much .
    does any one know voltage coming out of the heater????

  2. #2
    Senior Member
    Join Date
    Jun 2009
    You do know that a tube amp runs on lethal voltages?
    Heater has 6.3V but plate Voltage is around 480V. I am still trying to decifer what you treid to explain about, give me one more day and I might have an idea...

  3. #3
    Junior Member
    Join Date
    Jul 2009
    yYes i work with high voltage often I dont want to say it to loud because i don't like to be bitten but voltage on pin numbers would be helpful any test i could run on rectifer also helpful sorry aboot the long thread ny typing stinks so i drag it from my other post thanks in advance 101

  4. #4
    Old Timer oc disorder's Avatar
    Join Date
    Jul 2006
    Land Down Under
    Hi Jay101 and welcome to the forum.
    Now that you are a member have you tried the search engine ?
    If you search JTM 60 or JTM 30 you may find some info thats useful.
    Firstly did you try plugging a guitar into the effects return ?
    Although you say the loop is ok this can help pinpoint if its the output stage
    or the preamp.
    Also if you have access to another amplifier maybe you could try plugging the send from the loop into the seperate (working) amp to make sure the pre-amp is ok (output tubes can be removed for this).
    I suspect the fault is in the power amp but lets double check that first.
    bluesfreak has some good advice re the lethal voltages so make sure you have someone there to make sure you don't fry yourself !
    Which fuse blew ? The main back panel one or one of the internal ones ?
    You should check all those internal ones first .If you can't see them clearly
    take them out and check with a meter on low ohms setting.
    From memory there are heater fuses and an HT one.
    After the above lets check the bias at pin 5 on both the tube sockets with one probe on the chassis the other in pin 5.
    The standby switch can be left in standby for this.
    When you said "bias pot checks ok " did you measure its resistance or did you
    measure the bias at some point while turning the pot ?
    See the attached octal socket diagram and post your findings back
    including the "send to another amp" test.
    I guessed at the HT bluesfreak thinks its about 480v.
    Not too fussed at the accuracy of the readings at this stage just want to get a better overall picture.
    Attached Thumbnails Attached Thumbnails el34-octal-socket.jpg  

  5. #5
    Junior Member
    Join Date
    Jul 2009
    Thank you so much for your responce i had only checked oa the bias pot to see if it worked it did i will preform these test you sugested when i can maby tomorrow?? I donot have it with me i have it at a friends to have access to an ociliscope and other good tools and knowladgeable help.
    The fuse next to the bias pot is the one that blew it has not blown since the first time i had problems and retubed the power .I do thankyou for your help.P.S. this is a cool forum.i like the emails

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