JTM-30, Different Problem, PLEASE HELP!

Discussion in 'Marshall Amps' started by Gene Ballzz, Feb 19, 2013.

  1. Gene Ballzz

    Gene Ballzz Well-Known Member

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    Hey All,
    This is my first post, so please be gentle:p. I've been using and loving my JTM-30 combo (usually through an ancient 4x12 with green backs) since about 1992. I've almost always run the master flat out/full level and set the gain and channel volume to suit my various Les Pauls with PAF type (some actual PAFs) pickups and no pedals except a wah. Even after many years of hard use, I have not had any overheating and never even had a need to replace the tubes!:shock: A bud used it for a short rehearsal and upon firing it back up a few days later nothing but low (headphone-ish) volume. All controls work as normal and the sound/tone is exactly normal, it even stays at clean sound up to the normal level that it would start breaking up.:eek: I have replaced all tubes and replaced all of the solid state units (except for one that I couldn't cross) and all is still the same. Does anyone know an expert on these amps? I have no desire to modify this amp or re-invent the wheel. I realize, this is a "ONE SOUND" amp, but it is the perfect sound for me.
    Thanks,
    Gene
     
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  2. sccloser

    sccloser Well-Known Member

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    Sometimes the fx loop jacks on these get dirty and cause intermittent output and/or reduced vol levels. Try running a patch cable in the fx loop. Jacks may just need to be cleaned.

    Also, are you sure the tubes you put in are good?
     
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  3. Vinsanitizer

    Vinsanitizer 1st new member 2020 VIP Member

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    +1 on sccloser's comment. I had a JTM30 and ran it thru a 4x12 with the master on 10 (or close to it) all the time.

    I had to regularly clean the contacts on all the jacks because they would tarnish. Other than that I would look at:
    • Tubes
    • PC board-mounted fuses or other fuses
    • Tranny
     
    Last edited: Dec 22, 2019
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  4. Vinsanitizer

    Vinsanitizer 1st new member 2020 VIP Member

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    Also welcome to the forum and stuff.
     
  5. Gene Ballzz

    Gene Ballzz Well-Known Member

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    Ding-Ding-Ding,
    None of the above! It turned out to simply be a broken treble pot:eek2: At least it prompted me to completely re-tube, clean everything and install a fan. I know there is very little love for these on this forum, but this amp has given me a MONSTROUSLY big signature tone (through 4x12 greenbacks) for over 20 years and now that I'm doing a 3-piece using a loop machine (nothing pre-recorded/all live on the fly) this amp does "loop only duty" with a Class5-H as the main amp and, of course, the JTM30 is the go to for grab and go stuff. For my opinion, this is the most versatile Marshall I've ever owned. Thanks to all for the suggestions, as they were more than I expected.
    Gene
     
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  6. Force235

    Force235 Well-Known Member

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    Awesome, I think the only people who don't like these amps are non-owners. They got a bad rap early because they do have the loop jack contact issue, which everyone thought were other problems, it was a long time before it was fully understood and I think the frustration is what caused the negative reputation. As you can see from my sig, I have a number of these JTMs, I agree, the tone/sound is excellent, my first was my JTM60, bought it new in 96/97, it has been nothing but reliable and great all these years. It might sound crazy, but I have not changed the tubes yet... I usually run it through a 4x10 cab, but the 1x12 is great for gigging. I've only had trouble with the JTM30s and it was almost always the loop jack, I just purchased a pack of small 3" patch cables and leave one in the loop jack, never a problem after that, basically you don't need to install any fans if you run a cable in the loop jacks. The problem is, as the amp heats up, the loop jack metal connections start to twist and deform from the heat, losing contact and thus the cutout problem. Had they vented the top of the amp, they probably wouldn't have had any of the trouble with this amp.
    I would also encourage you to play around with the clean channel, you can get some great classic JTM45 overdrive sounds if you set the channel and master volumes correctly, the amp is more versatile than you think, I'd say, dead clean, JTM45, JCM800

    Which version of the JTM30 combo do you own?
     
  7. madh4ttr

    madh4ttr New Member

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    I used a JTM30 for many years (until my brother took it back, which prompted me to get a Class5). While it wasnt exactly what I was looking for, it had one of the best clean channels and would take pedals extremely well. I never had an overheating problem with it but the pots seemed to get dirty quicker than any other amp I owned. Kinda miss having it around
     
  8. Gene Ballzz

    Gene Ballzz Well-Known Member

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    Force235,
    It's a 1x12 combo. I just wish I could find an imaginative amp tech willing to help me figure out how to properly and safely turn the reverb loop into a parallel effects loop so that I could use something other than the awful (and fragile) reverb tank that it came with. While it sounds best thru one of my 4x12 greenback boxes, the original 12 isn't too shabby for grab & go, guest appearances, etc. It can get pretty damn loud when needed. Oh wait, the Master and channel volume are almost always on 10, so it's always kinda loud!:p
    Thanx,
    Gene
     
  9. jtm-series player

    jtm-series player New Member

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    hey there force,

    do you really have a jtm-series 30 watts head?
    I'm almost a 100% sure they didn't make a 30w
    top, but I was thinking about swapping the internals
    of a JTM600 and a 30w combo myself.
    did you already get it that way?

    greez jtm
     
    Last edited: Apr 21, 2013
  10. Force235

    Force235 Well-Known Member

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    JTM, yes, but I made it from gutting a Mosfet 100 lead cab and installing the JTM30 head pulled from a 2x10 combo. I call it my frankenamp, I need to post pics, it looks great, sounds great, glad I did it.
     
  11. ibmorjamn

    ibmorjamn Well-Known Member

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    I hate to reincarnate this old thread but just wondering Gene , how did the JTM 30 hold up and do you still have it ?

    I am looking at one and doing a little research.
     
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  12. Gene Ballzz

    Gene Ballzz Well-Known Member

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    @ibmorjamn
    Yes, I still have it and you betcha it still works great. Turns out the problem was an impact damaged treble pot (last part of the circuit before the PI) and all is good now. This amp model gets a bad rep as they tend to get lumped in with the assumption that they are almost the same as the JTM60. Nothing could be further from the truth, as the only main thing they have in common is the cosmetics!

    Also, they have a reputation for overheating issues, but that is quite often the JTM60 & NOT the JTM30. There are however a couple issues that can cause problems:
    #1 - Incorrect power tube choice. These are non-adjustable/fixed bias and designed very specifically around the SOVTEK 5881/6L6WGC tubes, which are not really a 6L6 or 5881! Yeah, the amp can get modded to have adjustable bias but this is not the best choice for multiple reasons, one of which is the fragility of the printed circuit board, as well as asking the barely up to spec output transformer to do anything more than intended!
    #2 - The speaker jacks are wired, labeled and decribed in the manual in a very poor and difficult to understand way that can lead many folks to use an 8 ohm load on the 16 ohm tap of the already barely adequate output transformer. It took me almost a 1/2 hour of pouring over the schematic to get a real handle on how they actually operate. The 8 ohm tap can only be accessed in two ways:
    A - Two 16 ohm cabs, plugged one each out of the two speaker output jacks.
    B - One "dummy" jack plugged into the 16 ohm output and then plug an 8 ohm speaker out of the other jack.
    With those few caveats, these are still amazingly great sounding amps and much louder than their "rated" 30 watts would suggest. I'm betting they actually put out every bit 35 to 40 watts! As a combo (IMHO) the original speaker needs to get replaced by something like a 65 watt Creamback, as I feel the OEM unit to sound like absoulte A$$, all harsh and non-musical!

    Let me know how it works out for you.

    Rock On!
    Gene
     
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  13. ibmorjamn

    ibmorjamn Well-Known Member

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    I thought I missed it but it is going to happen !
     
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  14. slide222

    slide222 Well-Known Member

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    I have the JTM 60 2x12 and run it tru a JTM 2x12- 4 off v30's, it has a fan pulling hot air out of tubes area and a diode on off switch. off its more tube driven and less fizzy, also I sent it to marshall where they checked the mods and new tubes and bias last year, and I agree channel 1 is great cleans , channel 2 is ultra gain , and I find I have to have the gain low to get the sound I want , but a very underated amp i'd say ,I did not pay a lot for it and it will stay in my amp collection

    edit :-I forgot this bit ,while it was at marshall I spoke to Joanna green , and she went away to ask and came back and said they had never had one returned because it over heated
     
    Last edited: Dec 26, 2019
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  15. wmachine

    wmachine Well-Known Member

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    I bought a JTM60 head years ago because it was local and cheap (still my avatar). I really like it. Enough that when I got a 2555X full stack and a JVM410HJS, I still can't bear to part with it. Didn't install a fan, but do keep it well ventilated. Also (as below) on old advice, I keep a jumper in the loop.
    Yeah, not a lot of range, but really really sweet where it is. Never see much posted on these.

     
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  16. ibmorjamn

    ibmorjamn Well-Known Member

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    cool , thanks
    wow , nice to be close enough to go there!
    Glad to see you still have the amp , I will be back to a Marshall.
     
    Last edited: Dec 26, 2019
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  17. slide222

    slide222 Well-Known Member

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    yea i'm close , at worse an hour an a bit, but they arranged to pick it up and drop it off anyway , and it was decent price ,can't remember how much , but ok , they are still great
    edit - sorry I did nt go to marshall I just phoned but its close enough to go , I made a mistake in the previous post -my sugarlevels have been all over the show today-diabetic type1
     
    Last edited: Dec 26, 2019
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  18. ibmorjamn

    ibmorjamn Well-Known Member

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    Good to know slide that there are members that have them and have kept them a number of years . I never heard much about them and as was mentioned in the previous posts a lot of negative , mostly miss leading info.
     
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  19. slide222

    slide222 Well-Known Member

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    man if you don't have one ,you should get one they are a bit like the vintage modern cheap and hugely underrated
    Ive been playing on mine today , a 2x12 combo + 2x12 jtm extenstion = 4v30's , and it has a great channel 1 , I was playing "where do you think you are going"dire straights and it was very close , and then on channel2 I was playing "Nantucket sleigh ride"mountain and that was very close , a lot of gain on the red channel 2 , I had that on less than half on gain
    I played a lot of other stuff as well , and my 4 v30's are nicely broken in so its magical ,you could easy do that ultra gain stuff ,especially if I flip the diode switch to on
    mine has had some mod's before I got it , a fan under the power tubes pulling the heat out and a diaode on/off switch
     
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  20. Mitchell Pearrow

    Mitchell Pearrow Well-Known Member

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    My son Jerry was a type one diabetic, as is one of my granddaughters, she was recently on a segment of good morning America about juvenile diabetes, she was diagnosed at age 6 she is 16 now. Sorry for the derailment.
    Mitch
     
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