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Thread: Jtm30 output tubes not lighting up?

  1. #1
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    Jtm30 output tubes not lighting up?

    Hi All

    I just popped over from the pup makers forum. I have a question . I lent my Jtm 30 combo to a friend and he just told me that after a month the amp stopped working. The main tube were not lighting up. I haven't had the amp back yet to check it out but was wondering if the output transformer might have blown. I realise that with such limited info it's hard to make a diagnosis, but I thought I would enquire before I get the amp back to check out.

    Cheers

    Andrew

  2. #2
    Old Timer Tom Phillips's Avatar
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    Tube heaters being off (If that even turns out to be correct) is not a symptom of a blown output transformer. You just need to wait until you have the amp so you can do first hand troubleshooting.

  3. #3
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    The JTM30 has an internal fuse for the heaters (F101).
    Attached Files Attached Files
    Chuck H and Alan0354 like this.
    Quote Originally Posted by Justin Thomas View Post
    In my ideal world, I'm not too loud - your room is too small!

  4. #4
    Old Timer oc disorder's Avatar
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    Waldo .. there is a T6.3amp fuse internally for the heaters, (the filaments) which is what you see glowing.
    Usually it (the fuse) blows for a reason eg the output tubes/valves are kaput !
    In this situation I wouldn't even consider fitting a new fuse until the two output tubes are replaced.
    Regarding the output transformer ... Last time I contacted the Marshall agent here in Aust. I was told they were no longer available
    and that was at least 5 years ago. As Tom has pointed out no glow in the tubes doesn't mean the output transformer has blown.
    BUT if there is a serious problem with the o/p tubes and you refit the fuse you may find within the first few seconds you have just burnt out your transformer.
    The fuse is one of the smaller ones M205 (5mm x 20mm) and a slow blow or timelag as it has a "T" before the amperage-T6.5 amps.
    A non timelag would probably go open circuit with an initial cold switch on with the high current surging through the cold filaments.
    Incidentally the pre-amp tubes also depend on this fuse , and as they operate on a DC supply , a bridge rectifier is connected after the fuse .
    It's harder to see the pre-amp tubes glowing due to the design of the tubes and also when the amp is in the cabinet.
    Good luck and maybe your friend could chip in towards the replacements.
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  5. #5
    g1
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    What kind of power tube fault could cause failure of the heater fuse? Heater to cathode short?
    Quote Originally Posted by Justin Thomas View Post
    In my ideal world, I'm not too loud - your room is too small!

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    My JCM 900 blew the HT fuse for no apparent reason too!!! I troubleshoot all the way to the fuse and found no problem. Replaced the fuse and was good to go. Don't ask me why.

  7. #7
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    Sometimes they blow from the turn on surge, especially if the line voltage is high. I recall one of the JTM models was prone to this and sometimes needed a little bit of over-fusing on the heaters. But I believe it was the JTM600 which had EL34's drawing more heater current.
    torcamaniac likes this.
    Quote Originally Posted by Justin Thomas View Post
    In my ideal world, I'm not too loud - your room is too small!

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    Hello All

    Well I got in touch with my friend who had borrowed the amp and guess what,? he had flipped the standby switch to quickly before the valves had a chance to warm up DUH!!! . So anyway thanks to all the people that responded.

    All the best

    Andrew

  9. #9
    Old Timer Tom Phillips's Avatar
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    Quote Originally Posted by the great waldo View Post
    ...he had flipped the standby switch to quickly before the valves had a chance to warm up DUH!!! ...
    That should not cause a problem unless he continued to flip the SB switch multiple times in quick succession.

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    That could not cause the problem.

    But I am not surprised it blew for no reason either. Look at the big tubes, make sure it's not glowing on the side( the greyish metal), if it's not glowing in the dark, and let it go for now.

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    Quote Originally Posted by Alan0354 View Post
    That could not cause the problem.

    But I am not surprised it blew for no reason either. Look at the big tubes, make sure it's not glowing on the side( the greyish metal), if it's not glowing in the dark, and let it go for now.
    Hi Alan

    There was actually nothing wrong with the amp. It was just that he wasn't waiting long enough for the amp to warm up !! It reminds me of a customer who bought a Martin acoustic guitar in for a setup, the fingerboard was swimming in lemon oil. I mentioned to him that he shouldn't oil the fingerboard as it was made out of plastic (corian or whatever martin are using these days) but we saw the funny side of it.

    Cheers

    Andrew

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    It's a different application, but after years and years of hvac work I've learned to accept nuisance over current tripping as just that, a nuisance. If I come across a blown fuse or tripped breaker, I perform simple checks to rule out grounded or resistive shorts, replace or reset the protection, and move on.
    ~Semi-No0b Hobbyist~

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    FWIW...I worked on a JTM60 recently that blew it's T6.3A fuse at random intervals. Replaced the bridge rectifier, floated it a bit higher off the pcb than stock in an attempt to get better airflow/cooling & no more problems.

    These amps have the bridge rect & main filter caps nestled amongst a few cement block 5W resistors, all of which sit directly over the horizontally mounted power tubes. Warm in there. Just a thought.

    Archie
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  14. #14
    g1
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    The JTM60's have EL34 which draw much more heater current than the 5881/6L6 of the JTM30. As a result they are much more prone to blowing the heater fuses.
    I seem to recall they were blowing during operation rather than at startup, so it may be heat related as you mention.
    Thanks for the tip Archie.
    Quote Originally Posted by Justin Thomas View Post
    In my ideal world, I'm not too loud - your room is too small!

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    Supporting Member Chuck H's Avatar
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    Quote Originally Posted by the great waldo View Post
    Hi Alan

    There was actually nothing wrong with the amp. It was just that he wasn't waiting long enough for the amp to warm up !! It reminds me of a customer who bought a Martin acoustic guitar in for a setup, the fingerboard was swimming in lemon oil. I mentioned to him that he shouldn't oil the fingerboard as it was made out of plastic (corian or whatever martin are using these days) but we saw the funny side of it.

    Cheers

    Andrew
    So is that to say that the amp is working correctly? No blown fuses and heaters are warming correctly? How long did the guy wait to see if the tubes were glowing, like two seconds?!? The tubes will show a glow after only a few seconds. Standby has nothing to do with the heater supply. Just sayin'. This got confusing and mislead somehow. And more importantly...

    Are you saying that Martin is using Corian instead of wood to make fingerboards?!?
    "I've heard magic defined as "a technology you don't understand". By that aphorism, the folks in this forum are practicing wizards, able to summon AND control the lightning demon, and make charms to allow others to use the demon in certain ways." R.G.

    "...less ear-friendly but handy for jazz." Leo_Gnardo

  16. #16
    g1
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    Quote Originally Posted by Chuck H View Post
    And more importantly...
    Are you saying that Martin is using Corian instead of wood to make fingerboards?!?
    Some models are using something called "richlite", which doesn't sound like it would soak up much oil.
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    Quote Originally Posted by Justin Thomas View Post
    In my ideal world, I'm not too loud - your room is too small!

  17. #17
    Supporting Member Chuck H's Avatar
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    I dunno... I certainly don't know as much about how to make a guitar as the good folks at C.F. Martin, but I'm still partial to wood under my fingers. I like this even less than the resin backs on Ovation instruments.

    "Yeah, I got the new Martin with the latest technological advancement. A recycled paper and plastic resin fingerboard!" I had to write this to see what it might look like since I don't expect anyone else is ever going to.

    Richlite - Wikipedia, the free encyclopedia
    g1 likes this.
    "I've heard magic defined as "a technology you don't understand". By that aphorism, the folks in this forum are practicing wizards, able to summon AND control the lightning demon, and make charms to allow others to use the demon in certain ways." R.G.

    "...less ear-friendly but handy for jazz." Leo_Gnardo

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    Supporting Member eschertron's Avatar
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    Quote Originally Posted by Chuck H View Post
    I dunno... I certainly don't know as much about how to make a guitar as the good folks at C.F. Martin, but I'm still partial to wood under my fingers. I like this even less than the resin backs on Ovation instruments.

    "Yeah, I got the new Martin with the latest technological advancement. A recycled paper and plastic resin fingerboard!" I had to write this to see what it might look like since I don't expect anyone else is ever going to.

    Richlite - Wikipedia, the free encyclopedia
    If the cellulose component of the richlite is sawdust from rosewood or ebony cutting operations, then the maker could advertise the fretboard is made "from" that exotic tonewood.

    When I saw the word "richlite" I thought it referred to people who'd buy a Martin just to say they had one.
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    Quote Originally Posted by Steve Conner View Post
    If it still won't get loud enough, it's probably broken.

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    Richlite is just the neck. It gets deeper when you look at the 'High Pressure Laminate' used for the back & sides of some models. I saw one smashed in & it sure looked like Formica. Maybe old growth Formica....

    Archie

  20. #20
    Supporting Member Jazz P Bass's Avatar
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    This article states that the 'fingerboard', at least on the OMC-160GTE, is made of "Richlite".
    Martin OMC-16OGTE Acoustic Guitar Review
    The neck is mahogony.

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    Martin 'X' series instruments...Richilite fb, Stratabond neck (hardwood ply) and HPL sides & back. An effort at cost cutting & sustainability. Different. Play & sound? Don't know 1st hand, but Martin put their moniker on them,so I'm betting they are ok.
    X Series | C.F. Martin & Co.

  22. #22
    Supporting Member Chuck H's Avatar
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    I'm sorry, but if I'm spending 800 bones on a guitar it'd better be made of WOOD. I'm just trying to wrap my head around this as a reality! "C.F. Martin is making guitars from laminated scrap wood, paper pulp epoxy resin and photo finished Formica. This is disgusting, 90's Gibson like behavior and I hope no one buys these. Too late for a save though. Once you go there you can never get all the way back. Martin is surely facing some tough times. since the market for top end manufactured instruments is in the tank and there are highly revered custom makers that are only about 25% higher than their top line products. I understand their consideration to target a different demographic. This though, for me, just takes a crap on the prestige of their whole product line. Probably because they have the nerve to set the price relative their reputation. So I have to make the consideration that decisions on the construction of their higher end instruments are also made by the same people. I'm just not putting my money in those hands.

    Sorry... Rant over.

    EDIT: I just had a minor secure! In their top line products they now offer a "John Mayer" model for 10k

    Maybe I should grow a thicker skin but I feel like one of my childhood hero's died
    Last edited by Chuck H; 01-21-2015 at 06:28 PM.
    g1 likes this.
    "I've heard magic defined as "a technology you don't understand". By that aphorism, the folks in this forum are practicing wizards, able to summon AND control the lightning demon, and make charms to allow others to use the demon in certain ways." R.G.

    "...less ear-friendly but handy for jazz." Leo_Gnardo

  23. #23
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    Yeah! Even Esteban guitars are made from "real wood" .
    Quote Originally Posted by Justin Thomas View Post
    In my ideal world, I'm not too loud - your room is too small!

  24. #24
    Supporting Member eschertron's Avatar
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    I'm not defending Martin on this, but to make an instrument out of real live wood, with all its imperfections and variation, takes a whole lot more time and effort simply to control quality in the build. Synthetics? Easy. Press the button and watch the gauge as material is carried down the production line.

    I'm suggesting that there's no other way to get a consistent quality product in the price gap from U$200 to U$2000. Either the production quality suffers due to inconsistencies, or the aesthetic quality suffers due to alternate (some may say inferior) materials.

    Perhaps Martin could have marketed these as the 'economy' line guitars, branding them differently the way Fender/Squier or Gibson/Epiphone does?
    Quote Originally Posted by Steve Conner View Post
    If it still won't get loud enough, it's probably broken.

  25. #25
    Supporting Member Chuck H's Avatar
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    I do know a little about the inconsistency of wood and the perils it poses to the end product. I understand the appeal of synthetic materials. There's something else in play here. What I'm saying is that if Takamine, made a guitar from these materials it would cost $250 and sound/play about the same, cause, well, the materials are consistent. But if Takamine made a guitar from these materials with a correspondingly appropriate price everyone would instantly brand it a cheap turd made of chip board and plastic.

    I'm not even considering any "hand made" aspects. Since only the more expensive premium guitars feature a herringbone inlay I have to wonder how they can offer it on select cheaper models. CNC? Made overseas and only assembled here? Something is amiss and Martin is charging a premium for a product that has about as much to do with their craftsmanship and reputation as that Takamine (except the Takamine probably has more actual hand built construction). I'm not saying you shouldn't make a guitar from synthetic or recycled materials with CNC construction. I'm saying Martin shouldn't. Especially if their going to price them in the "Stop! You're tearing me!" range.
    "I've heard magic defined as "a technology you don't understand". By that aphorism, the folks in this forum are practicing wizards, able to summon AND control the lightning demon, and make charms to allow others to use the demon in certain ways." R.G.

    "...less ear-friendly but handy for jazz." Leo_Gnardo

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    Quote Originally Posted by Chuck H View Post
    So is that to say that the amp is working correctly? No blown fuses and heaters are warming correctly? How long did the guy wait to see if the tubes were glowing, like two seconds?!? The tubes will show a glow after only a few seconds. Standby has nothing to do with the heater supply. Just sayin'. This got confusing and mislead somehow. And more importantly...

    Are you saying that Martin is using Corian instead of wood to make fingerboards?!?
    Hi Chuck

    The amp is working fine, I think he just panicked when he got no sound out of the amp straight away, 2 seconds was probably right and might have even had the volume turned down as well.!! I have been making guitars for over 30 years now and yes Martin is making fingerboards out of synthetic materials corian or similar, it's all down to making a buck corporate thinking (profit is king) and if you want real wood you'll have to pay through the nose. Set a precedent and away you go.!!! I thought that the standby switch was there to allow the 6.3 volts from the heater supply to warm the Valve plates up before hitting the valves with the full voltage to reduce plate stripping/degragation. I'm afraid i'm not that hot on amps, much better with holz!


    Cheers

    Andrew

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