JTM60 - Fan or Power Tube Bias Adjustment?

Discussion in 'The Workbench' started by TPMCD, Aug 30, 2011.

  1. TPMCD

    TPMCD New Member

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    I'll be getting a JTM60 in a couple of days, pretty excited about it. As anyone who knows these amps and has read these forums knows, they have an overheating issue (having to do with the way the power tubes are laid out and other issues with the transformer?).

    I've heard that adjusting the bias for the power tubes will take care of the problem (or at least has the possibility of taking care of the problem) and I'd rather do that than install the fan...something about a fan whirring in the background while I'm playing doesn't sound that great to me.

    All that being said, the guy I'm buying it from said he hasn't had any problems with the amp overheating and he sent me a pic of a clean circuit board...maybe I won't need to do anything, but I'd like to take care of the problem before it happens.

    As said, I'd like to just do the bias adjustment and not deal with the mucky-muck of the fan. Any thoughts? Thanks.
     
  2. TPMCD

    TPMCD New Member

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    Well, since no one has commented on this (which is understandable considering the JTM60/30 overheating issue has been beat to death), I'll reply to my own thread!

    Jammed out on the JTM60 for about an hour and a half and so far no issues...I'm not too experienced with Marshalls, so I know there must be better sounding ones out there, but I absolutely loved the sound of this sucker. The clean channel is fantastic...I've got it set up so that with normal/soft picking it's pretty clean, but when you dig in it there's a real nice creamy crunch to it. The gain channel is good, but I noticed if the GAIN knob is too high, it's too fizzy sounding...as others have said, this is not the amp you want if you're going for a great high gain sound. I use the gain channel more as a boost/slight crunch sound. On top of this, the reverb isn't half bad either.

    Anyway, I'm definitely digging it...I traded my NOS Blues Jr. tweed for it, and I'm perfectly happy. For the time being, guess I won't worry about the overheating issue...hopefully, it won't happen to me.
     
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  3. hwy1strat

    hwy1strat New Member

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    Hello.....I saw your thread and I just HAD to answer.
    I, too, am a recent JTM 60 owner..,..and I posted TWICE on this very forum with the same issue as you.
    NOT A SINGLE soul replied to my post either!:mad::confused: Don't feel so bad....

    In spite of it's overheating issue....I LOVE the sound of this amp and it's portability and power to keep up with a drum kit! And you're right...the CLEAN channel is surprisingly good!!! The reverb, as you mentioned is really good also!!:headbanger:

    This week...I'm bringing it to the famed ANDY FUCHS (of FUCHS amps fame) to have it worked on. I'm going to have him give it the 'once over', change out the tubes, adjust the bias (thanks for THAT idea!) and such.

    The previous owner installed a set of KT-88's in place of the stock EL-34's.
    The KT's have a bigger 'bottle' making it necessary for him to remove the tube cage.
    I'm going to have it restored to factory specs and have the cage reinstalled.

    From one JTM 60 owner to another.............ENJOY!!

    [​IMG]
     
  4. JayCM800

    JayCM800 Well-Known Member

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    Hello both of you!

    I don't have any experience with that amp model, so no advice regarding heating issues either...

    I just wanted to congratulate you on your amp brand choice. I'm glad you're digging your tones. Thank you for posting reviews and pics for i'm just still learning, as many others do around here!
     
  5. TPMCD

    TPMCD New Member

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    Thanks for the replies JayCM800 and hwy1strat!

    Since we're talking about it, might as well continue my story...while playing the amp, even though I loved the sound, it seemed very quiet for a 60 watt amp...well, it turns out that one of the power tubes was out! Gotta take it in to the shop now...going to a reputable guy here in Los Angeles and have him put it new power tubes and give it a once over. I discussed the heating issue, and he didn't seem to think there was a problem with that, but I'm going to press him on it and see if there's anything I should do about it.

    OK, thanks again all, good luck with everything!
     
  6. nedcronin

    nedcronin New Member

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    I used to have a JTM 60 head, sorry I just ran across this post or I would have replied. Anyway, 2 things. First, they are a great sounding amp. The clean channel is great. The gain channel being fizzy, well if you turn the master up that does make it less fizzy and you get a pretty good tone out of this quite under rated Marshall. Second, they DO overheat. However if you put a little muffin fan in there with it's own ac cable, you won't hear anything at all when you are playing. And it DOES keep the tubes cool. It is an awful design to have the tubes positioned the way they are. 20 bucks for a fan and you won't have to worry about it though. I will say that filling that small area up with KT88's would only serve to make the overheating problem worse, I would think.
     
  7. OutofStep

    OutofStep New Member

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    I've had the 2x12" JTM-60 for a few months now, and I absolutely love it's tone! I replaced all of the preamp/power tubes with JJ gold pins right after I bought it, and that really made it come alive. I have never had any overheating issues, but I do plan on installing a Radio Shack fan in the amp just in case. I love this amp, and I'm planning on buying the JTM-C212 extension cabinet to make it into a little half stack.
     
  8. TPMCD

    TPMCD New Member

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    Awesome! I love the sound of mine as well, and I'm also looking for the cab! Let me (us) know how it goes with the fan...I want to install one myself, and I'd like to hear how you do it. Thanks!
     
  9. 68_Mustang

    68_Mustang New Member

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    I love mine, no issues. Just keep it ventilated (away from the wall). I also got a small fan from Radio Shack for mine.

    [​IMG]
     
  10. tomfun

    tomfun New Member

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    Hi

    I have too had no replies when I have mentioned the letters and numbers - JTM 60. (please see my last post). They are great for the price, extremely cool looking. They look 40 years old when they are brand new!

    I bought the head version with the 2x12 cab. The head version has vents on top so I think the overheating problem is mainly with the combo versions? I think the gain is great. It's nice to push the volume a bit along with the gain, sounds more natural for a tube amp, rather than just up the gain alone. Turn the boost volume up with the gain then turn the master down to control overall volume.

    The only thing I'm getting is it could do with a bit more bottom end. That's why I'm asking if I linked up a 4x10 or small version 4x12 would it add more bass? Anybody? (please see my post - MG412 with JTM60 & 2x12 cab.

    The thing is these amps get compared to the likes of 900s etc. Why? They ate not in the same product range so they should be praised for the price and value for money don't you think?

    Cheers
     
  11. shredless

    shredless Well-Known Member

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    The reason you get no replies is that the "elitist" people on this forum think that the JTM30 and 60 are dog shit, so they ignore the peasants that own them

    I know it sucks but its true, hey at least they arent bashing your shit.

    My advice on the overheating issue....quit reading the internet and play the amp
     
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  12. tomfun

    tomfun New Member

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    Couldn't agree more... Is the reason they think they are dog shit because the gain doesn't go to 50?!

    I can't stand these narrow-minded so called 'musicians' who's idea of great tone is "hey dude, this monster's got heaps of gain." How many times have I read on this forum about certain models (e.g. JTM60) not having much gain?! Sometimes saying 'gain' when actually they, in reality, mean more 'fizz'!

    Maybe we don't all want to play through 20K output hellbuckers playing a thousand notes a second. Or play Drop D or chug away on muted, bottom E Metallichords and sound like 'every-other-shite-excuse-for-a-metal-band-rehearsing-in-the-next-room-that won't-shut-up-for-five-minutes'... type band! A shite guitarist with a pro level, expensive high end rig is still a shite guitarist, but just annoyingly louder. Believe it or not the sound that comes out of your amp ALSO has a lot to do with your guitar, pickups and fingers - yes fingers folks! Ask Schenker, Adrian Smith, Iommi (Laney, I know)... Also, the lord forgive me, I may want to play a Telecaster through a Marshall (yes, I said Telecaster, run for the hills - single coils!!).

    I'm surprised these people can hear their JCM200000000s from their knuckles scraping the floor!

    Now, I've got that off my chest maybe some open-minded, nice, civil Marshall expert could give us all some advice on other Marshall products that DON'T begin with the letters J, C and M? Let's keep it relative, you wouldn't compare a revolver with a machine gun? Maybe they could begin their sentences without using the words "these suck".

    Cheers
     
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  13. buzzy

    buzzy New Member

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    Hrmph! . . . need a hug? I read this forum for two years before joining and then didn’t post for another 9 months; now, 2 posts in two days! After much entertainment derived from this and other forums over the years, I can tell you that you need to wade thru a lot of mud to get to the river. And, when you jump in, it might not be an entirely pleasant swim.
    Some people don’t like any solid state in their signal path and they’re very vocal about it.
    Some people don’t like that Marshall shops out some models and/or releases models with design flaws. They too can be adamant in their opinion.
    Some of the most knowledgeable people here would tell you to use the search function for information on your amp.
    I read as much as I could stomach about the JTM30 before I bought mine. I like it for what it is and I don’t think I’ll put another dime into it. I use a fan – not a computer type fan installed in the back, an 8 inch rotating fan that I point at the back of the amp. It ain’t elegant and it probably ain’t the right solution, but it works for now.
    The bias question is separate from the overheating question; it will no doubt be necessary to solve that problem when I change tubes and I’ll deal with that in the future.
    I have amps for different purposes. I think the JTM30 has a superior fit for some tone situations. Sometimes, I also think that it is the BIC lighter of Marshall amps – there’s a chance it’ll start on fire and when it stops working, I’ll replace it with something else.
    My apologies if I’ve offended anyone.
     
  14. OutofStep

    OutofStep New Member

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    I haven't had a chance to install the fan yet. I'm waiting until I have a couple days off of work for Christmas to do it, because I may also re-cap the amp while I'm at it, and do a couple of other mods. I did manage to track down the JTM-C212 extension cab, though. There was another guy on this forum that offered to sell me his after I had already purchased mine (he was from Spain, so shipping would be up there!). If you're interested, PM me and I'll try to get back in touch with him.
     

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  15. macaroni

    macaroni New Member

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    Hi.
    I got one and the previous owner already put a huge fan kit in it .
    It has it's own power cord and he cut a hole in the cab .
    I took it to a tech and with proper bias adjustment he's not running that hot.
    I'm glad that the fan is there cause after 30 minutes or so when i'm giging with it , i start the fan just to make sure the tubes will be ok.
    By the way this amp , according to a vintage guitar article i've read ,is really sensible when you change the v1 and v2 for good tubes ( nos nib). It's true and it's fun to experiment with.
    I've also done the mod to bypass the diode in the boost channel and the difference is huge, the sound of that amp is superb on clean and boost.
    So for the question you adressed , i would say both: have the amp bias and put good power and preamp tubes in it. Go for the fan too but don't cut a hole in it...
    I have a 1981 2203 head with 4/12 matching cab ( g1265) and a jcm 800 1982 combo master vertical input with g1265 and this jtm60 have something on it's own. I'm sure it would be interesting to experiment with some good speaker . I tried hemp cones ( canabis rex(12) and litlle buddy(10) on other amps and they are great .
     
  16. fastpaddy

    fastpaddy New Member

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    Sorry, I'd forgotten the actual question when I made my last post. It is true that the JTM 60 has an over-heating issue. The socket of the 12ax7 nearest the EL34s in my amp is shrunken and deformed from excess heat getting to it. This problem will not be solved by bias adjustment. The valves need to have a sufficient amout of idle current going through them (think of it as the tickover setting for a carburettor) to respond properly to the modulation of the grid voltage that happens when you play your guitar into the amp, so it is best to run them with enough bias to make them give 65-70% of their max "plate dissipation" (35mA for EL34s, 19.6mA for 6V6GTs at 495v). Reducing the bias current too much can make them sound thin and I doubt if it will affect the heat from the valves very much. If you are going to get full use from a JTM60 and take advantage of its great tone, you need to fit a fan to it, which is what I have done. You can buy a 230/240v ac computer fan from M. Aplin for £16.99 and fix it to the valve-protection cage with four long bolts, suitable washers and nyloc nuts. Make sure it blows downwards, away from the valves. It is a simple matter to wire it into the amp's mains switch, bringing the wire down through the ventilation grid (use a rubber grommet around the wire where it passes through the grid) and you can fit a small in-line plug and socket so that it can easily be disconnected when you are removing the cage. The job will take an hour or two and you can then go out and gig with the amp to your heart's content and marvel at its reliability while enjoying that great sound without worrying that it could be slowly grilling the main circuit board. It will now be a piece of professional kit. Get that soldering iron out and have fun. Just make sure the filter caps are fully discharged before you go poking around in there or you might never again have to think about your guitar sound. A shock from one of those things can kill.
     
  17. hwy1strat

    hwy1strat New Member

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    Well....I just HAD to resurrect this old thread that I had replied to a while ago.
    I am sooo psyched!!!:headbanger:
    After months of intense searching....I finally found the rare Marshall JTM C12 extension cab......companion to my Marshall JTM 60!!!
    I am beyond happy with my find!!:dude:
    I also had my JTM 60 modded with some very helpful tips from a more than generous gent named GUNNAR from Sweden on the Metro Amp forum
    This thing is a BEAST now!!!
    And those overheating issues common to the amp.....GONE!!


    [​IMG]
     
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  18. pappashank

    pappashank New Member

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    Yup, I can also vouch for Gunnar's mods. Performed all of these on my JTM60 2x12 plus changed one speaker to a V30 and replaced the output xformer to a Mercury Magnetics American Pro series (fried the Dagnall). You need the fan on the tube cage! This amp is used for gigging, not bedroom playing, and I couldn't be happier with the sound of this thing. Gunnar's mods made a huge improvement towards that vintage Marshall tone.
     

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  19. lowinput

    lowinput New Member

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    If you like steve vai's 'eat em and smile' tone, get one of these amps!

    "David" "what" "waywompouwhap" "well, let more roll on to the sidewalk and take a look" "WOOW" "she's beautiful" "wha wha wha wha" "an she looks wild, wild, WILD!"

    Are you ready for the new sensation?
     
  20. okbassplayerguy

    okbassplayerguy New Member

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    I just picked up a JTM60 a few weeks ago with the 3X10 cabinet, and love it's sound, both green and red channels. I added a fan below the output tubes. The fan came with a variable speed control which is nice, since the fan on max speed is noticable even in a decent-sized room. i usually run it about 50-60% max speed.

    [​IMG]

    Marty has given me some good info on tubes to consider trying in the amp. I'm getting ready to place an order.

    Regards,

    Ed
     
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