Jtm45 Questions

Discussion in 'Building the Classics' started by black knight, Oct 24, 2018.

  1. black knight

    black knight New Member

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    Hello.

    I've been a member here for quite some time but have never participated in discussions. I didn't feel as if I had anything technical to offer. I have mostly resourced this forum as a learning tool by reading...a lot. Still, I have a lot to learn. For example, only recently did it really sink in that chasing that elusive "magic" Marshall tone involves many factors, but largely comes down to circuit specs. And of course to complicate things further, the transitional periods make it difficult to say "I'd just like to have the tone from a 1986/7 circuit from 1968". But at the same time, this is what fascinates me and keeps me eager to learn. I plan on building my first amp within the next year, after much research.

    For reference, I am not a young man, ahem, 50, and I have owned many, many Marshall models through the years (did I mention many?). At the moment, I only have a 1987x and two clones, a Ceriatone bass spec 1967/8 jmp50, and a recent acquisition, a JTM45 from GC, which brings me to the reason for my post.

    I saw this amp listed for a good price and I've been looking for a decent JTM45 clone. It was advertised as a Union Jack JTM45 and only had one picture of the front of the amp. I went for it as the return process is easy enough to navigate if necessitated. Though it is definitely not a Union Jack, it is well built and sounds really, really good. Like silly good.
    Being somewhat of a novice when it comes to circuits, can you guys tell me what I've got here? And maybe who built it?

    From what I have been able to identify, it appears to be a JTM45 circuit but with higher filtering? (2 - 50+50 and 1- 32+32 cans on top, and 1 - 16uF on the board). Sozo caps, carbon comp resistors throughout, Sprague Atoms, F@T and JJ filter caps, Hammond PT and choke plus what appears to be a Heyboer RS spec OT. The transformers seem small but I recently learned that it's not necessarily the size that matters as much as it's minimum handling capacity. In any case, the amp sounds terrific. I'll try to post some clips if I can manage to get a quality recording. The tone, if I were to describe it, sounds full and blossoming, with each note, even in a chord context, getting equal attention. It leans easily to beautiful harmonic feedback when notes are sustained. I've only tried it through a 4x12 with G12M-25's so far. And it only sounds better the louder it gets, with minimal "point of diminishing returns". And it's difficult to dial in a "bad" tone.

    Thanks for any information anyone is able to provide. I really just curious about the circuit (any variations from the original JTM45) and who might have built it. IMO, they did a great job. Opinions also would be welcome.

    I know it is not meant to be a dead nuts on replica. And although I am still a recovering vintage and vintage correct addict, these days I'm happy to have the tone, regardless of aesthetics and circuit tweaks. 20181024_074445.jpg 20181024_075247.jpg 20181024_080218.jpg 20181024_080314.jpg 20181024_080348.jpg 20181024_080437.jpg 20181024_080517.jpg 20181024_080603.jpg 20181024_080839.jpg 20181024_080919.jpg
     
  2. black knight

    black knight New Member

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  3. johnfv

    johnfv Well-Known Member

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    I'll let the experts comment on the circuit details but it sure looks like a nice build, congrats!
     
  4. neikeel

    neikeel Well-Known Member

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    Looks like a tidy JTM45 clone with good quality components d cent quality Hammond trannies not sure how good the OT is (interesting that both trannies are 90degrees from normal Marshall orientation). Slightly unusual jack/pot arrangement but maybe what works for the builder.
    Oh and 50+50 is too much for a GZ34. 64 is max (50 ideal).
     
  5. vivanchenko

    vivanchenko Active Member

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    Hammond transformers are not decent. They are excellent, over-engineered (according to my phone conversation with their designers and my personal experience) and extremely reliable. The one you have has the right, that is high enough, secondary voltage for a JTM amp. I believe your B+ should be about 450 V. 50 uF caps make the amp sound tighter and a bit cleaner. 32 uF is good for a rawer and squishier sound, more along the lines of Jimi Hendrix. Looks like a nice, clean work.
     
  6. neikeel

    neikeel Well-Known Member

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    I’m English - understatement applies.
    Mercury Magnetics make overengineered transformers too. Their OT sounds like poop in a JTM45 but great in a metal monster.
    Heyboer and Classic tone make transformers that sound like the originals.
    By all means use the 50+50 (ie 100uF) mains filters like a post 69 JMP50 but it will kill most GZ34s.
    Do some reading if you don’t believe me!!
     
  7. black knight

    black knight New Member

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    Thank you guys for your input. Just using my ears, I'm not sure I'd change anything about this amp. Sitting at GC for so long, 100's of players must have plugged in and tried it out. I can't understand why no one bought it.

    I should note that the rectifier tube is a JJ branded 5U4GB. Would this tube be appropriate from what you can discern from the photos of the circuit?

    Neikeel, regarding transformer orientation, is it standard practice for them to be oriented in such a way as to reduce noise? And what makes the jack/pot arrangement unusual?

    20181026_105543.jpg
     
  8. neikeel

    neikeel Well-Known Member

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    Normally the JTM45 has laydown PT. Obvs this is clone. The builder used a 100w chassis and decided to put the choke between the transformers - different strokes I guess.
    Normally in JTM 45 the OT is aligned with lams along chassis (early 100s did not but got changed around 69 to decrease coupling hum). The PT usually goes the other way eg
    upload_2018-10-26_17-20-39.jpeg
    It will probably work fine as it is - just different!
    The input jack 1M ground reference seems to be on high and low jacks, not just high.
    Have you read up about blowing up overstressed rectifier tubes yet?
    https://www.diyaudio.com/forums/tubes-valves/180064-5u4-rectifier-maximum-filter-cap-value.html
     
    Last edited: Oct 26, 2018
  9. black knight

    black knight New Member

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    No, but I'll give you one guess what I'm gonna do right now...
     
  10. vivanchenko

    vivanchenko Active Member

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    The JJ 5U4GB is not a proper JTM 45 rectifier tube. Unlike the JMPs the JTMs "like" high B+ voltage and the best ones had something like 470-490 V B+. As far as I remember with the 5U4GB you are loosing about 20 V and it is considerably spongier than the GZ34. The Chinese GZ34 tube is the best inexpensive option today and with it you are getting much closer to the real thing. As far as I remember they are manufactured using the original Mullard equipment which the Chinese bought when it was about to be scrapped. That's another one of those urban legends. In any case this is what I am using and it never failed on me. You will need to re-bias the amp if you use the GZ34. With it you will be considerably closer to what a JTM 45 should feel and sound like.
     
  11. vivanchenko

    vivanchenko Active Member

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    This is a very good point about which I forgot. A JTM should have a 32+32 uF power amp filter cap. Apart from altering tone and feel a 50+50 cap will over-stress the rectifier.

    You actually do have a 32+32 uF can capacitor installed on top of the chassis, but there should not be any 50 uF capacitors anywhere at all and you have two 50+50 can caps! This does not look like a JTM 45 spec. Looks like somebody else's take on the classic. What those 50+50 uF can caps are even doing there? Do you have the schematic? Are they a part of heater wiring? Does this amp have direct current heaters? This does make sense because it reduces the hum.

    Actually, a correctly made JTM 45 is virtually hum free even with AC heater wiring. Mine is. I wouldn't bother with DC heaters for a JTM of all amps. So, what are those 50+50 can caps? What are they doing?
     
    Last edited: Oct 27, 2018
  12. ampmadscientist

    ampmadscientist Well-Known Member

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    50uF + 50uF no it is not too much for a rectifier tube. It will not over-stress the tube.

    50 + 50 is OK and will reduce more hum compared to 32+32.
     
  13. vivanchenko

    vivanchenko Active Member

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    As I mentioned above, the JTMs are very quiet, low gain, amps even with 32+32 caps. I can hardly hear any noise at all. May be just a very little bit in a very quiet room. And that's with 32+32. You will have a very hard time detecting a difference in noise levels with 50+50 caps. I tried both 32+32 and 50+50. It is a matter of taste. To me 50+50 filter caps make the amp sound noticeably thinner and more sterile, but it is just me. Others might like it.

    The most interesting thing here is that the amp should have either 32+32 OR 50+50 (as Marshall did for one of its reissues), but in reality it is 32+32 AND two 50+50! This is two can caps two many. What are they doing there? The only thing I can think of is that they are used for preamp DC heater wiring. But unlike the JMPs the JTMs are very quiet even with AC heaters. Why bother?
     
  14. vivanchenko

    vivanchenko Active Member

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    According to GZ34 rectifier tube data-sheet limiting capacitor input value is 60 uF. With 50+50 you will exceed it: https://drtube.com/datasheets/gz34s-jj2003.pdf . I believe that one of the reasons why Marshall started using 50+50 filter caps in the JMP amps was that at that time they started using SS rectifiers which could handle mush higher capacitor input values.

    So it's not just a matter of taste. It's also about complying with the data sheet.
     
    Last edited: Oct 27, 2018
  15. neikeel

    neikeel Well-Known Member

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    vivanchenko and plexifier like this.
  16. vivanchenko

    vivanchenko Active Member

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    The data-sheets you linked show exactly the same thing - 60 uF max capacitor input. 50+50 =100 uF which is way above the maximum (limiting) value. The first amps where 50+50 uF caps showed up were the JMPs all of which which had solid state rectifiers.
     
  17. vivanchenko

    vivanchenko Active Member

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    neikeel likes this.
  18. black knight

    black knight New Member

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    Again, thank you for taking the time to help me with this. To remind you, I am nearly clueless when it comes to circuits and design parameters. I am eager to learn so please bear with me. I read and read and watched videos on this topic of filtering and still don't fully comprehend it. Maybe it is not as complex as I think it is but after seeing so many different configurations, I'm even more confused. First, it seems the PT in my particular amp is ok with the 5U4GB rectifier tube (6.3V, 5A). But...the two 50+50uf caps exceed the 40uf max capacitance rating of the tube which will presumably stress the PT. Is this correct? And another weird thing is it looks like they are grounded in series to the presence pot? (See the unsheilded wire in the photos in the initial and second post). Also seems different than anything I've seen. And how did the Marshall JTM45RI get away with 50+50 filtering without doing any damage?

    http://forum.metropoulos.net/viewtopic.php?t=38402

    Also, stupidly, I played the amp for a while last night and this morning before working this out. Well hopefully I haven't done damage to the PT but today I noticed a static sound on the decay of some chords. No volume loss and not on all chords. (Worst culprit was a barred F# chord FWIW). I turned it off immediately and would like to resolve this first before doing any more harm. I went from pretty excited to have good tone again to pretty bummed. Basically I just want to get the amp within safe design specs and go from there.

    I can change caps no problem. They're actually very accessible in this build but I don't know circuits enough to understand what else is going on in this amp. As you guys know, it's really not that simple. Primary's, secondary's, PI, mains, screens, dual caps vs single, etc. I'm just scratching the surface at this point. Was there a valid reason the builder had the 50+50 caps there? Was it experimentation? I wish I knew more and was able to make an educated guess based on tracing the circuit.

    Right now, I just want a good sounding amp to play so would like advice on what caps to use based on what you can interpret or if necessary, I can take more pics and provide more detail.
     
  19. ampmadscientist

    ampmadscientist Well-Known Member

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    "I noticed a static sound on the decay of some chords."

    The static sound in the decay of some chords is caused by cold bias, usually.
     
  20. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    You know Ampmadscientist is more right than most know. I was just wondering if there was going to be a rebuttal.


    Is it my eyesight or did any think the first two 50/50uF were paralleled? That would make it 200uF.

    I think the intention was to stack them, in series, for a total of 50uF. :)
     

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