Need advice on JTM45/plexi clone build

Discussion in 'Shock Brother's DIY Amps' started by itsGiusto, Aug 19, 2019.

  1. itsGiusto

    itsGiusto Tele-Meister

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    I'm going to be starting a build of a JTM45. I'd like to be able to have some things be switchable to the more high-gain, early 70s plexi specs. For example, I'd like to put a switch to switch between tube rectifier and solid state rectifier, and have two separate bias pots, so I can bias the tubes properly for each of the different rectifiers.

    I'm looking for some advice:

    1. What is a good kit to use? I've used mojotone kits in the past and liked them (for parts-quality, not instructions, which were terrible, so I referenced the BYOC instructions a lot), but are there any other ones I should consider?

    2. I'd like to be able to swap between EL34s and KT66s. Which transformer should I use for this purpose? Is it better to go with a transformer designed for EL34s if I want to be able to swap like this?
     
  2. Wally

    Wally Telefied Ad Free Member

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    • In what way is there more gain in say a JMP Md. 1987 that’s in the first Marshall? Or...the earlier Plexi circuit? Have you charted the differences in the two circuits? I am curious, too.
    As for kits, ???? I don’t know. Maybe this bump will get the ball rolling. I figure you will get some suggestions for Ceriatone. A quick look at the Triode kit looks good as far as trannies.
     
  3. itsGiusto

    itsGiusto Tele-Meister

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    Sorry, maybe "high-gain" might not have been the best word to describe it. It is slightly higher gain, but maybe just "tighter and brighter" are better words to use.

    The major differences as far as I can tell (I'm no authority, though, and there are also smaller changes I don't care as much about, like the bright cap value):

    • JTM45 uses a GZ34 tube rectifier, early 70s plexi uses a diode bridge.
    • Plexi splits the cathode of the first preamp tube stage, using different bias for each triode.
    • Plexi uses EL34s instead of KT66s, and has higher value and higher wattage screen resistors (1k instead of 470r)
    • Plexi has much higher filtering, using three 50+50ufs, instead of a 32+32uf and two 16 ufs in the JTM45
    • Some difference in NFB which I haven't fully worked out yet (plexi has 100k coming from 4ohm tap, instead of 27k coming from 16ohm tap)
    • Lower PPI coupling caps in plexi, 22nf vs 100nf in the JTM45. This definitely would limit the bass and create a tighter, but less bassy sound in the plexi.
    • Plexi has different output transformer, I think higher-rated for wattage, and also with a smaller primary, something like 4k instead of 8k, I think.
    • I think the power transformer is different too, but I don't know how exactly.
    • Some plexis also seem to have V2's cathode bypassed with a cap, probably to make it higher gain and brighter.

    I have definitely considered Ceriatone, but IDK, I don't necessarily trust their part quality. Maybe that's uncalled for and people should feel free to correct me if I'm wrong.
     
    Last edited: Aug 19, 2019
  4. bftfender

    bftfender Friend of Leo's

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    i have 3

    jmp50 1987 now 2204..started as Weber kit
    jtm50 made by Union Jack called The 45..now Rola..el34 ss/tube rec
    jtm45. weber kit 6l6 fender spec'd power

    I can only state as a player what i hear..these were all kits at one point but my tech made em incredible. We have a/b'd vs vintage...

    I can def tell you what they all do tho. Those things you ask for,,good idea..the circuits lend themselves to really customizing what you want out of it..especially paired with speakers for certain sound
     
  5. wtk0315

    wtk0315 Tele-Holic

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    I went to valvestorm for my small parts for my Frankenstein 4xel84 Plexi. I don't think they have transformers though. I liked them because their kits have 1 watt resistors standard which is a suggested mod to reduce hiss.
     
  6. Paul G.

    Paul G. Friend of Leo's

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    Definitely prefer Ceriatone and Triode over the Mojotone kit for the JTM45. Mojotone has their own take on power supply and location of caps that is different enough from standard as to make mods a bit more difficult -- their layout is hand drawn and difficult to follow, though the schematic is clear enough.

    The differences are split cathode in first stage opposed to JTM shared cathode, bypass cap on the bright channel went from ~250uf to .068, added filtering, tone stack changes, SS rectifier.

    You can use an output transformer for EL34s with KT66s, but -- you compensate for the difference in the tube's impedance by adjusting your output impedance to the speaker. The 8 ohm tap will become your 16 ohm tap for KT66, 4 ohm becomes 8, and you don't use the 16. You need to have screen resistors for KT66 tubes - 470R to 1K. The bias dropping resistor may need to change.


    The thing to do is print out schematics of both and use a highlighter on all the differences.

    The JTM will be a looser more open sounding amp, the JCM will be tighter, brighter, more aggressive.
     
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  7. itsGiusto

    itsGiusto Tele-Meister

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    Are there any negative consequences to the tone for using an EL34 transformer with KT66s? Does it sound worse than using a standard JTM45 transformer with KT66s?
     
  8. Paul G.

    Paul G. Friend of Leo's

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    I've built 2 JTM45s. One from a Ceriatone kit, using Mercury Magnetics transformers which I picked up from someone who found himself in over his head. These are for 3.4k input (EL34), but I wired it up for KT66s, adjusting the output taps. The second came from a Mojotone Small Parts kit, plus a chassis I had (came with the Xformers) and Classictone transformers with a 6.8k input (KT66).

    The amps sound more the same than different, and more filtering and a larger choke in the second amp may have more to do with the differences than the transformers did. You said you were interested in going back and forth, so that's why I said what I did with the transformers.
     
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  9. Wally

    Wally Telefied Ad Free Member

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    ...should be interesting...
     
  10. dan40

    dan40 Tele-Afflicted Silver Supporter

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    Valvestorm took over the selling of amp kit parts when Metroamp got out of the kit business. They don't actually have a kit but they do have a complete list of needed parts on one page, and you can pick and choose various components (new and nos) to complete your build. If you do go with Valvestorm, be sure to use the Metroamp instructions provided at the top of their JTM45 page. Trying to mix different kits with different layouts and instructions can lead to a headache.Unfortunately they do not sell transformers as many builders like to use their preferred brand for these. I really like Marstran and Classictone but Merren also makes a great OT if you don't mind the wait. I have purchased several items from Triode in years past but lately they seem to be having some issues with shipping promptly and can be very hard to reach if you do have a problem. Quite a few recent threads about bad experiences lately but hopefully they get the issues ironed out.

    http://valvestorm.com/JTM45 Kit

    http://www.marstran.com/
     
  11. dan40

    dan40 Tele-Afflicted Silver Supporter

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  12. Nickfl

    Nickfl Tele-Afflicted

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    Personally I wouldn't go to the trouble of having switchable tube solid-state rectifier option, just not enough of a difference to justify the considerable trouble.

    If you want to make it versatile I would consider switching for cascaded gain stages on the input and a switchable cold clipper stage. There have been a couple of schematics posted over here in the last couple of years with options like that.

    There was a recent thread called Marshalls greatest hits or something to that effect I don't remember who the original poster was and I can't be bothered to search for it right now but I'm sure you can find it with little effort. The schematic he had going in there for switching between several classic Marshall preamp models was pretty well thought out.
     
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  13. D'tar

    D'tar Friend of Leo's

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  14. PCollen

    PCollen Friend of Leo's

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    Use the Weber WZ34 CopperCap Rectifier, and be happy. Plugs right in to the JTM45 tube rectifier socket
     
  15. bftfender

    bftfender Friend of Leo's

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    the ss/rube rec is like having 2 dif amps..way worth it to my ears
     
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  16. bftfender

    bftfender Friend of Leo's

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    4hole (2).jpg
     
  17. tubedood

    tubedood Tele-Meister

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    I am all ears (and eyes) on this thread!

    My next amp project might be to try a way to get two Marshalls in one cabinet.
     
  18. BobSmith

    BobSmith Tele-Meister

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    I did a Valvestorm jtm45 kit earlier this year. Totally recommend! They have a great forum too on Metroamp too with bunches of experts to help you along (perhaps not quite as great as TDPRI of course). I got some very good suggestions as I made my way thru the build and the amp turned out great.

    Iron: I went with Mercury (not radiospares because I wanted the versatility of an output impedance switch). I’m VERY happy with their performance. Merren (aka ****** on metroamp forum because the name is banned there, not sure the story there but I’m sure it’s silly) is suppose to be fantastic (never heard one in person) but it took him forever to respond to inquires so I passed. You can’t go wrong with Mercury I think, but it has been described as “too clean”, but I also have a handwired JTM45 reissue from Marshall with a Drake made tranny that sounds about the same, so there you go...

    Mine is on top (Mojotone small head box) and Marshall handwired reissue is on the bottom.
     

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    Last edited: Aug 19, 2019
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  19. Nickfl

    Nickfl Tele-Afflicted

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    There's also the preamp schematic posted by @printer2 a year or so ago in this thread. A lot of clever switching between marshall variants and even a little Trainwreck for good measure.
     
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  20. printer2

    printer2 Poster Extraordinaire

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    S3 is a two pole three position, center off, switch. The center position is Fender Blackface, the other two are two of the Marshall variants from what I recall.
     
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