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Bassbreaker 45 or Mojotone 5E3 Kit?

Discussion in 'Amp Input - Normal or Bright' started by Dudeman7, Sep 19, 2020.

  1. fezz parka

    fezz parka fezz parka

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    And the Pro Jr. does the tweed thing pretty well. The Blues Jr just has too much going on.
     
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  2. Andrew Wasson

    Andrew Wasson Senior Stratmaster

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    Put a 12ax7 in V1 on a Bassman, increase the negative feedback and you’ve got a JTM45.
     
  3. pierce

    pierce Strat-Talker

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    you arent buying a JTM45.. you are buying a KIT, modern iron etc.. If you are building a kit they ASSUMING that you are cognizant and are capable of not electrocuting yourself and possible have the skills to complete/modify it. Modern line voltage will take you over spec on any vintage tube amp readings unless you are using some kind of power regualtion
     
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  4. dante1963

    dante1963 Senior Stratmaster

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    Forgive me, but I’m not sure I understood your post.

    My post was a simple warning that the Mojotone “JTM45” kit, is not, in fact a kit for a JTM45, but for some sort of hybrid amp later in Marshall’s line.

    In Mojotone’s ad copy on their site, they list some of the legendary performers who used a JTM45.

    But the truth is, none of them used an amp like Mojotone is offering.

    A JTM45 is an historic amp with a great reputation. (Which is why I bought the reissue from Marshall, and why it’s the only amp I'm using right now.)

    What Mojotone is selling—either as parts, or pre-assembled—is NOT a JTM45. Not even close.

    It may be a great collection of parts, but no one can build an actual JTM45 with those parts. No matter their knowledge or skill level.

    I believe the Ceriatone kit may be more authentic. (Other than the addition of an effects loop and half power switch.)

    At least they are using the correct tube and transformer values.

    And a bunch of other companies also have good JTM45 kits, as well.

    (No offense implied, btw.)
     
    Last edited: Sep 24, 2020
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  5. Andrew Wasson

    Andrew Wasson Senior Stratmaster

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    Now that you're piqued my interest, I had a look at the Mojotone JTM45 kit to see if there were any glaring issues. I'm an electronics guy and I've been soldering and building electronics since I could read a schematic (many, many years before I learned to spell correctly). Funny thing is my "The Tube Amp Book by Aspen Pittman" has all sorts of obscure amps, every Fender amp ever dreamed of and in every variation and a good representation of the Marshall lineup but no JTM45. It's like it never existed so I had to compare it to the 5F6/5F6A Bassman which the JTM45 was lifted from. I didn't see any errors or omissions. It looks like it's true to form. A Bassman circuit with a 12AX7 in V1. Then I found the 1966 schematic for the JTM45 and it's identical to this Mojotone kit.

    Is your concern that it has EL34 instead of KT66, 6L6 or KT88's on the outputs?

    It appears that in around 1968 Marshall was using EL34's but if a guy wanted to use any of the others, it's just a matter of changing a couple of pins for a pair of KT66's or a pair of 6L6's. Oddly enough the screen grid resistor values and the grid stoppers on the Mojotone amp are the same as the 1966 JTM45 schematic so as far as I can see, if you connect pins 1 & 8 together on each output socket and you could use KT66, 6L6 and EL34's interchangeably.

    The power supply appears to be the same too but should work better with our higher 120 VAC vs the old 110 VAC of the past. I think they're using the same value choke. Are you concerned about the output transformer? I don't know their output transformers so I can't pass judgment on them.

    Unless I'm overlooking something obviious, it seems to me that this Mojotone JTM45 is as good as you could ask for and if you like, you can adjust it for any output tubes you choose. It is not a gross variation on the JTM45 theme. They've even gone a little higher in power handling for the resistors on the power supply for the second and third nodes of the power supply circuit. I do see an error in the schematic at that point where they mislabel the nodes for the high voltage power but it's probably mentioned elsewhere and most people build using the layout diagram anyway. It looks fine.
     
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  6. dante1963

    dante1963 Senior Stratmaster

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    Hi Andrew,

    Thanks for that info—it’s really interesting and informative. (I’m not an electronics guy, so this is really helpful.)

    Not being an electronics guy, all I could do was compare the basic specs of the original JTM45 with what they were selling. (Plus, researching a couple of other boards where at least a couple of guys way more knowledgeable than I expressed similar concerns.)

    The biggest two differences, as I said, were the use of el34s (actually they use E34Ls—which I’m assuming are the same) and the wattage itself.

    Which really brings us to the interesting question of “what, exactly is a JTM45?”

    It evolved over the years—and apparently Marshall wasn’t adverse to swapping out resistors and caps with whatever he happened to have on hand.

    But it started out roughly 30 watts, with 5881s. (Which is what Marshall’s reissue uses now.) He later switched to KT66s. The power didn’t go up until the short-lived JTM45/100, I think.

    Then from 1966-1968 (if I have my dates right), they introduced the JTM50. That was when he switched to EL34s. That was the (so-called) Black Flag. At first it had the tube rectifier, but halfway through the run, they switched to a solid state rectifier.

    So the Mojotone specs seem way closer to the Black Flag than what most of us think of as a JTM45. (Which would basically be a Bassman circuit with more negative feedback and a few other changes.)

    So, I guess that’s the question. What did Peter Green use? What did Clapton use on The Beano album? Combo versions of the earlier design, not the later.

    I’m wondering if I could paraphrase Fezz Parka, who when talking about the 5e3 always says “If you mod a 5e3 it stops being a 5e3.” I’m wondering if the same could be said of the JTM45?

    Ceriatone (not that I’m a shill for them, or anything) offers both a JTM45 and a JTM50 Black Flag.

    I wish Mojotone would do the same, because I’d probably wind up buying the JTM50.

    But, again, I think this is an interesting discussion and I’m interested in your thoughts. I’m also happy to hear that at least the tube changes can be undone.

    Personally, I love the look of the offset cabinet they offer. It looks very similar to the very first JTM45 heads. (But, I’m pretty sure the amp they offer would not sound very much like the Marshall RI I have upstairs.)
     
    Last edited: Sep 25, 2020
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  7. Andrew Wasson

    Andrew Wasson Senior Stratmaster

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    I was just looking at the Mojotone kit a little more. I’ll look at the Ceriatone one too. There are some really good tribute amps available in the tube amp realm.

    To me it looks like the Mojotone JTM45 should do a good representation of an early Marshall JTM45. It is literally the same circuit as the 1966 JTM45 with bigger output tubes. Swap a set of 6L6 Or KT66 tubes and it’s identical.

    I still can’t speak for the output transformer. I’d want to look into that just for completeness but IMO, if it’s a good match for the tubes and voltages, it won’t make a lot of difference. In terms of amp tone, I'm of the opinion that the tone that makes an amp is primarily in the preamp and right up to the phase splitter. That’s where all of our favourite amps make their magic. Since this circuit uses the same tubes, the same component values and the same power supply voltages, I'm pretty sure it's going to sound very much like a JTM45.

    I hadn't noticed that they ship with E34L's since the schematic they list shows EL34's but that might not be such a bad thing. They’re a little louder and have tighter bottom end. People tend to compare them favourably with 6L6’s. Regardless, we can run it with most any power tube we want anyway. I think if I were going to build one, I'd take your lead and use something smaller and period correct. Maybe 6L6's or KT66's. My dad (departed) started building tube amps when he was 14 and only stopped when he died 2 years ago. I've inherited his interest and knack for electronics along with his hoard of tubes so I'd spend some time figuring out which worked best.

    You say you’re not an electronics guy but I’ll bet if you gave it a shot, you’d get the feel for it. A lot of it has to do with attention to detail. If you ever get the itch, I’d suggest trying your hand at building an amp. Build a 70’s silver face champ. They’re single ended so really simple to build and you’ll get your hands in the game. I’ve got the itch for one of those right now. The power supply is the area I tackle first and I’m very careful about discharging the capacitors before I get my hands in the Chassis. That’s the only caveat. There’s high voltage and you have to respect it.

    Anyway, this has been a good diversion. Now I’ve got another amp on the bucket list :thumb:
     
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  8. dante1963

    dante1963 Senior Stratmaster

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    :)

    Thanks again for this info!

    This is one of the reasons that this the only forum I participate in--really knowledgeable users willing to share their experience.
     
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  9. jumpbluesdude

    jumpbluesdude New Member!

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    First let me say I have been working on amps since 1989 and have experience with older amps. The 5E3 would be a great amp for you. It sounds like you want a platform for effects. Make sure you put a 12AY7 (like the schematic calls for) in the first tube socket to keep the headroom. Change the .1 cap in one channel to a .047 and jumper the channels. Adjust the volume controls for eq. Weber alnico vintage series. You are set.
     
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  10. Dudeman7

    Dudeman7 Strat-Talker

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    That's exactly what I am looking for: a great pedal platform that I can control the range of dirt on with my guitar's volume pot.

    Overdriven 5E3s sound glorious to me. Now it's just a matter of saving the shekels for one.
     
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