So what makes a JTM45 different from a Bassman?

Discussion in 'Amps and Cabs' started by soldano16, Aug 3, 2006.


  1. soldano16

    soldano16 Member

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    I assume different

    transformers
    resisters/caps
    speakers

    make a diff but how much circuit change was done? Was it those tweaks/changes that created a new sound?

    I guess I could find all this out in a Marshall book so this is sorta the $20 question.
     
  2. cnardone

    cnardone Supporting Member

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  3. sgmarshall

    sgmarshall Member

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    In Michael Doye's book, Marshall, the sound of rock, there is a pretty in depth discussion on the differences, the 2 that stand out are
    1. The feedback loop comes from a different tap on the output transformer, giving more negative feedback (in the presence circuit).
    2. The polarity is reverse on the speaker outputs on the JTM 45 (the origional version, not sure about the reissue).
     
  4. Tag

    Tag Gold Supporting Member

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    Tonally speaking, the Marshall has way more gain. That leads to a better crunch tone, but the cleans are not nearly as good, and the JTM is smaller sounding.
     
  5. Fuchsaudio

    Fuchsaudio Member

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    A few factors: El-34's (or 6550's) and a unique output transformer design and feedback loop, changed from the original Fender/Western Electric circuits.
     
  6. Red Planet

    Red Planet Member

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    Yeah I allways knew those JTM 45's were high gain amps. Not much difference from one of those to something like a Krank. :roll
     
  7. PaulC

    PaulC Member

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    Actually the bassman is the higher gain amp. The JTM came with a 12AX7 for the first tube, and the bassman came stock with a 12AT7. Pop in a 12AX7 in the bassman (which a lot of guys do) and the preamp gain is the same. The bassman has more gain in it's output section due to it having less feedback. It's output section is more aggressive than the JTM's due to this.

    The choice of pwr tubes plays a bit in the amount of headroom in the output section. The pwr tubes in the JTM will break up sooner than the Bassman, but the bassman is being hit with a hotter signal.

    The cab is the biggest part in the headroom issue. An open back 4x10 vs a 2x12 is the biggest difference IMHO. The 10's will roll back the bass a little, and spread the pwr between 4 speakers for less speaker breakup. 2 low wattage 12's will break up more, and their stronger mid range content will rock better than what's used in the bassman.

    The 2 ohm output on the bassman makes it pretty hard to find a good 2x12 setup, but if you did that, and put some EL34's in it it would out rock the JTM.

    But don't go putting EL34's in a tweed bassman - it'll stress the heater supply...

    Later, PaulC
     
  8. Tag

    Tag Gold Supporting Member

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    Hey Paul,
    I have to respectfully, but TOTALLY disagree. I had a bassman RI for over 10 years. I compared it to way to many JTM 45s to count. While I have never had them into the same speakers, I had a 12Ax7 in mine, as well as a tube rectifier. The JTMs always had FAR more gain to the ear. They break up way earlier, and also seem to have MORE bass and upper mids than the JTMs. I have done this with originals, the Marshall HWRIs, the standard head, and my favorite, the bluesbreaker combos. Also did it with the Blockhead clones. As far as overdriving, it is not even close. The JTMs have WAY more distortion, and at much lower volumes. The amps also sound far different to my ear, I do not care what the schematics show. These amps sound as different as night and day.
     
  9. rwe333

    rwe333 Supporting Member

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    Perhaps a Bassman RI circuit differs from the original tweed?
    (I wouldn't know - just throwing it out for conversation)
     
  10. PaulC

    PaulC Member

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    Tag,

    You are right - they do sound as different as night and day, but the choice of output tubes and speaker setups is where it's at. Unless you've heard a bassman with el34's running into a 2x12 you can't make your claims. I have, and I also understand the electronics of the amps.

    The circuits are pretty much dead on with the difference in gain going to the bassman - it's the hotter circuit, but it's dumbed down with it's choice of tubes and speaker setup. Put EL34's in a bassman, and hook it up to a 2x12 celestion cab and it's the meaner amp. There's no question about it.

    Later, PaulC
     
  11. japhy

    japhy Member

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    Just a quick thought . The JTM 45 isn't an El-34 amp either . It should run some type of 6l6 ( 5881, 6l6 , Kt66 etc.)
     
  12. PaulC

    PaulC Member

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    You're right - a real JTM would have KT66's in them - cleaner sounding than EL34's, but a little less than 6L6's.

    PaulC
     
  13. Rusty G.

    Rusty G. Member

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    This topic is just perfect for a Fender BASSBREAKER. That's right. My brother's got one and it is one of the sweetest amps I've ever played. It's the Bassman circuit, handwired from the Custom Shop, with a tube rectifier AND 2 X 12's in a tweed cabinet. Awesome amp. It was supposed to be Fender's answer to Marshall's bluesbreaker amp.

    If you ever see one, scoop it up, because it's destined to become a classic!:RoCkIn
     
  14. waxnsteel

    waxnsteel Member

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  15. michael.e

    michael.e Member

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    The Bassbreaker had a pair of Greenies in it right? Have you pulled the chassis? As far as I have heard and read, these amps are still PCB.

    M.E.
     
  16. pbass2go

    pbass2go Member

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    I just got my origional Tweed Bassman back from a well deserved Cap job and Bias set. I would love to compare it to a RI.

    The amp sounded great before, (I had to put in re-issue speakers, the origional ones were MIA when I got it), but now it is a MONSTER!! I brought it home and plugged my strat and turned it up.... OMG!! :eek: I think it would stand up to a Marshall now.

    Cheers...
     
  17. Rusty G.

    Rusty G. Member

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    No I haven't pulled the chassis and looked. My brother told me it was point to point wired and I took him at his word. Now, he's not the gear nut that I am, so I may just have to call him on it. Still, it's a GREAT sounding amp. Sounds really good with P-90's!

    :RoCkIn :RoCkIn :RoCkIn
     
  18. nashvillesteve

    nashvillesteve Member

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    I know a lot of people here have more experience with amps and circuitry, but I have to share one very eye-opening experience I had with my amp- trying out different speakers.

    I have a Carmen Ghia head that came with a more powerful ceramic version of a Celestion Blue by WeberVST... (C12MZ- "MZ" for "Mr. Z"?)... anyway, I wanted to experiment with speakers. I think it's a 30 watt speaker.

    I got an alnico (15 watt) WeberVST P12B and the amp went from breaking up about halfway a bit when digging in with the C12MZ to barking and breaking up almost instantly. It just didn't work for me, as it was very crunchy sounding, and I am too big into Albert King and Freddie King for that much crunch... for a Tom Petty or Led Zep type sound, it was awesome!

    I also had a WeberVST C12CA 85 watt JBL clone with the biggest friggin' ceramic magnet. I had purchased it for my Leslie cabinet, where it still resides happily, but as an experiment, I put it in my Dr. Z 1x12" cab. The Carmen Ghia would not break up. I dimed the volume and it was still clean (I understand this might be more from the speaker being vastly more powerful than the amp and the P12B being a speaker that purpotedly handles less power than the amp puts out- 15w through an 18w class A amp that's more like 30 watts in a Class A/B- it kept up with a 4-10 Super Reverb (SF) at a jam that was set at 6 with the Ghia set to about 1:30...(that was with the C12MZ)...

    So, I would say that speakers possibly have a more pronounced effect on gain than anything else. I have swapped out tubes (I have a 5751 and a 12AX7 in the preamp- 5751 for a bit o' headroom and 12AX7 for breakup, according to "Mr." Z, anyway), but that difference was miniscule compared to the speakers.

    The 4x10 combo when compared to 2x12" might make a difference, too, and I would suspect the transformer would be another major player.

    Fender Bassman:
    45 watts into 2 ohms
    Ohms: 2 ohms
    Speakers: 4-10" JensenĀ® P-10R, 8 ohm Speakers with AlNiCo Magnets

    The JTM-45 had 3 ECC83 tubes in the preamp instead of 3 12AX7s that were in the Bassman, not sure how those compare... EL34s (obviously in the output section). I don't remember what else is inside those off the top of my head, but there are other differences, some of which have been pointed out, but I'm just not that familiar with (capacitors? hell!) However, it's only a 30 watt amp, so at the same volume it will be working harder than the Bassman.

    Celestion Greenbacks are 12" and 25 watts. If there are four of them that the amp is pushing 30 watts through 24" of speaker in a 2x12" or 48" of speaker in a 4x12" cab...
    P10R Jensens are 10" and 25 watts... That's 45 watts through 40" of speaker...

    Therefore, I might throw out there as a question for the techies that could the Bassman might be overdriving with more from tubes and circuitry and the 2x12" cab with a JTM-45 is getting more breakup from speakers and smaller size? The ceramic celestions should have more headroom than the alnico Jensens, but there are lots of other factors for speakers (cone type, magnet size, dust cap, exact materials, did they dope speakers back then?).

    The JTM-45 in a 2x12" situation is putting 12.5% more wattage to the speaker area than the Bassman, correct? I know the magnets on the Greenbacks were much smaller than those on alnico speakers of the day, right? I'm sure the output transformer has a lot to do with how efficiently the power hits the speakers as well.

    In addition to having a higher wattage-to-speaker-area ratio than the Bassman, the JTM-45 is not as powerful an amp as the Bassman so it would have to be turned up even more to play over a drummer, correct? There are possibly too many component differences to even point to one factor, but an objective experiment would be to run the JT-45 through a Bassman's 4x10 speakers... unless you are just comparing overall package. A JTM-45 through a 4x12" cab might make up for some of the power difference with 8" extra inches of speaker, I don't recall the equations from the "Physics of sound" lectures... The way the speakers were wired might make a difference as well.

    I am no expert but was very profoundly surprised with my personal speaker experimentation in the past and am very curious... though I'm not sure I can fully absorb all of the electronic variables in components that non-experts like me don't deal with (us simpletons who love the gear but are more players and such seem to put the importance on the stuff *I* (we) can change ourselves like tubes & speakers, while there are myriad other factors... Was the 2x12" on the Marshall cab closed back? I think they were, that would make a big difference right there... Sorry if this post was a waste of space, but I am trying to understand a bit more about these classic amps...

    ... I'll take one of each, please. :)
     
  19. DocRock

    DocRock Supporting Member

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    Are you kidding?! That post was *not* a waste of space at all IMO. That's the type of critical listening, experimentation, and sharing of ideas that makes me happy to have TGP. Thanks for putting forth the effort to type all of that. I'm no electronics guy either, but I have good ears.

    Certainly more helpful than some of the threads I've seen on TGP lately, sheesh.......

    Cheers,

    Doc :)
     
  20. StompBoxBlues

    StompBoxBlues Member

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    I could be totally wrong, but I can't see that making any difference. The only place it ought to matter is in the power supply, and as long as the filter caps for the power are correct (getting rid of ripple) it ought not make any difference. Or did I miss something?

    I haven't read further than this post yet, but MAN is this ever a welcome thread. I have a JTM-45 Germino (Classic 45) coming most likely TODAY :) and just last year bought a Bassman, point-to-point handmade up in the north of Norway.

    I am SO looking forward to finding out for myself what the differences, but I would say (and certainly somene else further downstream from your post has mentioned) the main difference probably is tubes! And trafo...

    Since it is a head, the speakers...well I have a 2 X 10" I could try it one to get it near the 4 x 10"s in my Bassman (the only thing I didn't like, I think about changing is he used the original RNC cables instead of phone jacks for the speaker outs...he stayed true to the original which is a slight pain in the ass as far as trying out external speakers, or attenuators. I heard there is a harness from Weber that converts, I may go for that or else hard wire it to be more modern.

    But this thread came at JUST the right time!
     

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