Marshall JTM45 6L6/KT66/5881 Schematic????

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by TimDude88, Aug 1, 2020 at 8:04 PM.

  1. TimDude88

    TimDude88 Member

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    So I'm diving down the JTM45 rabbit hole. I'm trying to find a good looking schematic/layout for an original-ish JTM45 that was done with 6L6/KT66/5881 and 4 x 12AX7's. I went to MojoTone, but their schematic is for EL34's, and those are quite a bit different. I went to this website here, Dr.Tube, and I found this schematic here, but it's kinda rough to read. And I don't want to botch a build. If anyone can steer me into the direction of a JTM45 schematic that utilizes the 6L6/KT66/5881 and 4 x 12AX7's, I would greatly appreciate it. I'm also aware this schematic is supposed to be similar to the Tweed Bassman, but again, without a schematic that's legible, I can't really know. I also found this schematic, but it only has 3 x 12AX7's, so I'm not sure how close it is to the original, and it doesn't say what types of tubes are used in the power section. However, upon looking at it a bit further, it looks like this schematic is basically identical to MojoTone's schematic so I'm assuming it utilizes EL34's, not 6L6's. I'm sorta surprised I'm having a hard time finding this considering that so many people build clones of these amps. Maybe everyone just does the EL34 version...?

    Or, if I'm wasting my time looking for the original and I should just look at MojoTone's "British 45" that's more or less a JTM50, let me know.
     
  2. BearBryan

    BearBryan Member

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  3. oneblackened

    oneblackened Member

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    Can't think of any. It always had 3 I thought.
     
  4. TimDude88

    TimDude88 Member

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    Well as far as I can see, this schematic here has 4 x 12AX7's. Regardless, I'm more so just looking for a schematic that features the 6L6 power tubes than the EL34 power tubes.
    https://drtube.com/schematics/marshall/jtm45tr.gif
     
    Last edited: Aug 1, 2020 at 9:34 PM
  5. oneblackened

    oneblackened Member

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    Oh, I see - you're looking at a tremolo circuit there. JTM45s didn't have that IIRC.
     
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  6. TimDude88

    TimDude88 Member

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    Okay, thanks! Sorry if this is a newbie question, but what's "IIRC"? I understand some tube amp terminology, but I'm not totally familiar with all of the acronyms yet.
     
  7. BearBryan

    BearBryan Member

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    Not only did they not have four 12AX7s but they never had EL34s either, so I don’t know what is happening right now.
     
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  8. TimDude88

    TimDude88 Member

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    I know, the JTM50 which came along afterwards, came with the EL34's for a little more power.
     
  9. BearBryan

    BearBryan Member

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    That’s the transformer not the EL34s.

    That schematic is a bluesbreaker schematic. It’s tough to read some of the values but it does appear to have a bass tone stack and shared cathode so I don’t know why it says 1987t on it as well.
     
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  10. Ben S.

    Ben S. Member

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    There were some heads out there that had the 1962/Bluesbreaker chassis in them IIRC. I could go digging through the Marshall book and see if it had a model number, but that's all the way downstairs.
     
  11. oneblackened

    oneblackened Member

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    IIRC just means "If I Remember/Recall Correctly".
     
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  12. TimDude88

    TimDude88 Member

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  13. gldtp99

    gldtp99 Member

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    This JTM45 schematic by Mark Huss (who is better known for his Hiwatt schematics and a very good Marshall 2204 schematic) may be of help.

    I'm not an expert on the extreme details of exactly what Marshall changed and when the changes occurred in the early days of production and development of their amps.

    But this mhuss "JTM45" schematic seems to be a reasonable representation of what many think of when they talk about a JTM45 amp.

    Even Marshall's own JTM45RI (2045) isn't exactly what some people consider a true JTM45--- but I liked the one I had and slightly modded to be closer to "stock"---- even though I now know that an "Original Stock JTM45" circuit means different things to different people.

    Mhuss JTM45 schematic:

    https://mhuss.com/amps/JTM45/JTM45Classic.pdf
     
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  14. pdf64

    pdf64 Member

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    The JTM45 is not just one thing, it was a range of amps, and their circuit and components used evolved over time. So there's no 'one true' JTM45. As it's just an 'add on' feature, it seems reasonable to include the trem version (1987t) within that range, as what appears to be an original Marshall drawing is titled 'JTM45' and includes the trem. The same applies to the use of EL34.
    But even with that, I've never seen a schematic that's 100% accurate to amp that I've seen in person or pics of (only had one on my bench, and that was 35 years ago, when they were just 'early Marshalls'), eg the choke in series with the HT feed to the OT CT is surely a drawing error, and the 1k safety resistor across the output jacks (similar to that on an AC50) is never shown.
    This Mark Huss schematic https://mhuss.com/amps/JTM45/JTM45Classic2a.pdf and the Brian Wallace one linked in post #1 http://raw-sewage.net/images/jtm45-readable.jpg are about the best, with the handwritten notes here http://raw-sewage.net/images/jtm45.jpg being a useful supplement regarding some of the model differences.
    Regarding the R32 (shared screen grid resistor) note on Mark Huss' schematic, I suspect that 1k is by far the most common value, with 8k2 perhaps being an occasional build error (as it would significantly reduce power output) and 1k5 perhaps being used as a one off, eg if 1k 2W were out of stock.
    Whatever, between the power tube type, OT type, preamp type and screen grid supply arrangement, there are a lot of possible versions.
    If building one then you need to choose which version you want, but minor mods such as grid stoppers, eg 5k6, and 1 ohm cathode current sensing resistors for the power tubes are a very good idea.
    And a better, non 'hot switching' standby arrangement, eg http://www.valvewizard.co.uk/standby3.jpg or just omit that feature and use the switch for something else.
    As a reference, see the Marstran database http://marstran.com/Historic Data Base.htm and photos http://marstran.com/Marshall Pictorial Page.htm
     
    Last edited: Aug 2, 2020 at 7:27 AM
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  15. stryker59

    stryker59 Supporting Member

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    For instructions layout and schematic check out valvestorms's JTM45 kit instructions under their build checklists.
     
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  16. BearBryan

    BearBryan Member

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  17. pdf64

    pdf64 Member

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    That’s a rather different trem implementation that the original. Even if off, the latter will significantly affect the normal / trem channel’s vol control taper, max gain, and at high vol settings, that channel’s bass roll off.
     
    Last edited: Aug 2, 2020 at 10:56 AM
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  18. Ben S.

    Ben S. Member

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    FYI, the Doyle book lists the Tremolo options of the JTM45 head as: model T1987 and T1989
    With the 1989 being marketed for organs. The catalog picture with the models is on page 165 of the first edition of Doyle's book.
     
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  19. BearBryan

    BearBryan Member

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    Marshall and their numbers...
     
  20. TimDude88

    TimDude88 Member

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    SERIOUSLY! Not even like I'm dying to copy Peter Green's, but at this point I'm just curious now what the heck he had. Apparently it was an MkIV. But I don't know what that translates to with regards to schematics. I assumed since so many people build JTM45 clones that there MUST be a readily accessible schematic for the "classic BluesBreaker." I guess there may have been a lack of record keeping for whatever reason.
     
  21. pdf64

    pdf64 Member

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    Try Brian Wallace’s redrafting of the 1987t http://marstran.com/1987T.gif
    As mentioned above, a ‘64 model would probably have had the RS OT, which he reports were apparently always hardwired to 15ohms.
    Doyle’s book may clarify whether the 4x10 cab was more likely to be 8 or 15 ohms, and if it was specific to the organ or lead trem version. But if you hardwire a 100pF bright cap and add a 390pF switchable bright cap in parallel, then according to the notes attributed to Ken Bran, you’ve got both available :)
     
    Last edited: Aug 4, 2020 at 4:19 AM
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