Is a JTM45 with…

Discussion in 'Marshall Amps' started by Johan, Nov 23, 2021.

  1. Johan

    Johan Member

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    … EL34s and SS rectifier still a JTM45?

    Just wondering. There’s one (not a genuine Marshall) for sale not too far away, and for a decent price.
     
  2. lespaul339

    lespaul339 Well-Known Member

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    In my opinion no. You need that tube rectifier to get that sag the JTM is known for. Also kt66 in JTM just sound so much better IMO.
     
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  3. Ken Underwood

    Ken Underwood Well-Known Member VIP Member

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    A true JTM45 will not have KT66s but 6L6s or after that EL34s, originality is key
     
  4. saxon68

    saxon68 Well-Known Member

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    There’s the official opinion folks.
     
  5. Eric'45

    Eric'45 Well-Known Member

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    That's a 1987 with JTM45 Badge :wave:
     
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  6. jimmyo

    jimmyo Member

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    Some like them natural and saggy, while others prefer more silicon with a tighter bottom end. You’ll have to try a few to see which you prefer……….
     
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  7. Obi Plexi-nobi

    Obi Plexi-nobi Well-Known Member

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    Bought this book DECADES ago. Shows how to take a dead rectifier tube base and solder up a bridge rectifier in it, using silicon diodes. I used to have low funds & used and old RCA school intercom w/ 6L6 tubes as a power amp for my Boogie preamp. I would randomly switch back and forth between the original tube rectifier and the solid-state one I rigged up. You could REALLY tell the difference in sag/sponginess between the two. BOTH sounded excellent in different ways... Book.jpg
     
    Last edited: Nov 27, 2021
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  8. Dblgun

    Dblgun Well-Known Member

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    Wow, I was under the impression that many were produced by Marshall with KT66's. So much information shared here that dispells inaccurate beliefs.
     
  9. lespaul339

    lespaul339 Well-Known Member

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    Same. But I still like my kt66's in mine!
     
  10. neikeel

    neikeel Well-Known Member

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    The KT66s were fitted ex-factory after Ken left the company. I believe that Dudley and Ken’s used 5881s as they were ex-military versions of 6L6. KT66 were used once the military (probably cheaper) valves got used up and GEC/Marconi being a U.K. company had the nearest spec.
    Marshall first used EL34s with the output transformer changes in early 1967 (Drake 784-128 in the 50w and 1202-119 in 100W) as these had lower primary of 3.4K and 1.75k respectively compared with the earlier 8k and 4k OTs in 65 and 66 supplied by Drake.
     
  11. dro

    dro Well-Known Member

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    My first one had 5881's The latest RI I've had for the past 10-15 years is set up with 6L6's.
    Never seen a JTM45 with EL34's and they all have GZ34/5AR4 tube rectifiers.
    Sounds more like a 1987x to me too.
    Played a 1987x once. Didn't fit the bill for me.
     
  12. Dblgun

    Dblgun Well-Known Member

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    Neil, was there a PT or circuit change during the change from 5881's to KT66's? Seems like 450-475v on the plate is more than is normally used with the 5881 variant of the 6L6.
     
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  13. Eric'45

    Eric'45 Well-Known Member

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    I run my 2245 RI still with 5881s- it was setup with those from the factory and I like it this way. yet many claim the first reason why the JTM45 Reissues aren't "authentic:noplease:" is that they don't use KT66s. Circuit changes aside, I am no expert on this and i would like not to go down that rabbit hole. interesting to hear it from one that actually knows, that 5881 tubes are, indeed, a proper choice for a JTM45 :thumb:.
     
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  14. Kinkless Tetrode

    Kinkless Tetrode Well-Known Member

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    Or a 1986 if it has the shared V1 cathode and the typical JTM45 cap and resistor values.
     
  15. Kinkless Tetrode

    Kinkless Tetrode Well-Known Member

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    I have ran EL34s in mine. I liked the result.
     
  16. Dblgun

    Dblgun Well-Known Member

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    5881/6L6's in a P/P AB want an OT with 6-6.6k depending on which variation. A pair of KT66's 8k and EL34's 3.4k. Seems like it would not be advisable to interchange them in the long term without other changes.
     
  17. gregr

    gregr Well-Known Member

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    There’s more to it than just output tubes and a rectifier. There are many other differences between it and a 1987, notably the topology and voicing at the first valve as well as the tone stack. These directly affect the tone every bit as much as the rectifier and output valves, arguably more so. Then there are differences in transformers, plate voltages, a slew of other component values and whatever else I’ve failed to mention. The JTM45 is basically a Fender Bassman, just made with parts that were available in the UK.

    I know most of you already know this, but it bears repeating whenever someone implies that the difference between a super lead and a jtm is the rectifier and output valves.
     
    Last edited: Nov 26, 2021
  18. Ken Underwood

    Ken Underwood Well-Known Member VIP Member

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    With any comment i make then you must appreciate that i mean Original JTM45, lots happened between 63 and 67.

    I think a lot of comment and opinions come from reading books, well you know what i feel about that
     
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  19. neikeel

    neikeel Well-Known Member

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    The first PTs were the self lead RS with hammerite paint and selector on top. You also see the open frame lug type but AFAIK same specs. The Drake successor 1202-55 is also broadly the same. All approx 450vdc post rectification. The only high voltage 50w PTs I know of are the Drake -133 on the late JTM Black Flags for 470vdc or thereabouts.
     
  20. neikeel

    neikeel Well-Known Member

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    Agreed.
    By definition a 1987 is a lead spec 50watter. Key features are the split cathode on v1 and upper mid focussed tone stack as well as smaller pi output couplers.
    A 1986 is closer to a JTM45 but still sl different.
     
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