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Building a JTM 45/100 (Malcolm Young/KT66)

Discussion in 'Building the Classics' started by _Steve, Nov 23, 2020.

  1. _Steve

    _Steve Well-Known Member

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    Interestingly it's just blown two V2 valves - one soon after another. Before it happened I got some brief sound dropouts before coming back.

    Voltages look "correct" based on the old schematic, but I saw that the CF cathode pin is 215 (225 in schematic). Could I be having heather-cathode arcing? Datasheet says 200V max.
     
  2. neikeel

    neikeel Well-Known Member

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    Some 12AX7s cannot handle the cathode follower position
     
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  3. neikeel

    neikeel Well-Known Member

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    “Spiral filament tubes do not work well in cathode follower circuits and are prone to failure. This appears to be due to the closer spacing of the spiral filament to the cathode. Spiral filament tubes include the 12AX7LPS, 12AX7EH, Tung-Sol 12AX7, Mullard 12AX7 reissue, and JJ ECC83s/12AX7”

    For cathode followers and phase inverters you will need a robust tube such as any old Mullard/Brimar or one of the Sovteks.
     
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  4. _Steve

    _Steve Well-Known Member

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    They were both Tung-Sol reissues :) I have a Sovtek somewhere...
     
  5. adew1

    adew1 Active Member

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    Good to know! Thanks.
     
  6. roef

    roef New Member

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    see here on arching and elevated heaters: http://www.valvewizard.co.uk/accf.html
     
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  7. Chris-in-LA

    Chris-in-LA Well-Known Member

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    What about the Sovtek 12ax7WA, WB, or Wc? Do they have the spiral filaments?
     
  8. playloud

    playloud Active Member

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    Hope you managed to get the amp running again Steve? It looks like a really nice job!

    I'm currently contemplating trying the "Dickinson spec" JTM 45/100 as my first build. I know it's an ambitious first project, but I'm willing to go slow. I considered building a JTM50 instead, but I've already sort-of owned one of those (Germino Club 40), and the JTM 45/100 is something I know will keep me motivated.

    From what I can gather, this Brian Wallace schematic is the one to use? It seems to line up with the pics on Amp Archives, and neikeel and William Vogel's very nice builds.

    The major lingering question I have is about the KT66s. For those who have used both the real GECs and the Gold Lions, how substantial is the difference between the originals at ~560V and the reissues at ~490V? If the build is a success, I'll probably want to spring for the GECs at some point in the future, but it's a big investment up front (not to mention that finding a reliable quad is pretty hard at any price!)

    A more minor question is about the bias filter caps. Given the high plate voltages, I'm thinking reliability is even more important than usual. Any recommendations on what to use here?

    First step will be ordering the transformer set from Marstran (1204-43, 1202-84, and 3H choke). That should give me some time to get the other parts together...
     
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  9. _Steve

    _Steve Well-Known Member

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    Hi yep I definitely got it up and running some time back. It is a very, very special amp! Im actually just in the middle of elevating its heaters and decided to take the opportunity to tidy up some of the wiring so the boards are currently out :)

    Mine is (mostly) switchable between the Dickenson spec and original. TBH I keep it almost entirely in Dickenson-spec mode so going with that schematic is probably a good choice. It's also by far the clearest most readable schematic!

    Sorry I can't help you with the NOS KT66s. I did however make mine switchable between 560VDC and 500VDC. Im not sure I'd bother with that again. It sounds freak'n awesome no matter what. Its not enough of an audible difference to care about really.

    Good luck with the build! Personally I wouldn't do it for a first build, but Im sure folks in here will help you along the way!
     
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  10. playloud

    playloud Active Member

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    That's excellent to hear!

    And good-to-know re the different voltages. I assume you used the JJs at 560?
     
  11. _Steve

    _Steve Well-Known Member

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    Yep, JJs.
     
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  12. william vogel

    william vogel Well-Known Member Gold Supporting Member

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    I built a Dickenson spec 45/100 about 2 months ago and I’m loving it. It’s running 490 volts with TAD KT66. Running a Mullard I61 in V1, a Sylvania long plate in v2 and a Telefunken smooth plate in v3. It’s my second 45/100 build and these amps are a top favorite of Marshall circuits.
     
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  13. playloud

    playloud Active Member

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    That looks fantastic!

    I'm thinking I'll go a similar route in terms of parts. Pihers, Mustards and an F&T for the 32+32 dual cap in the preamp. Considering the Rifas sold by Valvestorm in the power section.

    I have some 10uf 165V Philips electrolytics for the bias section. I'm not sure how old they are, but they're definitely unused. Looks like you are using something similar?
     

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