JTM50 build finished

Discussion in 'Building the Classics' started by AndyD, Dec 8, 2021.

  1. AndyD

    AndyD Well-Known Member

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    Some of you may have seen my original thread which started as a Plexi bass build, which then evolved into a JTM50. The build is now complete, but not tested.
    From the start I have tried to use as many old parts as possible and tried to capture the vibe of an original build, but incorporating some more up to date features such as the "Larry" grounding scheme. Some of the wiring routes I have altered if i felt a different way was more efficient. So please don't treat this as a vintage clone.
    I must give a huge thanks to @StuC for providing me with a first rate chassis, circuit board and speaker output selector and @neikeel for supplying me with the Merren output transformer, Marstran power transformer, couple of resistors and the vintage power selector.....plus some very much appreciated layout information. If it weren't for the generosity and knowledge shared on this forum, there would be no JTM50 build. Its been a great project. I now have to go through everything again to check for errors and connections before firing it up...... (btw, front and rear panels on the way.)
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  2. South Park

    South Park Well-Known Member

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    Nice job I have one on my build list . If you get a variac you can power things up on low voltage and test everything as you build any low voltage AC power supply will work to . So the next build going to be
     
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  3. AndyD

    AndyD Well-Known Member

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    Thank you very much! No plans for another build.
     
  4. pedecamp

    pedecamp Well-Known Member

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    Wow! A very professional looking build you are talented! :yesway:
     
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  5. Marcomel79

    Marcomel79 Well-Known Member

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    Looks beautiful! Would love to hear how it sounds
     
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  6. lespaul339

    lespaul339 Well-Known Member

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    Looks great. Nice work!
     
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  7. adew1

    adew1 Active Member

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    Great job!
     
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  8. Ivan H

    Ivan H Active Member

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    Great job, really nice build. I have one that could be it's twin, the only difference being I didn't use an AC voltage selector. Cheers
     
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  9. neikeel

    neikeel Well-Known Member

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    Excellent work. Have you fired her up yet?
    Are you modding the other selector label to be able to use for impedance?
    Will be interesting to hear how it performs vs the lead original.
     
  10. AndyD

    AndyD Well-Known Member

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    Thank you for your kind words, Neil. I haven't fired her up just yet. Its been a while since I've built an amp from scratch so i'm checking everything over very carefully. I might get a chance tomorrow, we will see! I haven't modded the other selector label yet, either. Don't want to ruin it!
     
  11. AndyD

    AndyD Well-Known Member

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    Finally had the opportunity to fire the amp today after reminding myself how to it (its been a while since I have built an amp from scratch. Everything went smoothly, thankfully. All the voltages were spot on and no fires! The rectifier valve is a NOS Blackburn made valve which I bought recently. This give 405V on the plates of the output valves. I tried a pair of recently purchased RFTs but they were not very well matched. Threw in a pair of Marshall branded EL34s that I had in stock. Mazda in V1, Sovtek LPS V2 and a balanced LPS in V3. Housed in a cab that I made a little while ago with original old handle and the logo from Denmark St. It sounds fabulous. Plenty of headroom and has a smooth transition into overdriven tones. Very usable eq and just "feels" lovely. Only spent about a half hour on it so early days. I have thoroughly enjoyed the build....though it is good to reach a conclusion so that I can stop obsessing!
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    Still need a back panel!
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  12. Pete Farrington

    Pete Farrington Well-Known Member

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    Excellent, well done! With the amp and the cab :)
    When you’re next in there, what’s the VAC from the HT winding, between pins 4&6 of the rectifier?
     
  13. AndyD

    AndyD Well-Known Member

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    Thank you, Pete. Btw, you’ll be pleased to hear that I used the bulb limiter when firing up! The only reading I took on pins 4 & 6 were when I had the bulb in series and that was about 315v each side. These are obviously going to be low because of the bulb. I promise to take a reading the next time I’m inside it!
     
  14. Spanngitter

    Spanngitter Active Member

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    Nice build...and pls ensure that your Bias Supply branches off the Standby on the HOT (not the Switched) side!
     
  15. neikeel

    neikeel Well-Known Member

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    I presume that you used the lower voltage pairs of HT? The high pair would normally give about 420v.

    @Spanngitter - the bias is hard wired off the PT secondary (white wire).

    oh and ps - good job on the cab too!!
     
    Last edited: Dec 16, 2021
  16. AndyD

    AndyD Well-Known Member

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    Hi Neil, I have definitely come off the high HT taps C and G. I guessed that 430v would be achieved with a solid state rectifier. I wasn’t too surprised to find I was getting 405v with the valve rectifier. You think that’s a little low?
     
  17. AndyD

    AndyD Well-Known Member

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    Just realised that I have added a couple of diodes as above. Could these contribute to the voltage drop?
     
  18. Pete Farrington

    Pete Farrington Well-Known Member

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    Only to a negligible degree.
     
  19. AndyD

    AndyD Well-Known Member

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    @neikeel I did a bit more digging into HT values. Found a couple of Uncle Doug videos in which he is working on two JMPs, one valve rectified and the other solid state. The valve rectified amp had 385V on the plates and the other 405V. Also, on the layout you posted there is a note next to pin 3 of the output sockets that says "390v - 430v range". Also, I wonder if I might be on the wrong primary tap. At the moment I have hooked up to the yellow wire (240V). Think my wall voltage is around 228V (i will double check that), so maybe change to the orange wire (230V)?
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    Please chime in if you think differently!
     
    Last edited: Dec 17, 2021
  20. Go Loud Early

    Go Loud Early Member

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    Looks great!
     

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