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JTM45 build

Discussion in 'Shock Brother's DIY Amps' started by Tom Kamphuys, Oct 10, 2018.

  1. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    I've been reading and researching for a long time now. First I looked into building a JCM800, but thought I didn't have enough time to actually built it. After that I thought I might build a 18W clone. But now I finally bought myself a JTM45 clone kit!

    I would like to alter a few things and as advised by robrob's fantastic website I'm planning to post the schematics here hoping I'll get some help from you guys and girls.

    In the mean time I've build a amp cradle. Below is the image as a teaser for what's to come!

    cradle.jpg
     
  2. coldengray

    coldengray Tele-Meister

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    Subscribed. JTM45 is my favorite amp.
     
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  3. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    I've got the first iteration of the altered schematic ready. It might look like a lot has changed, but it boils down to:

    - Limit noise: 'preferred input network' from Blencowe.
    - Split inputs to prepare for 'one wire mod'.
    - Lower most capacitors to tame the bass. Cutoff frequencies around low E (82 Hz). Should reduce blocking distortion also (Blencowe).
    - Arc protection for Cathode Follower (Blencowe).
    - Generous use of grid stoppers (Blencowe).

    SchematicNew.png

    Red dots mark a crossing, not a connection.
    Feedback phase might be wrong. Will follow manual for that.

    Looking forward to your (positive :)) feedback!
     
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  4. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    And now the turretboard layout:

    TurretBoardWithComponentsNew.png

    - Grid stopper moved to tubes.
    - Bias circuit still a bit rough...
     
  5. coldengray

    coldengray Tele-Meister

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    Lowering the PI caps is going to take away some of the soul of the amp, IMO. You might as well just use a lead spec board.
     
  6. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    Can you give any insights into the cause of that? Cutoff frequency is still well below the low E. Bias excursion is milliseconds in both cases.

    Lead spec... Can you please explain?
     
  7. coldengray

    coldengray Tele-Meister

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    Do some research between lead and bass spec Marshall amps. A JTM45 is bass spec, whereas a JMP50 was available in lead and bass spec. They way you have that board setup is essentially lead spec. Look at the Metro 50W kit instructions on the Valvestorm site, it shows the differences.

    http://valvestorm.com/sites/default/files/50_WATT_KIT_INSTRUCTIONS.pdf
     
  8. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    Here is the result of my homework assignment ;)

    Input: Yes, input looks a bit more like the Lead spec. My cathode bypass capacitance can be easily increased by putting another capacitor in parallel.

    2nd gain stage: Here the Lead spec has a .68uF cathode bypass capacitor. This will increase the gain for the higher frequencies. I follow Bass spec here.

    Tone stack: Here the Lead spec actually some more bass and middle o_O. I follow the Bass spec here.

    (output) Coupling caps PI: I have even smaller capacitors here than the Lead spec. Cutoff frequency is however lower than the Low E for all versions. The same holds for the other coupling caps I made smaller.

    As for what you call soul: I can think of two ways how the lower caps can alter the 'soul' even though the cutoff frequencies are lower than the guitar frequency spectrum.
    1) Lower frequencies (than the Low E) could arise from intermodulation distortion and these can 'suffer' from the lower cap values.
    2) Bias excursion and recovery (Orig: 7, 29ms vs. Mine: 1, 3ms) also differ. This change is intentional however; it limits the chance of blocking distortion. I follow the advice of Blencowe on this one (and several others...). I admit, he focusses on a scientific approach and questions some common (scientifically unfounded) knowledge amongst guitarists. I guess I'm that kind a guy too, however I'm impressed at what a good set of ears can hear (e.g. Rob Chapman in the blindfold challenges).
     
    Last edited: Oct 14, 2018
  9. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    I made LTSPICE models of the JTM45: one with the original cap values, the other with the new cap values.

    BodePlot.png
     
  10. coldengray

    coldengray Tele-Meister

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    To me the cathode bypass cap of 330/220uf is key to the vintage tone. But if you’re going for less bass that’s the way to do it.

    I would leave the first two coupling caps as .022uf and follow the lead spec PI setup.
     
    Last edited: Oct 18, 2018
  11. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    Ok, I'm lost...

    Below is a Bode plot of the complete amp for the original (old, normal channel) schematic, Ch. I (Normal) and Ch2. (Bright) as calculated by LTSPICE of my adjusted schematic. The smaller window on top of it is just the input stage (with formula from Radiotron Designers Handbook Rev. 4, p. 484).

    BodePlot+cathodeBypass.png

    Why is the 'new Ch. II' actually above all others just above 100Hz, before diving down as the frequency goes down?

    Can anyone explain this?
     
  12. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    An update...

    First, the (bottom) graph in the previous post is wrong. I had a DC power supply with filter resistors, but no capacitors...

    Second, I've been changing a lot of parts and simulating the result. Based on the advise here and parts availability I ended up quite close to the original now, apart from the input.
    I've made a layout in diylc. It was a lot of work and I (of course...) found a chance to change things.
    I tried to implement a multiple star grounding scheme. I think it looks very similar to Larry's Grounding Scheme (http://www.tdpri.com/threads/dc-heaters-on-jtm-style.884748/#post-8690391), apart from the fact that I have only one connection from the circuit to the chassis near the input.
    I also moved a filter cap further (physically, not electronically) towards the input stages as it makes the wires to the grounding point shorter. Is there any negative side effect to this?

    Layout_ChangedFilter_28102018.png

    Legend:
    Red wires: B+, Line
    Black wires: ground
    Green wires: grids
    Yellow wires: cathodes
    Blue wires: plates
    Gray wires: other

    Black turrets: present
    Red turrets: present, but to be removed
    Blue: new, to be added


    Comments, tips, remarks, criticism; it's all welcome!

    Ps. I couldn't upload the diy file, but if anybody wants it I can send it to you.
     
    Last edited: Oct 28, 2018
  13. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    Finally did some practical work again. I mounted the chassis-mounted bias and humdinger pots. Also made a chassis connection dedicated for the line ground (if that's how it's called).
    Besides that I think I have doubled my soldering experience soldering the heater wire. Getting two wires through the pre amp tube socket lugs was no easy task.

    IMG_20181104_213926685.jpg

    Given the nearly non-existing experience I'm pretty happy how it turned out.
     
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  14. gusfinley

    gusfinley Tele-Afflicted

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    Where dud you get your chassis? I'm contemplating a 50W Marshall-style build but I want one of those 4 Octal sized holes instead of the more prevalent ones with the 3 Octal size holes.
     
  15. Outlaws

    Outlaws Tele-Meister

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    Valvestorm sells them.
     
  16. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    I got the kit from ToneFactory (NL), but its very similar to MableAudio (China), Jukebox Revival (NL) and BYOC (USA?)
     
  17. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    Time for my weekly update:

    Slowly, but steadily, worked on the amp this week. I've mounted the face plates together with all the dials, switches and jacks. Besides that, I made a dummy load for future testing of the amp
    I've also been soldering mostly around the power transformer and output transformer. I've intentionally left out the standby switch as there is no need for it. I had already ordered a new 500V cap for the supplied 475V cap to withstand the full voltage when the valves are not conducting yet.

    IMG_20181111_215704943.jpg
     
  18. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    I've been waiting for some parts in the mean time. They have arrived.

    I mounted the components (minus 1x 100nF cap, 1x 22nF cap and some jumpers) to the turretboard. Before I start soldering I would like to ask you guys if you spot any errors. I've seen some impressive examples lately on this forum!

    IMG_20181124_132320340.jpg IMG_20181124_132313222.jpg IMG_20181124_132256295.jpg
     

    Attached Files:

    Last edited: Nov 24, 2018
  19. harleytech

    harleytech TDPRI Member

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    I built one from Valvestorm and I just built it to JTM45 specs and it sounds fantastic,glorious... My opinion would be to build it to spec, then if you don't like some things you can change things as you like. I think you will like it stock as I have... JMHO...
     
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  20. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Just catching up on this thread. Nice work.

    Agree build stock, then mod if you want. Makes troubleshooting way easier, and then you know what a real one sounded like and what to change — or leave alone.

    OTOH things like moving the filter cap to be nearer its work is elegant and often recommended. Not an electrical mod, as you note.

    As a detail, IIRC you should be able to Export to PNG in DIYLC, and pngs nicely upload here.

    Edit: gotta love the orange caps in any amp built in the NL. :)
     
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