Build Thread - Marshall 18 Watt | The Canadian Guitar Forum

Build Thread Marshall 18 Watt

Discussion in 'Amp Building/Technical/Repair' started by Lincoln, Apr 18, 2019.

  1. Lincoln

    Lincoln Gold Member

    Joined:
    Jun 2, 2008
    Location:
    Fort Saskatchewan, Alberta
    I've started on a Marshall 18 watt TMB build. First amp I've ever built with EL84 power tubes. A new adventure with an endless supply of learning opportunities. I've never played one of these, but I've heard they are tone monsters and that's all the encouragement I need.
    Chassis & face plates from ZH Music Parts, board from Antique Electronics, transformers & misc parts from Next Gen. Pretty sure it's going to be a single 12" combo. But it could end up being 2 x 10's. You never know......

    Chassis with tube sockets and iron mounted, ready to put the turret board in place
    18watt-2.jpg
     
  2. johnnyshaka

    johnnyshaka

    Joined:
    Nov 2, 2014
    Location:
    Sherwood Park, Alberta
    *Thread watched*

    I've never played one either so I'm curious to see how this one turns out...good luck!
     
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  3. greco

    greco Gold Member

    Joined:
    Jul 15, 2007
    Location:
    Kitchener, Ontario
    Same here!

    Enjoy the build and thanks in advance for the updates and (hint, hint) lots of pics.

    Cheers

    Dave
     
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  4. Frenchy99

    Frenchy99

    Joined:
    Oct 15, 2016
    Location:
    North Pole
    Will be enjoying this thread. Good luck on the build. El84 sound amazing!
     
  5. Lincoln

    Lincoln Gold Member

    Joined:
    Jun 2, 2008
    Location:
    Fort Saskatchewan, Alberta
    Gentlemen, a few questions if I may......... :D

    18watt-3.jpg

    I've never seen diodes in line before the rectifier tube before. Is the EZ81 not a full rectifier tube like say a GZ34 or 5Y3? I've looked at as many EZ81 power supply schematics as I could google, and none of them had diodes in that position. What's up with that?

    The .01uf capacitor across the HV terminals of the rectifier tube? 3KV? Any idea what the purpose of that is?

    The 5 watt 1.5K resistor between the sections of the filter cap. I've got 3 watt, 1.5K resistors. Would that do? Hammond says that transformer is only good for 14o mA, where is the rest of the current going to come from?

    All new to me, I just want to have some sort of idea what the heck is going on and why.

    *#*(
     
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  6. jb welder

    jb welder

    Joined:
    Sep 13, 2010
    Location:
    Melonville, Canada
    The diodes are protection if the rectifier shorts. I think a few of the modern Fender amps might have them. Should the tube short, you still rectify the AC rather than allowing full AC on the caps. And no stoppage in play. ;) Having the tube in series means you still get the 'sag' effect from tube rectification. The little cap there is for noise suppression as far as I know. (diode switching noise)

    140mA is plenty to create wattage at that kind of voltage. 'I squared R' gives almost 30W if it were all going through there. However, it's not going to get all that. You would have to calculate with the voltage drop in the circuit. I have a feeling 3W will manage, but 5 is probably more reliable long term.

    edit: 64 Vibroverb Custom is one of the new fenders that has the diodes at the tube rectifier:
    https://www.thetubestore.com/lib/the...nder/Fender-64-Vibroverb-Custom-Schematic.pdf
     
    Last edited: Apr 20, 2019
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  7. Lincoln

    Lincoln Gold Member

    Joined:
    Jun 2, 2008
    Location:
    Fort Saskatchewan, Alberta
    Super. Thank you. 5 Watt resistor it will be. With a side of diodes, and a topping of ceramic cap.
     
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  8. greco

    greco Gold Member

    Joined:
    Jul 15, 2007
    Location:
    Kitchener, Ontario
    Hold the pots and LEDs. Considering a transformer for dessert.
     
  9. Lincoln

    Lincoln Gold Member

    Joined:
    Jun 2, 2008
    Location:
    Fort Saskatchewan, Alberta
    A couple updates as things progress. Both sides now
    18watt-4.jpg
    18watt-5.jpg
     
  10. reckless toboggan

    reckless toboggan

    Joined:
    Mar 9, 2019
    Location:
    CA
    Subscribed from your first post.

    This is amazing.
     
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  11. Lincoln

    Lincoln Gold Member

    Joined:
    Jun 2, 2008
    Location:
    Fort Saskatchewan, Alberta
    Here's the progress, or lack there of it. Most of the wiring around the power transformer is in place now. Those 2 diodes before the rectifier tube are in place, and the 5 watt resistor between the sections of the filter can is in place. Grounds in too.
    It's kinda nice how it all fits and how Marshall kept all the AC in the same little area rather than spread it halfway down the chassis like Fender did.
    18watt-6.jpg
    Only thing I'm not enjoying is the tiny 9 pin tube sockets. I think if I ever build another 18W, I'm going build it with a GZ34 (or solid state rec.) and 6V6 power tubes. That's how much I dislike 9 pin sockets.
     
  12. Frenchy99

    Frenchy99

    Joined:
    Oct 15, 2016
    Location:
    North Pole
    Looks good!
     
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  13. greco

    greco Gold Member

    Joined:
    Jul 15, 2007
    Location:
    Kitchener, Ontario
    Progressing well...Congrats!

    That area is total gridlock!
    Screen Shot 2019-04-24 at 10.48.13 PM.png Screen Shot 2019-04-24 at 10.47.31 PM.png
     
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  14. Lincoln

    Lincoln Gold Member

    Joined:
    Jun 2, 2008
    Location:
    Fort Saskatchewan, Alberta
    Finished my least favorite part of any amp build. The tube heater wiring. Seem here as the tightly twisted wires along the top edge of the chassis.
    18watt-7.jpg
    I ended up turning the power tube sockets around 180 degrees so the heater wires would be shorter/more direct.
     
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  15. reckless toboggan

    reckless toboggan

    Joined:
    Mar 9, 2019
    Location:
    CA
    Okay.

    Fine.

    Just stop.

    How much for you to make me one of these*?


    *must be impervious to drool.
     
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