Marshall's greatest hits

Discussion in 'Shock Brother's DIY Amps' started by shortcircuit, May 18, 2019.

  1. shortcircuit

    shortcircuit Tele-Meister

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    JMP 50 + JCM 800 + 4PDT switch + @robrob + fixed bias 1974 output = heresy. Good thing this is a Fender forum. If this was a Marshall forum, I'd be burned at the stake.

    apoplexi.jpg
     
    Last edited: May 18, 2019
  2. Whatizitman

    Whatizitman Tele-Afflicted

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    Yes, please


    Sent from my iPhone using Tapatalk
     
  3. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Looks really interesting. You've got 5 triodes using B+4 in a relatively high gain amp. I would suggest you add another power node to cut down on interaction.
     
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  4. BigDaddy23

    BigDaddy23 Tele-Meister

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    Subscribed!!
     
  5. shortcircuit

    shortcircuit Tele-Meister

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    Roger that, thanks. Recalculated the power supply with my load lines and measurements from Marshall's schematics. The 6AV6 is not going to work as the cathode follower as its Vhk is only 90V. I'll move it to the second position and use it as the cold clipper.

    Apoplexi.png
     
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  6. D'tar

    D'tar Tele-Afflicted

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  7. shortcircuit

    shortcircuit Tele-Meister

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    According to Marshall's schematic, there's 167V on the cathode of the cathode follower with a plate supply of 290V. I'm looking at about 175V on the cathode with my higher plate supply, so I'll need at least 85V of elevation at a minimum. I would like to have a 10V cushion, so I'm looking at 95V of elevation. The Vhk of the 6BQ5/EL84's is 100V. Kind of cutting it close. I think it will also be easier to wire up with the 6AV6 as the cold clipper.
     
  8. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Any elevation is better than none with a cathode follower in cuircuit so you don't have to push all the way to 95v. 70v is a happy, safe medium.
     
  9. printer2

    printer2 Poster Extraordinaire

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    Put some 1 ohm resistors in the output cathodes.
     
  10. shortcircuit

    shortcircuit Tele-Meister

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    I like to live dangerously and probe the high voltages on the plates.:D
     
  11. shortcircuit

    shortcircuit Tele-Meister

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    Gonna use a Frondelli PPIMV instead of the traditional pre phase inverter master volume. The bias voltage should only be around -14 V, so some cheap Alpha's should do nicely.
     
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  12. shortcircuit

    shortcircuit Tele-Meister

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    I have had all the parts for a couple of weeks, but I've been procrastinating. Finally got the layout drawn up with my usual pencil and paper. I fired up DIY Layout Creator hoping to make a nice looking layout, but I got bored after the first socket, so pencil and paper it is. The 4PDT switch should sit right behind the 6AV6 and the 12AX7 to the right, so the 470K resistor and the 470pF cap should connect to it directly. I forgot the 47pF cap between the plates of the phase inverter.

    Layout.jpg

    Here's the updated schematic with corrections of some typo's, addition of the PPIMV, and option for cathode/fixed bias on the power tubes. I also redid the power supply so that the screens of the power tubes are seeing less than 300V. The rest of the voltages now more resemble Marshall's 50W amps.

    Apoplexi.jpg
     
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  13. shortcircuit

    shortcircuit Tele-Meister

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    Chassis ready to go. There sure are a lot of holes in the front....

    01 Chassis.jpg
     
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  14. tele_savales

    tele_savales Tele-Meister

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    More aerodynamic that way.
     
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  15. shortcircuit

    shortcircuit Tele-Meister

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    Speed holes...
     
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  16. BigDaddy23

    BigDaddy23 Tele-Meister

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    Looking good, mate! What size/type is that chassis?

    I have an 18w Marshall on the bench at the moment. ....same transformers but turret board construction. I'm not confident/learned enough to give P2P/tagstrip construction a crack yet but I must say that your layout drawing and the schematic make it easy to follow.

    Looking forward to seeing it progress!
     
  17. shortcircuit

    shortcircuit Tele-Meister

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    12"x8", Hammond 1444-22.
     
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  18. shortcircuit

    shortcircuit Tele-Meister

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    Wired up the heaters last night. The 12A_7 family of tubes are just a pain in the butt with respect to the heater wiring. Did the first socket tonight, but I was distracted by basketball so I didn't get much done.

    02 First Stage.jpg
     
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  19. shortcircuit

    shortcircuit Tele-Meister

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    Calling it a night.

    03  Tubes 2 & 3.jpg
     
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  20. shortcircuit

    shortcircuit Tele-Meister

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    All done, overall gut shot:

    04 Overall Gut.jpg

    Detail of the preamp section:

    05 Preamp.jpg

    Power section and bias supply:

    06 Power tubes and bias.jpg

    Power supply:

    08 Power supply.jpg

    The amp:

    09 Outside.jpg

    Final schematics and voltages:

    Apoplexi.jpg

    The power output for a clean sine wave at 440Hz was 13.6W for fixed bias and 12W for cathode bias. The whole point of this amp is not a clean output though. And yeah, now I finally understand why the plexi and the JCM800 are so revered. I have a little Marshall Code 25 modeling amp with all of Marshall's amps programmed, and although it's fun to play with, I was never impressed by the sound. This amp + Les Paul + 4x10 cab = 42.
     
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