70s AND 80s Marshall in one box

Discussion in 'Shock Brother's DIY Amps' started by JohnnyCrash, Aug 19, 2019.

  1. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

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    I just finished building a 70s JMP 2204 circuit that can switch (via a pull switch on the gain pot) into an 80s JCM800 2204 circuit.

    Contrary to popular internet misinformation, not all 2204s are the same circuit (the early JMP 2204s [but not 2203s] don’t cascade gainstages on V1 and run V2 hotter than normal [330k shared plate resistor], whereas the JCM cascades a gainstage on V1 and uses the typical 100k shared plate resistor on V2).

    All in a TINY generic aluminum chassis. I couldn’t believe how quiet it runs, given the messy wiring, the high gain, and cramped space.

    V2 is JMP circuit, which makes for insane gain in JCM800 mode. In either mode it has a kind of “chewy” kind of late compressing overdrive (if I had to explain it slightly exaggeratedly... if that makes sense). I suspect because of this stage.

    The (less) Presence/Negative Feedback control is also JMP. Which, in my opinion, makes the Bass pot more useful than the JCM800 (more) Presence/Negative Feedback allows. It still has plenty of tightly controlled bass, but not as insane of an amount as some JCM800s have (depending on speakers). Its less NFB make it a little more dynamic than JCM800s.

    One “mod”: I used a 47k slope resistor in the tonestack. I’m sure some plexis and JMPs went out with this value (a common subbed value at the factory due to supply), but it’s not stock/standard on the schematics. I like the slightly different midrange character of the 47k instead of the “stock” 33k (or JTM45’s 56k).

    Finally, the output transformer is a 35 watter. I purposely got an under-powered OT for tonal reasons: transformer core saturation at slightly lower volumes.

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    Last edited: Aug 19, 2019
  2. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

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    This is the head cab I’ll be repurposing.

    I built it from birch ply (back panel is just cheap ply) years ago for a tweed Champ type build.

    I’ll be covering it in black and red tolex with white piping.



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  3. Dacious

    Dacious Poster Extraordinaire

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    That looks supercool.
     
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  4. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

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    It nails that ‘70s Marshall sound everyone has heard on the classic tracks.

    In ‘80s mode it can do drop-D tuned heavy metal better than my other JCM800 builds. It’s not exactly a JCM800, but something hybrid (I think the different V2 circuit and lower Presence/NFB make it filthy dirty in a cool, but familiar way).
     
  5. telemnemonics

    telemnemonics Doctor of Teleocity Ad Free Member

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    I like the slightly smaller but still sorta big!
    Does it bake bread too?
    I keep thinking about a downsized 50w Marshall that still runs fixed bias el34 but weighs less and has a little louder Marshall cleans than an 18w, plus the dirt.
    This is pretty much it!
    I like the older circuit but otherwise I bet it's killer as lunchboxes go...
     
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  6. telemnemonics

    telemnemonics Doctor of Teleocity Ad Free Member

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    So how did you squeeze all those guts in there?
    Not much room for a board, wired some PTP?
     
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  7. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

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    I had room for a small eyelet fiber board, which I used, but most of the circuit is PTP as well as tag strips.
     
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  8. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

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    Speaking of lunchboxes...

    I built a 18watt “Stout” type amp in an old PA chassis for my brother.

    I called it the “Lunchbox!”

    Solid state rectification, cathode biased EL84s, and simplified controls.

    Sounds killer, looks ugly as hell.

    A4E884E6-D768-436D-A975-05EBCB80F21D.jpeg 7C5E8662-A06F-4674-ACE2-D5206B81D82D.jpeg
     
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  9. AxRookie

    AxRookie Tele-Meister

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    Can you make me one ? lol
     
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  10. Guitarteach

    Guitarteach Poster Extraordinaire

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    Me too?
     
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  11. Chunkocaster

    Chunkocaster Poster Extraordinaire

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    Awesome, i'd like one mounted in a line 6 spider cab.:)
     
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  12. Tonetele

    Tonetele Poster Extraordinaire

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    Clever!:)
     
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  13. shortcircuit

    shortcircuit Tele-Meister

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    Super sweet! Show us the guts, please.
     
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  14. Whatizitman

    Whatizitman Friend of Leo's

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    I will gladly pay you Tuesday...!!

    Seriously. When do they go into prod? :D
     
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  15. telemnemonics

    telemnemonics Doctor of Teleocity Ad Free Member

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    I was guessing based on the tube placement and limited space it had to be largely PTP.
    That takes a good amount of conceptualization, translating a classic turret board style pcb circuit into PTP!

    I guess PTP is less pretty to some but I love the look in general, even though it looks less regimented and more insane.
    I'd sure like to see some wiring pics!
    If you plan to build them though, maybe your layout is proprietary?
    Don't you have a day job now?
     
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  16. telemnemonics

    telemnemonics Doctor of Teleocity Ad Free Member

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    Just in case you’re not sure you want to post pics of wiring that ain’t pretty, here’s an amp I bought recently that was I guess the first by a Detroit area builder known as Hawkins amps.
    It was built in an old radio or stereo chassis and used the PT and tube sockets but the rest of the wiring is a new mix of PTP, turret and tag strips.

    So let’s see yours!
    It has to look better than this and I guarantee we will love it!

    IMG_0995.jpg
    IMG_0996.jpg


    Sent from my iPad using Tapatalk
     
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  17. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

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    It’s not pretty... in fact, it’s downright sloppy. It’s a trainwreck to try to rewire stuff! I had to rewire the entire bias supply already.

    The circuit was sorta accidentally built in layers. To get to lower layers I have to unsolder higher layers. I’m an archaeologist, so I thought it was fitting that there are “stratigraphic layers” to this amp :D

    Perhaps it’s the small size (shorter lead dress), or pure luck, but even with all the high gain, it’s probably the quietest amp I’ve ever built.



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  18. Paul-T

    Paul-T Tele-Meister

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    that's very cool... a Hieronymus Bosch kind of cool.
     
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  19. shortcircuit

    shortcircuit Tele-Meister

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    That's some naughtiness there, man!
     
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  20. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

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    I have red and black tolex. I’m thinking of doing something like this to the head cab (with 3/32” white piping to border the red and black strip)—except black with a red strip in the middle:


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