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DIY 18w marshall jcm800

Discussion in 'Amps [BG]' started by The_Lucek, Feb 27, 2014.

  1. The_Lucek

    The_Lucek

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    Here is the deal,
    I recently came across a 12w hi-fi amp using 2 el84 tubes. It's a mono block and had volume bass and treble. But my father decided to bypass it and also took out the mic pre it had in it. (In former Yugoslavia many amps had this feature.) And made it a good looking paper weight. So i plan to salvage all the parts i can (manly the transformers) and make a proper guitar amp. So i was planing to get the ac dc-ish tones. They used the plexis and jcm800s but that needs much bigger transformers. So i was wondering if there was a way to take the preamp of the jcm800 and the power section of the 18w model. Is that possible or am I just fantasizing? And if it is possible, what is needed in between the two?

    PS I'm looking to keep the phase tube of the jcm800, if it is possible. I know and apologize for asking too much but its a waist to let the components go to waist. And i don't have the scheme for the amp that got gutted. I tried the marshall forum but it isn't as visited as tb. I am sincerely sorry if i brake some rules, but i really need a wizard with tube amps. :help:
  2. nashvillebill

    nashvillebill

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    It certainly is possible. Basically, from the monoblock you've got, you need to keep the power supply (the power transformer, rectifier, and filter caps) and the power amp section (the phase inverter, power tubes, and output transformer). Then you add the preamp section of your choice. Oh, I see you want to use the Marshall phase inverter. It would take some investigation to see what changes would need to be made there.

    You will need to learn a lot more to make this work, here is one good resource:

    http://www.valvewizard.co.uk/

    Also go to www.ax84.com They have lots of EL84 projects.
  3. The_Lucek

    The_Lucek

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  4. nashvillebill

    nashvillebill

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    The Presence knob controls the amount of global NFB being applied, it appears. You could certainly implement the Marshall PI design but you may have to change some component values if your supply voltages are greatly different.
  5. The_Lucek

    The_Lucek

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    I was first planing to combine the schematic of the jcm800 pre and the power stage of the 18w Marshall had made back in the day.
    http://www.drtube.com/schematics/marshall/18w.gif
    I just needed conformation that it is possible, and maybe if someone could sort the thing out. I mean, I'am still in high school so my understanding of these amps is quite limited. But I'm willing to learn. :D
  6. InsanityAmps

    InsanityAmps

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    Definitely possible but if you have never worked on something like this before then you may be over your head. Even with schematics you could have a nice door stop after hooking it all up. The way the amp is wired can make all the difference between something that is dead quiet with no signal and totally rocks cranked or hums badly and oscillates when turned up. Grounding and lead dress is very important. I would suggest you start researching building tube amps and maybe join the diy audio forum or ax84.
  7. The_Lucek

    The_Lucek

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    Thank you for the info. I've already started to go trough the valvewizard. Really nice stuff. But something caught my attention in your post. You mentioned that the amp could oscillate when turned way up. Well I already have an amp with this problem. It's three ecf80 in the pre, 2 el84 in the power section and an ez81 rectifier. And probably that doesn't help you, but what is the problem in most cases? Some sort of feedback?
  8. Chainsaw Willie

    Chainsaw Willie

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    It has been a long time since I visited the site, but 18watt.com was a forum with people building all kinds of variants of the 18-watt out of all kinds of stuff. A sister site full of good diy tubey info was sewatt.com, mostly about single ended stuff, but still a great resource for DIY and learning.

    If you have a good feeling for what you are doing, you can marry any preamp to any phase inverter to any powersection. My last build was a 6G3 Brown Deluxe pre with a 5E3 tweed deluxe power, built on the chassis and transformers of an old dead Magnatone.
  9. InsanityAmps

    InsanityAmps

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    Ecf80 huh? That's a funky tube. Usually oscillations are caused by the wiring to the inputs of preamp tubes. On a high gain amp you want to keep connections to the inputs as short and direct as possible using shielded wire. And try to keep input wires away from output wires. A series resistor to the input can tame some oscillations but the larger you go, the more high frequency content you will start losing due to Miller capacitance of the input grid. Start with 100k and work up from there. If there is just too much gain in general then no amount of series resistance will help and you need some sort of voltage divider to cut the signal down a bit.

    Feedback can also be caused mechanically. This is easily checked by flicking or grabbing the tube with your fingers while it is oscillating. If the oscillation stops when you grab the tube then you have a microphonic one and it should be replaced.
  10. The_Lucek

    The_Lucek

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    I would never thought of that. So the fact that the inputs are right next to the output transformer and speaker connections isn't that great. I'll try to open her up and see what is really going on. Thanks for the advice. Really appreciate it. :beer: :beer: :beer:
  11. Vince Klortho

    Vince Klortho

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  12. The_Lucek

    The_Lucek

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    Bought links are clickable and very appreciated. The 18w forum has very helpful stuff, but is a bit less visited than tb. Nevertheless, thank you. :)

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