Rob’s JCM800 vs 18 watt lite Iib

Discussion in 'Shock Brother's DIY Amps' started by Mongo Park, Jan 19, 2020.

  1. Mongo Park

    Mongo Park Tele-Holic

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    I have not really ventured far outside the Fender comfort zone. And thought it was time to take a walk on the wild side.
    Rob’s JCM 800 Micro, the 20 watts or so, looked like a good place to make something different but good sounding. Before I committed to hardware I wanted to ask how this compares to the 18watt lite or is it apples and oranges here. If so I will back Rob and go that way.
    any experience would be appreciated
    Cheers Ron.
     
  2. dan40

    dan40 Tele-Afflicted

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    Rob offers two version of the 800. One is a one watt micro version using a 12au7 and the other is a 15-20 watt 6v6 version. The preamps in both are a cascaded gain stage/cathode follower style just like a Marshall JCM 800.

    The 18 watt lite has a slightly different preamp architecture and gets most of it's overdrive from the el84 output section. They have to be pretty loud to sound their best in my opinion. The cascaded preamp in the 800 will allow for OD tones at lower volume levels thanks to the master volume.
     
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  3. Mongo Park

    Mongo Park Tele-Holic

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    Thanks Rob’s sounds like the one for me.
     
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  4. jtcnj

    jtcnj Tele-Holic

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    I'll be watching!!
    i have the 1W version on deck when I can get the time for it.
     
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  5. Mongo Park

    Mongo Park Tele-Holic

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    Looking forward to it. I am just lining up the Iron . Got a request in with Matt at Musical Power Supply, he makes great Transformers. I will collect together the other many small parts in the mean time, and considering the speaker.
    Cheers Ron.
     
  6. tubedood

    tubedood Tele-Meister

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    I would like to try that 1 watt JCM also when time allows.
    Will be watching to see how others builds go and sound!
     
  7. jtcnj

    jtcnj Tele-Holic

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    I used his
    http://rover.ebay.com/rover/1/711-5...0001&campid=5338148343&icep_item=153333319317

    I believe this would also be suitable at 6.6k primary for 6v6, but check with him.
    The Allen TO-26 lists plate to plate impedance at 7k.
    The JJ sheet lists 6v6 Push-Pull Ra-a (I think this is the spec) at 10k.
    The Tung-Sol sheet I have lists push pull plate to plate as 10k@ 250v, 8k @285v.

    The impedance appears to drop as plate voltage increases; so the 393v on Rob's schematic probably pushes the predicted number down, where 6.6k is fine. Again, the Allen is 7k.
    I believe being in the ballpark is sufficient.

    I went with 8.5k primary for my last 6v6 build - 6G2- using the Hammond EDB1760E

    Both work fine and sound great; I couldn't tell you if the different primary impedance makes any noticeable difference while playing.

    This has been my experience, as I'm no expert.
     
    Last edited: Jan 21, 2020
  8. printer2

    printer2 Poster Extraordinaire

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    The Lite just has a volume and tone control, the 18 Watt TMB has a regular tone stack.

    [​IMG]
     
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