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EICO 324 signal generator into a JCM800 pre

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  • EICO 324 signal generator into a JCM800 pre

    As the title states, i converted an Eico model 324 sign gen into a jcm800 pre. I replaced the 12au7 with a 12ax7.
    attached is the stock schematic.Click image for larger version

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    My issue is a motorboating noise when i turn it on. Key things i changed are the power supply rectification and the heater wiring.

    First of all i replaced the single selenium rectifier with a bridge rectifier. i put in various value filter caps with larger dropping resistors between the two tubes. made no difference. i thought maybe the stages were becoming decoupled....but no. it has 2x47uf @ 300Vdc caps installed with a 10K dropping resistor currently. i then started looking at the stock schematic. would swapping it to a bridge rectifier give me these issue? I was thinking about returning it to the stock single diode half wave rectifier circuit and see how that fairs. im getting 180vdc after the bridge and the schematic says it should be 110vdc....thats a huge increase....
    Secondly i rewired the heaters and installed 100ohm balance resistors for a ground reference like in a normal tube amp. The stock unit just grounds pin 9 as the ground reference....

    I put this project down a few months back as i didnt have time to tinker but i have a friend who wants to use it for some recordings soon so i wanted to get it up and running.
    any suggestions are greatly appreciated.

  • #2
    What jcm800 pre schematic are you using? The few I looked at show voltages in the 300 range for the 12ax7s. Not sure that PT has the guts to do what you want to do with it.

    Binkie McFartnuggets‏:If we really wanted to know the meaning of life we would have fed Stephen Hawking shrooms a long time ago.


    • #3
      You will need to upload the schematic of what you have built.
      "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey


      • #4
        my apologies. its a 2203 preamp. and after going through it again last night i found my tone stack wasn't grounded and my output pot was wired backwards. DOH!
        After fixing those stupid mistakes its ALIVE!!! Because my supply voltage is very low i had to make some changes to the plate/cathode resistors. I will have to check and note the chosen values tonight. I also used a 1M output pot..i am going to lower it to attenuate some of the signal as it generates alot of volume and i have to keep it near zero to match the amp volume.
        I am going to have to mod it for more gain as the way it is, its more like a vintage overdrive/rock preamp with a mushy/fuzzy bottom end. I got 40uf on each supply nodes. Smaller coupling caps in the first stages will have to be implemented. pics and vids to follow.


        • #5
          also, i replaced the stock 12au7 with a 12ax7 and i left the 12av7. I didnt realize the 12av7 is a low gain tube....will swap and report back.


          • #6
            Still not much gain. So converted to a Mesa Bottle Rocket Pre. WHOAAAA!!! loads of heavy gain. Me likey :-) its alittle nosiey now but i will reroute things to keep it quiet.


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