Micro-JMP based on rob micro-JCM : NFB question

Discussion in 'Shock Brother's DIY Amps' started by nasdak, Mar 25, 2020.

  1. nasdak

    nasdak Tele-Meister

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    Hi!
    I just finished a micro JMP : 1959 preamp and then micro-JCM PI and power section (thanks rob for your website!!!)
    It sounds really good but something's strange though.
    Presence control has little to no effect at all...
    On this amp NFB is fed from the 16ohms tap and attenuated by a 22k.
    My guess is that with 22k the amp has about no NFB at all because of the very small output voltage (only 1W so maybe 1 or 2 V RMS?)
    I will experiment with lower NFB value but i'd like to hear what you think ?

     
    tubedood likes this.
  2. nasdak

    nasdak Tele-Meister

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    well, i made some tests
    reduced the NFB resistor down to 470R !!!
    Volume has dropped, but presence has still very little effect.
     
  3. sds1

    sds1 Tele-Afflicted

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    Got a schematic? Double-check the presence control wiring. A pic of that would be cool if you have it.

    Does this amp have a post-PI master volume? If so, make sure your tests are performed with that pot on 10.
     
  4. nasdak

    nasdak Tele-Meister

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    i don't have an exact schematic but it's a 1959 preamp coupled to the 1W JCM800 by rob so it's the same for the PI and the power section (without master volume) :
    [​IMG]

    can't take a picture of my wiring, can't have my camera at the right angle but i checked it 100 times...
    but this is how i wired it (only diff is my pot is a 20k lin)
    [​IMG]

    i also had to tweak the 12AU7 cathode resistor as it was biased at 140% with a 510R !
     
    Last edited: Mar 25, 2020
  5. dan40

    dan40 Tele-Afflicted

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    I recently built his JCM 800 circuit and the presence does work normally using the 22k value. It's not an extreme difference from 1-10 on the potentiometer but I do hear the high end increase as I turn it up. Mine was also biased up pretty hot with the stock resistor so I ended up around 650 ohm with a 12bh7a output tube.

    With a 20k pot you may want to try lowering the value of the 4.7k slightly to keep the relationship of the two in the same ballpark. You could also try the old style Marshall presence circuit using the 5k pot without the resistor.
     
  6. robrob

    robrob Poster Extraordinaire Ad Free Member

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    The 22k NFB resistor was used to scale the output voltage of the JCM800 Micro. The NFB should be very similar to a full size JCM800. I don't know why your presence control isn't working.
     
  7. Bitsleftover

    Bitsleftover Tele-Meister

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    I’m just finishing off a similar project. Switchable JTM45-1959 pre into a bassman micro PI and Power section.
    My presence control is barely noticeable either. I’m coming off the 8 ohm tap with switchable 15k/No FB/33k I’m not really hearing any change in FB from any position.
    Your amp sounds great by the way.
    It’s confirmed my suspicion that I’m down on gain. I’ve got less dirt on 10 than you have on 5. Does yours get much dirtier as you crank it up to 10? How’s it sound full tilt?


    Sent from my iPhone using Tapatalk
     
  8. nasdak

    nasdak Tele-Meister

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    Thanks rob!
    how did you calculate that value ? it seems way to high to have an impact ?


    About the 12AU7 dissipation : max anode dissipation is 2.75W per anode
    on your website https://robrobinette.com/Deluxe_Micro.htm "
    Power Tube Comparison" you say for a 12AU7 that 10,5ma per triode at 363V equals to 69% max dissipation... but it's about 3,8W, that's way more than max dissipation !
     
    Last edited: Mar 26, 2020
  9. nasdak

    nasdak Tele-Meister

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    yeah i got a lot of gain.
    i played with the nfb, it becomes noticable with a 1k NFB resistor. presence control has an effect but very small
     
  10. robrob

    robrob Poster Extraordinaire Ad Free Member

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    nasdak, 2.75w per triode.

    The JCM800 uses a 100k NFB resistor tapped from the 4 ohm tap so I used a 100k from the 8 ohm for the JCM800 6V6 then cut that to 22k for the Micro.

    The NFB resistor value can definitely be tuned to your taste though and a lower value will give the presence control more authority. You definitely want to sample the amp's clean and overdrive tone when tuning NFB.
     
    Last edited: Mar 27, 2020
  11. nasdak

    nasdak Tele-Meister

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    Thanks,
    yeah i think that NFB needs to be tweaked.

    About dissipation : yes that's 2.75 per triode max for a 12AU7.
    So 10.5ma per triode at 360V is way beyond max dissipation !


    I also added a RC network on the plates of the 12AU7, same you can find on the 1W marshalls (10k and 0.002uF cap) to reduce the high frequency
     
    Last edited: Mar 28, 2020 at 9:37 AM
  12. robrob

    robrob Poster Extraordinaire Ad Free Member

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    nasdak, it's 10.5ma total plate current.
     
  13. nasdak

    nasdak Tele-Meister

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    nope!
    with a 510R cathode resistor
    says you get about 10v
    that's 20ma
    10ma per plate
    3.5w per plate
    tube meltdown!
     
  14. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Yep, I messed up when using my own online bias calculator :oops:. I re-ran the numbers on my Deluxe Micro with a 680 ohm cathode resistor for the 12AU7 power tube and she's running at 114% of max. It's hot but she hasn't blown a power tube yet and she sounds sweet.
     
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