MC10 update

Discussion in 'Shock Brother's DIY Amps' started by andrewRneumann, Jun 25, 2020.

  1. andrewRneumann

    andrewRneumann Tele-Meister

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    Yeah, I should have made one long build thread. I’m new to designing and building amps, and I’m new to forums.

    Anyway, here’s where I’m at. I built a bank of potentiometers that clamps to the bottom of the amp (top in the image) for the testing and adjusting phase. I just labeled everything with a sharpie for now. I plan to run all the stages at near neutral bias, work on the clean tone, then start warming up and cooling them down until it sounds about right. Also have pots for interstate attenuation and negative feedback adjustment. This will be my first chance to hear, in the moment, what changes to the circuit do, so I’m really looking forward to learning a lot.

    DA2EACB6-1AC9-4158-973E-191BDCBFE936.jpeg

    Here’s the current gut shot. The power supply is done, tested, and working. Heater wiring is done and tested. A paper copy of the board design from DIYLC is helping me figure out any tweaks that need to be made. One issue is that I want to be able to remove the OT without disturbing the board, so there will have to be a hole in the middle to access a screw head. Having the paper tiger in there helps make sure it all works. (I left the leads on the PT long for now until I’m confident in the design and then I will trim them.)

    2FB31BB2-673C-4F28-BE7B-189D87D8CE01.jpeg

    Seeing it all come together is really encouraging for me. I’m already seeing things that I could have done better. Like maybe just mount the reservoir capacitors right on the end of the board instead of separately. That’s the fun of starting from scratch!

    Be good everyone,
     
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  2. NSB_Chris

    NSB_Chris Tele-Meister

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    Can you post a schematic of your entire circuit creation.
     
  3. andrewRneumann

    andrewRneumann Tele-Meister

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    I’m kind of a scatter-brain, so my schematic is what many people would call... untidy. I need to clean it up and will post soon.

    Big picture, it’s a 3+1 group of 12AX7’s driving a single-ended pair of 6V6’s in parallel. My goals with this project is to learn how to start with a clean sheet, pick a topology, design a working circuit, specify parts, build, troubleshoot, voice, and lock in a design.
     
  4. Preacher

    Preacher Friend of Leo's

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    That is a lot of knobs! I love it!
     
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  5. andrewRneumann

    andrewRneumann Tele-Meister

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    n x (Knobs, buttons, indicators) = qty. dopamine

    06443909-15B8-43BC-96FF-B81C5E9C7D76.jpeg
     
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  6. dan40

    dan40 Tele-Afflicted

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    I like your idea of using the potentiometer for testing and often use one or two myself when testing a new amp. Be very careful with your lead dress though. With so many extra potentiometers in the circuit, there will be many extra wires running to and from which greatly increases your chance of oscillation and odd noises from the amp. You will need to be very careful in how you route your wires to and from the pots to avoid noise problems.
     
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  7. andrewRneumann

    andrewRneumann Tele-Meister

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    Yeah, I’m not sure exactly what sound I will get when this is turned on... but to your point, I’m seeing 2 issues that need to be dealt with.

    1. Loss of shielding from chassis.
    2. Stray capacitance from wires running too close to each other.

    I’m considering an additional “hurdle” or guide plate that will keep the wires separated to and from the pots. Or I could use shielded wire for all the leads? That’s probably overkill.
     
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  8. tubegeek

    tubegeek Tele-Afflicted

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    Shielded wire for some/all the pot leads may well be necessary. If you are adjusting cathode R's you should be able to get away with those unshielded.

    Twisted pairs might also be good enough.
     
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  9. andrewRneumann

    andrewRneumann Tele-Meister

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    Thank you! I didn’t think about twisting. What do you think about the negative feedback test pot (it’s a 100K pot in series with a 12K resistor for test)? Since it connects to the driver cathode, probably doesn’t need shielding either?
     
  10. tubegeek

    tubegeek Tele-Afflicted

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    Agreed. Think high impedance vs low impedance. Also think coupling between input and output. I think the cathode factor (low impedance) outweighs the coupling factor - especially because it is coupled as NEGATIVE feedback, the better the coupling (on purpose or by accident) the less the gain & the more the stability.

    The only worry on the FB loop is if you somehow get an unwanted 180° phase shift at some unexpected frequency. That flips the neg fb into pos fb - uh oh.

    You'll know if that happens, unless it happens at supersonic freqs in which case you'll have weird problems that don't make sense - crazy bias shifts, mucho hum, like that. If you get anything like that switch to tack-soldering shorter connections instead of pots, one stage at a time.

    As I write this - it occurs to me - you may want to add the adjustment feature into one stage at a time to simplify the hunt if that becomes necessary.

    High impedance points will be your most likely potential trouble spots.
     
    Last edited: Jun 27, 2020
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  11. andrewRneumann

    andrewRneumann Tele-Meister

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    Ha, what are the chances that I escape this project without opening the wallet for a scope? I get your point. And “the hunt” is exactly what I’m trying to learn.

    After what you said, I’m going to run shielded wire for the interstage attenuator test pots. I only have 2 of them.
     
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  12. andrewRneumann

    andrewRneumann Tele-Meister

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    Here’s the first set of leads going to the V1A cathode resistor pot. Should I try to make them as short as possible or prioritize maximum spacing?

    A86C263C-488B-402B-9CB9-0C1D3B0246B1.jpeg
     
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  13. tubegeek

    tubegeek Tele-Afflicted

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    Don't think it'll matter @ 1K-2K. Short will keep things organized and other next steps away from them though.
     
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  14. andrewRneumann

    andrewRneumann Tele-Meister

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    AF20DCC7-75C5-44D2-8919-3F48AD2B640E.jpeg
     
  15. dan40

    dan40 Tele-Afflicted

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    It looks like the way you are wiring things up will work fine. Just keep the wiring short and tidy and cross at 90 degree angles if wires must cross each other.
     
    Last edited: Jun 28, 2020
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  16. andrewRneumann

    andrewRneumann Tele-Meister

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    It’s coming along. I decided to try the twist on the high impedance leads. Still have to populate the power amp circuit and mount and wire up the OT. Maybe I will turn this on tomorrow night!

    EEE0C8EB-650A-4587-AC52-8F5239AD09CC.jpeg
     
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  17. Kevin Wolfe

    Kevin Wolfe Tele-Meister

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    Should wind up being a nice, “Phat” SE amp. Lookin’ great.
     
  18. andrewRneumann

    andrewRneumann Tele-Meister

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    The wiring is complete! (Unless it isn’t...)

    BADD22F1-07B7-4BCC-88CC-EF1B1A4104CA.jpeg
     
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  19. andrewRneumann

    andrewRneumann Tele-Meister

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    ...and I wired my pots backwards. Not the test ones. The ones underneath.
     
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  20. NSB_Chris

    NSB_Chris Tele-Meister

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    Do you have two pilot lights? Looks like there is one on the front next to the on/standby/off switch driven of the AC mains and then there is another that is connected to the heater supply in series with a resistor. Is that correct?
     
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