Experimental DC-DC Deluxe Micro

Discussion in 'Shock Brother's DIY Amps' started by drneilmb, Dec 4, 2019 at 2:29 PM.

  1. drneilmb

    drneilmb TDPRI Member

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    Hi, first post in this forum. I'm a middle-aged ham radio operator who's getting back into electric guitars and got excited to build my own tube amp. Being super-stingy, I was inspired by Rob Robinette's Deluxe Micro and its use of a very affordable output transformer. And this https://marinesnow-jp.jimdo.com/ホーム/真空管ミニギターアンプ/dc-dcモジュール使用/ presented the idea of using a $4 DC-DC converter in place of the HV power transformer. Applying some deeply ghetto engineering, I built this with about $80 in parts from AES.


    1202191540(1).jpg

    The lower left is an IEC inlet, wired up to the innards of a 19V Samsung laptop supply. To the lower right are two voltage converters set at 12.6V to feed the filaments. Why two and why series? Because (19V - 6.3V) * 2 * 300mA was too much dissipation for a single module. I thought I was getting a true buck converter, but the cheap ones on eBay are just LM317s on little heatsinks. I tried only one, but it went into thermal protection (thankfully).

    The two tubes are up on "standoffs" inspired by https://heilsound.com/amateur-radio-post/the-pine-board-project/ and everything is wired truly point-to-point on the tube sockets and on the back of the pots.

    1202191540a.jpg

    And how does it sound? I'm too much of a rookie to know since I've literally never played through any other tube amp before, but I can say that it makes lots of fun tones depending on the knob positions. It's far more responsive to picking dynamics and volume controls than any multi-effects amp model I've used. Tone changes by a lot depending on the pickups from single coils (clean up through gritty to dirt) to P90s (light dirt up through heavy crunch) to humbuckers (almost convincing metal tones with everything dimed).

    I just have two questions left:

    1. There's a lot of hum and maybe some self-oscillation leading to a slight modulation effect. Is there something obviously wrong with my wiring (besides, like, everything)? Should I make a top for it and see if it helps? Shield the wood with foil? I had some confusion with grounding when connecting all of the DC modules as opposed to a more traditional star ground at the reservoir caps and the chassis.

    2. What next? There's mods for this, switchable negative feedback looks interesting, also switchable cathode bypass caps. Or move on to something like the Bassman Micro or JCM800 Micro. I've got a second one of these DC-DC converters, but both of those take real $40 output transformers and $45 of tubes. I have ideas for how to make a more normal wooden "chassis" with tube sockets going up through holes in the wood and a screw-on front and rear panel so I can follow better layout conventions.

    Thanks to @robrob for the design and the rest of his wonderful resources, and thanks for any comments that the rest of you mad scientists can offer.

    -Neil N0FN
     
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  2. FenderLover

    FenderLover Friend of Leo's

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    Maybe you should go through it a little more thoroughly and separate signal wiring frome everything else, stay away from the transformer, dress leads around the outside of the noisy bits, twist the heaters and route them away from DC nd signal wires. It's a cool project, but invites some trouble by mixing a switching circuit with audio circuits. Physical separation is your friend. A cover will shield outside signals away, and that is likely not your problem, the noise is likely from inside the box. Stick with it and tell us about any headway you may gain.
     
  3. D'tar

    D'tar Friend of Leo's

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    Cool, welcome, glad you finally made it through moderation!!!:)
     
  4. drneilmb

    drneilmb TDPRI Member

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    In a surprise development, I found the source of the oscillation/modulation. I was playing around putting some metal over the top to reduce hum and it was gone. Turns out that the front panel and back panel were floating with respect to each other. Laying a metal ruler from front to back makes it go away.

    Listen for the warble before and after the ruler is laid on.



    I think that I'll make a plywood top with some copper tape running from front to back.
     
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  5. FenderLover

    FenderLover Friend of Leo's

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    Cool!
    It might be nice to try a wire first if all you need is a ground. That way you can show off the gut and people can see how the sausage is made.

    Looking at the links you posted, it looks like everything in close proximity is not too much of an issue.
     
  6. drneilmb

    drneilmb TDPRI Member

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    I was probing around on the high voltage power supply voltage with the oscilloscope and one of the switchers must run at 72 kHz with about 300 mV of ripple. There's a larger ripple of about 2V but it has a lower frequency that isn't fixed.

    1204191617.jpg

    I'll check the output of the 19V supply to see which one it is.

    -Neil N0FN
     
    robrob likes this.
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