Bimodal Micro Amp - Build #2

Discussion in 'Shock Brother's DIY Amps' started by Asmith, Mar 18, 2020.

  1. Asmith

    Asmith Friend of Leo's

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    So yesterday I got a bit impulsive, I threw an amp design together in about 4 hours and then ordered a heap load of parts. I may not have spent a massive amount of time drawing the schematic but I've been wanting to do something like this for a while. Bimodal comes from statistics (least favourite area of maths) and is a special type of distribution with 2 peaks.

    Single ECC83/12AX7 preamp:
    Hi and Lo inputs (on a switch to save space inside the chassis rather than 2 jacks).
    Hi setting is both triodes cascaded.
    Lo is a singular triode gain stage.

    Switchable tone stack:
    Deluxe Reverb (Black)
    AD15 (Orange)

    ECC83/12AX7 Paraphase inverter - Need high gain for the Lo setting.

    ECC82/12AU7 Push-Pull Power amp - micro amp 1 Watt.

    Bimodal.png

    Input is V1 on the far centre left and the PT voltage is V2 in the lower right corner (rectifier diodes are missing).

    To save space I'm mounting all the large 450V filter caps onto an octal valve base to make some sort of Cap can.

    I still need to buy an enclosure but I'm going to wait for the parts to arrive so I can take measurements and decide on the best layout.

    I have a crap track record for all of the following starting projects, finishing them and documenting them but I'm going to do my best to document this build start to finish on this thread.
     
    Last edited: Mar 18, 2020
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  2. Asmith

    Asmith Friend of Leo's

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    Parts are here so its time to plan and measure out a chassis

    20200323_112255.jpg
     
  3. Asmith

    Asmith Friend of Leo's

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    And I have 4 valves instead of 3 because I got a 12at7 to try in the phase inverter in case it gets a bit too hairy on the hi input
     
  4. Badside

    Badside Tele-Meister

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    Just 2 cents of mine, if you need gain in the PI to compensate for the low gain of the low input, I would investigate the option of a cathodyne. It works well with a 12AU7 power amp (Marshall uses it in many 1W models) because they don't draw a lot of current, you can use massive 470k grid stoppers into the power amp which fixes typical cathodyne issues. By doing so you have one triode left as a gain stage (driving the actual cathodyne) so total gain is similar to paraphase.

    Because the paraphase is all over the place when driven. It sorts of take over the character of the amp at that point. I built a Supro clone a few years ago and I didn't like it until I got rid of the paraphase inverter. That's me of course.

    If you do keep the paraphase, I suggest not using a bypass capacitor on the top triode (it's counterproductive to keep gain high since you have to dump a lot of signal before it) and rework the voltage divider so you don't have 220k of series resistance before it. Because that will lose a lot of highs, with a -3dB corner frequency around 6KHz.

    But yeah, just making chat. I know I hate it when people tell me I should build my amps differently :)
     
  5. Asmith

    Asmith Friend of Leo's

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    And of course the diy octal cap can:
    1x 150uF @ 450v
    3x 33uF @ 400v

    I need to find an appropriate sized pipe to slide it in with some sealant.

    20200323_125601.jpg
     
  6. Asmith

    Asmith Friend of Leo's

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    Hey all suggestions are welcome I am new to this Frankenstein amp thing, this is build #2. #1 was also a complete scratch build, not based on a published schematic but it was simpler (until I modded it). I also have a spare parts from other projects where I bulk bought some resistors and caps so most changes should be easy to implement.

    I've always liked the sound of supros and the paraphase inverter seems quite common on the iconic models so thats why I wanted to implement it. If I hate it I'll look into other options.

    Good spot on cathode cap, I'll definitely implement the change. I think I just put it in there because of symmetry, not a good reason haha.

    I was also toying with the idea of removing both cathode caps anyway and putting in negative feedback on the cathode of the lower triode.
     
  7. Asmith

    Asmith Friend of Leo's

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    My enclosure arrived today, its a 1590DD sized enclosure (about double the size of a EHX Big Muff). I forgot to charge my drill so I have to take a break. But ive got this done so far.

    20200324_154629.jpg
     

    Attached Files:

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  8. Asmith

    Asmith Friend of Leo's

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    Drilled the face, so I couldnt resist a mockup. From left to right:
    Input,
    Hi/Lo and Black/Orange switches,
    Gain,
    Treble,
    Bass,
    Volume,
    Power.

    20200325_134508.jpg
     
  9. Asmith

    Asmith Friend of Leo's

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    So today I managed to wire the heaters, power supply and ground bus.

    Power supply was very fiddly its done though all on the octal socket.

    20200330_161729_001.jpg

    20200330_161643.jpg

    Im just going to take a moment to praise solid core wire, I just love the stuff

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  10. Asmith

    Asmith Friend of Leo's

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    I made the changes to the paraphase inverter as suggested by @Badside inverter. Removing the bypass cap reduced the gain to ~30 and allowed a 150k, 5k6 voltage divider, both results should be enough to move that high cut out of the speakers range.

    Cap can is wired up but definitely needs a cover, theres exposed high voltage. I have a spent can of deodorant that looks perfect for the job. Just have to chop to top off and give it a clean.

    20200403_125041.jpg

    I finished up wiring up the rats nest, I only ran into a few issues.

    I forgot to order 3 22nF poly caps, I didnt have a 150k resistor to hand. Luckily I had some 400v box caps I pulled from a unloved diy pedal and I made the 150k resistor using 3 100k resistors.

    I also made a some stupid mistakes that were a pain to fix, I wired both 1/4" jacks backwards, luckily I noticed before powering up. I also miss-wired channel switch which was buried under a pile of components but at least nothing got damaged.

    Not bad since I didnt mock up any layout and just went by the schematic.

    20200403_121416.jpg

    Overall it came out well, noise seems to be a concern with these micro amps but mine came out really quiet. Probably down to the toroidal transformer and the layout used for the grounding scheme.

    It sounds good still surprisingly loud for home use, I will see about posting a sample but it will be on my phone.

    20200403_125344.jpg
     
  11. Asmith

    Asmith Friend of Leo's

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    Does anyone have any advice for the unused leads on the output transformer? I'm sure I will never use the extra taps so Im tempted to give them the snip but I think it would be wiser to twist them together and tuck them into the chassis somewhere out the way.
     
  12. Asmith

    Asmith Friend of Leo's

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    I finished a few little details, cap can is now shielded and insulated and I tucked the unused secondaries away into chassis.

    20200404_132212.jpg

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