Blackvibe Micro: my first non-kit build

Discussion in 'Shock Brother's DIY Amps' started by DanceCanse, Jan 29, 2020.

  1. DanceCanse

    DanceCanse TDPRI Member

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    After having a ton of fun building a Boothill 5E3 and a Weber 6A14, I've decided to tackle Rob's Blackvibe Micro with a GZ34: https://robrobinette.com/RR763_BLACKVIBE.htm#BLACKVIBE_MICRO

    A big thank you is owed to all the guys and gals who post here, especially Rob. Though I'm sure he hears that enough! I've gotten so much out of this forum, but finished my first two builds realizing I barely took a single photo or any documentation of how the build progressed. So I'm hoping this time around my thoughts/ideas/mistakes might help another potential builder out there. I'll probably be slow to build and slow to post because ...life.

    I sourced the turret board from www.hoffmanamps.com using Rob's template, transformers from www.musicalpowersupplies.com, and almost all of my parts from www.tubesandmore.com, www.mojotone.com. I wanted to keep this as cheap as possible without major sacrifices, and I'm at about $350 total. The chassis I bought is going to be pretty tight for this build. Before I start drilling holes, I was hoping to get a few pairs of eyes on my layout. I've got the transformers crammed pretty tightly with the choke and OT 90 degrees from the PT. I hope there's no issue with the rectifier, choke, and cap can being so close together as well. I'm going to try to put the rectifier kind of in between (and behind) the PT and choke so it's easier to see when I (hopefully) show this amp off because well... tubes look cool.
    IMG_1794.jpg
    I tweaked Robs DIYLC layout to reflect these particular transformers since the wire colors are a bit non-standard. I also moved the power switch to the front, deleted an input jack, put an impedance switch in place of an external speaker jack, upped the cathode resistor value to 680ohms (I'll probably try a few values to get the bias right), and added a three-way preamp boost switch in-between the volume and treble pots giving me 270K, none, and 1M values for my vib ch load resistor to ground. I'm not sure if this will accomplish what I want without jumpering around the 220K channel mixing resistor, but I don't know how to do all that without some complex switching. Here's how Rob explains it:

    "Although the Blackvibe is a single channel amp I kept the AB763 220k channel mixing resistor because it has an effect on the phase inverter's overdrive tone and preamp gain. When testing the Blackvibe I realized it had quite a bit more preamp gain than my blackface Band-Master and blackfaced Bandmaster Reberb's normal channels. I soon realized I had underestimated the load presented to the normal channel by the vibrato channel. I added a 270k "Vibrato Channel Load" resistor to take the place of the Vibrato Channel's 220k mixing resistor and 50k Tremolo Intensity pot to ground(the Tremolo pot is also in parallel with a 100K plate load resistor to "AC ground"). The Blackvibe's 220k Mixing and 270k Vibrato Channel Load resistors form a voltage divider which attenuates the guitar signal by 45% just like in the two channel AB763. I did this to recreate the most authentic AB763 blackface normal channel tone but you can jumper around the 220k mixing resistor and delete the 270k Vibrato Channel Load resistors to add preamp gain if you prefer. Placing the Vibrato Channel Load resistor on a switch will provide a nice "Preamp Boost" when the resistor is disconnected. We can also raise the value of the Vibrato Channel Load resistor to increase preamp gain. A 1M load resistor will only attenuate guitar signal by 18% compared to the 270k at 45%." RR763_BLACKVIBE_MICRO_wjm.png
    So that's my plan. Sorry for being so long-winded. Anyone see any signs of self-sabotage in my layout?

    Cheers!
     
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  2. Mongo Park

    Mongo Park Tele-Holic

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    Iron from Matt at Musical Power Supply, nice score.
     
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  3. Mongo Park

    Mongo Park Tele-Holic

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    Mount the power transformer where you want it in the chassis. Connect up the power transformer as it should be and turn it on so you have power to it. NOTE : heat shrink any the exposed wire ends you are not using for this test.
    Connect the 16 ohm and ground from your output transformer to your Multimeter. Set your multimeter to milivolts. This will give you a reading of the magnetic field of the power transformer affecting the output transformer. You can move the output transformer around or rotate it to find the optimal position.
    cheers Ron
     
  4. DanceCanse

    DanceCanse TDPRI Member

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    Thanks For the tip Ron!

    That test sounds a bit over my head, but I’ll look into it more.
     
  5. DanceCanse

    DanceCanse TDPRI Member

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    Here’s another option if I can fit my power cord, fuse, power switch, and lamp around all those transformer wires! Not sure if this would cause problems, but I like how it looks.

    C5E0C350-1428-42FF-9453-6A02ADC0AD5D.jpeg
     
  6. King Fan

    King Fan Friend of Leo's

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    Look up the 'headphone test' for orienting OT since the PT is basically fixed...


    Sent from my iPad using Tapatalk
     
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  7. Mongo Park

    Mongo Park Tele-Holic

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    Yes what I described is much the same as the headphone method only way more useful.
    Cheers Ron
     
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  8. DanceCanse

    DanceCanse TDPRI Member

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    Thanks @Mongo Park and @King Fan. After a bit of searching I realize I was way over-complicating Mongo’s suggestion in my head when I first read it.

    Am I correct that for this test I ONLY hook up the two PT primary wires to power and safely secure the rest? Then I ONLY hook up two OT wires (ground and 16 ohm in my case) to either my meter or some headphones and safely secure the rest? Sounds easy enough.
     
  9. King Fan

    King Fan Friend of Leo's

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  10. robrob

    robrob Poster Extraordinaire Ad Free Member

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    The power transformer usually puts out the strongest magnetic field and the field usually comes out the bell ends of the transformer so I would suggest rotating your power transformer either 45 degrees (the other two transformers can then be square) or 180 degrees (and have the choke at 45 degrees as in the pic).
     
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  11. Mongo Park

    Mongo Park Tele-Holic

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    Yes that is correct, you got this give it a try. Keep in mind what Rob has given you to narrow it down.
    Cheers Ron.
     
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  12. DanceCanse

    DanceCanse TDPRI Member

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    Ok, so I'll settle the debate between headphones and multi-meter for this test: use whichever one you prefer, or try both! I found that the angle they were at made a tiny bit of difference, and distance between them made a huge difference. Went with Rob's second suggestion, but put the OT between the phase inverter and the power tube. I think I'm about ready to drill some holes! IMG_1812.jpg
     
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