100th Post Spectacular! RR763 Blackvibe Micro Build

Discussion in 'Shock Brother's DIY Amps' started by 72_Custom, May 8, 2021.

  1. 72_Custom

    72_Custom Tele-Meister Silver Supporter

    Aug 11, 2007
    I recently finished putting together what is now my 4th amplifier build, a RobRob Blackvibe Micro head with a solid state rectifier. I used a Hammond Lunchbox chassis. I wanted to keep the control panel layout the “normal” way (i.e. inputs on the left), so my layout was mirrored. This caused me more grief than I care to admit. The amp was built for a friend of mine who didn’t want the Mid/Raw pot, so the Mid control is just a 10K pot and the raw control is on a push-pull pot . He also wanted switchable NFB (another push-pull pot) and a 4/8 Ohm selector. He was trying to keep costs down, so he didn’t go for the custom faceplate I designed and will instead go with the label-maker approach.

    I initially wired in a presence control, but I felt the effect was minimal, so I replaced it with a Master Volume instead. In fact, several things that I thought would have a more drastic effect had relatively minor tonal impacts. NFB was one. The other was the B+ voltage. Rob’s schematic shows 365V at B+1 (with a 240V PT secondary output). I was using a 237V PT and had only 319V. As a test, I clipped in another larger transformer and, with a Variac, brought the voltage up to 390V (what Rob calls for on the tube rectified version). With the Variac, I could quickly switch between the two and was surprised at how subtle the difference was. Maybe it’s just because of the low wattage of the amp. The Raw switch, on the other hand, has a HUGE impact.

    I wasn’t over the moon about the sound right out of the gate, but I quickly learned that speaker selection was critical. The best pairing was with a Weber ceramic Signature 8. It also took a moment to learn how to properly set the EQ with this guy. I generally like a lot of bass in my amps, but with this amp, I had to keep the bass below 6 or so or else it would get flubby. After living with it for a few days, I definitely changed my tune.

    Overall, a very cool project and a great learning experience. It was the first time I punched out a chassis myself. I’d be very curious to try the newer EF80 pentode versions of these 1-Watt micro amps Rob has put together.

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