AB763 Micro Reverb amp builded! (mostly)

Discussion in 'Shock Brother's DIY Amps' started by alathIN, Jun 29, 2020.

  1. alathIN

    alathIN Tele-Holic

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    I'm not sure if any of my projects are ever really "done" and I don't have a cabinet yet, but my 2 Channel Micro Reverb is done to the point of being a very playable and nice sounding amp.

    This project started from Rob Robinette's 1-watt micro RR763 - then added a second channel, and an EL84 driven reverb mostly stolen from the Vibro King.
    All the tubes are Soviet era military tubes because I had a bunch lying around. And I've decided now I quite like them. They go smoothly into overdrive and produce pleasant musical-sounding distortion (to my ear at least).

    20200629_213956.jpg


    Schematic and Layouts way down at the bottom.

    Observations

    1) Channel 1 does sound quite Blackface-ish with the raw/mid knob all the way down and the push-pull bias switch on the "clean" position.

    2) Turn up the Channel 1 raw/mid knob and it starts getting tweed-y, louder, and a lot more breakup. This gets interesting tones up to about halfway and then becomes too much of whatever it's doing.

    3) Put the Channel 1 push-pull bias on the cold bias position and it starts to sound like a high gain Marshall. This is when you really need the "tight" switch because otherwise the bottom end starts to get too thick.

    4) Channel 2 is mildly disappointing. I love the Pentode channel on my Trinity TC-15 and this is basically a copy, but it's a lot more chimey and magical paired with the Vox-ish output stage. It does do some nice high gain sounds but the clean sounds are kind of "meh."

    5) The reverb is really nice. It can go very lush or very subtle. I usually struggle to make reverb work with high gain sounds but for whatever reason this reverb plays nice dirty or clean.

    6) I did all the "voicing" and tone mods with guitars I built - which all have single coil pickups and all of them more toward the vintage-y end of the spectrum than high output SCs. I get some nice rich clean sounds and some really robust high gain sounds that would almost make you doubt you're playing a single coil guitar. All three Strat pickups sound just lovely with this thing, from clean to max saturation.
    My one guitar with humbuckers is an Ibanez ES335 knockoff with Duncan Seth Lover pickups. This amp and the Seth Lovers do not combine for any clean tones at all. It starts at "crunch" and goes up to face-melting distortion and it's too fat and thick everywhere.
    I'm kind of a single coil guy anyway, so I don't particularly mind this.
    But if anyone who likes humbuckers were to try to build something like this you will want to voice it very differently.

    7) The "tight" switch is super useful to prevent high gain sounds from getting too thick/muddy.

    8) I am playing this through a 2x12 cabinet right now because that's all I have - it's got a Weber 12F150 which is a very Blackface-y sounding speaker and a Celestion G12H30.
    If I could do anything with a speaker change for this amp, I would want to tighten up the low end. It has a bit of a tendency to go "boom-y" - not bad, but if I could tweak something with a speaker change that would be it.
    My idea is to make it a 2x10 cabinet - and ask CJ at Weber what to speakers to put in to keep the "boom" tamed. I like punchy low end, but it has to be tight/precise.

    9) Other than the different heater wiring, I really have had no problems with the Soviet-era tubes. They sound really good to my ear.

    10) I originally designed this with a Merlin-style ground lift to allow wet-dry or stereo rigs without ground loops. For whatever reason the ground lift was a noisemaker on this amp so I just ran a conductor to bypass it. The ground lift worked like a charm on my ReVibe; no noise, no problem. And I have no idea what made the difference.

    11) The only real "bug" left is that there is hideous noise when I plug in the reverb footswitch. It's not the reverb circuit per se - I can get really nice reverb, or use the dwell and volume to turn the reverb off. But something about the dang footswitch just does not want to play nice. Again, odd, since it's the exact same foot switch circuit (and same actual foot switch) that works fine in my ReVibe.

    So still to make the cabinet. And there is a pointless entertainment feature I don't have quite sorted out yet. So I will probably post an update down the road.

    Overall a satisfying project and a very useful amp for playing at home volume levels.

    I learned a lot about all the tweaks you can do to change the voicing - coupling caps, tone caps, mixing resistors, to name a few.
    Also drawing load lines and fooling around with hotter vs colder bias. In the end I found a clean bias I really liked and a cold clipping bias that I really liked also - I wasn't willing to give up either one, so that is what inspired the push-pull bias switch.
    My other projects have all been very thoroughly sorted out by others before I built them - so not a lot of "I wonder what value this cap should be?" - pretty much build by numbers and start playing.
    The next time I build an amp and want to tweak the sound a little, I'll have a much better idea what to do.
    MicroAB763 schematic final.jpg MicroAB763 layout final.jpg
     
    Last edited: Jun 30, 2020
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  2. alathIN

    alathIN Tele-Holic

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    Stuff I'd do differently:

    1) skip the ground lift and put the output filter cap and dropping resistor in the doghouse. This was a later add-on - and there was plenty of room where I put it - but it would agree with my sense of orderliness to have it in the doghouse.

    2) This is a repurposed chassis and had the two jacks in the middle of everything, which made some unnecessarily long runs. I used shielded cable and it's really a very decently quiet amp, but I wouldn't repeat this arrangement if I was making a new chassis from scratch.

    3) similar thoughts about how the reverb and output transformers are arranged

    4) I would probably omit the pentode preamp channel, much as it pains me to say it. The mid/raw control and the push-pull bias give Channel 1 a lot of versatility, and it does just about everything I'd want from this amp. Channel 2 might get lonely.
     
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  3. andrewRneumann

    andrewRneumann Tele-Meister

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    Looks gorgeous. Nice work.

    What are the dimensions of that chassis?
     
  4. alathIN

    alathIN Tele-Holic

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    Thank you!
    The cabinet is 18" x 7" x 2 3/4"

    For the first chassis I made, I copied the dimensions from my TC-15 and I've never had a reason to change it. My idea always starts out that I'm going to build something simple and have a nice spacious super clean build. Then I start adding stuff.
     
  5. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Very cool amp design and build, congrats on getting it built and sorted.

    "It has a bit of a tendency to go "boom-y" - not bad, but if I could tweak something with a speaker change that would be it."

    Try dropping your phase inverter input cap from .01 to .001uF or even 500pF.
     
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  6. alathIN

    alathIN Tele-Holic

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    Here's the arrangement I have now:

    tight switch.JPG


    Does the PI input cap act like a series capacitor with whatever is going on in the tight switch before it?

    If so my total capacitance is currently 979pF in the "normal" position and 769pF in the "tight" position.
    I'm wondering if that calculation is wrong because it's a very noticeable difference between "tight" and "normal" and I'm not convinced I could hear the difference between 979 and 769 pF.

    Assuming the series cap calculation works:
    I have a 470pf on hand I could use for the PI input cap.
    Then I'd have 448 in the "normal" position and 411pf "tight." That feels like everything would be too tight and not much difference between the two switch positions.

    I could also replace the first coupling cap with .005uf.
    Then I'd get 715pF "tight" and 833 "normal."
    If my calculations and guesswork are correct, this would make both "tight" and "normal" just a slight bit tighter... which feels like the right answer... but maybe not enough difference between tight and normal to be useful?

    I'm tempted to try this:
    tight switch III.JPG

    This calculates to 535pF "tight" and 833pF "normal." So, both tighter and a bigger difference between the two.

    This is probably something that my ear will tell me better than my calculator.
    I'll give it a whirl and report back...
     
  7. mrriggs

    mrriggs TDPRI Member

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  8. robrob

    robrob Poster Extraordinaire Ad Free Member

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    The 220k mixing resistor helps prevent "series caps". Did you change the schematic or did I just misread it on the .001uF PI cap? Try a 500pF there. Fender used that size to cut some bottom end from AB763 amp heads that were designed to run closed back cabs. It will also tighten up the overdrive tone.
     
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  9. alathIN

    alathIN Tele-Holic

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    Didn't change the schematic - it's been .001 since I built it.
    I̶ ̶d̶o̶n̶'̶t̶ ̶h̶a̶v̶e̶ ̶a̶ ̶5̶0̶0̶p̶f̶ ̶l̶a̶y̶i̶n̶g̶ ̶a̶r̶o̶u̶n̶d̶ ̶b̶u̶t̶ ̶I̶ ̶d̶o̶ ̶h̶a̶v̶e̶ ̶a̶ ̶4̶7̶0̶p̶F̶ ̶w̶h̶i̶c̶h̶ ̶I̶'̶m̶ ̶g̶u̶e̶s̶s̶i̶n̶g̶ ̶i̶s̶ ̶c̶l̶o̶s̶e̶ ̶e̶n̶o̶u̶g̶h̶ ̶t̶o̶ ̶s̶e̶e̶ ̶w̶h̶a̶t̶ ̶t̶h̶i̶s̶ ̶c̶h̶a̶n̶g̶e̶ ̶w̶i̶l̶l̶ ̶d̶o̶
    I do have a parallel combo that gets me to 517pF - trying that first.

    Interesting you mention this mod for closed back cabinets... the 2x12 that I'm using now is a closed back cabinet.

    Also seeing your reply reminded me of something - early on in this project you commented on my schematic that I didn't need to use a bigger mixing resistor for the pentode channel - you said just let it be louder, the bigger resistor might take away some of the "magic."
    I thought that sounded like good advice at the time but just forgot to make the change.
    Going to make that change too...

    I only had two mild complaints about this amp and it looks like there's hope of fixing them both!
     
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