Champ Micro completed

Discussion in 'Shock Brother's DIY Amps' started by Lavochkin, Jan 18, 2020.

  1. Lavochkin

    Lavochkin TDPRI Member

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    IMG_20200118_154814.jpg IMG_20200117_193635.jpg IMG_20200117_193552.jpg

    About a year ago I embarked on building Rob Robinette's Champ Micro (first time builder) and just finished. Many thanks to Rob for all the work he put into his website! Start up went very well (all the smoke stayed inside the components where it belongs) and I like the sound. No hum except at very high volume where I won't be playing anyway (I live in a townhouse, neighbors just the other side of the wall). There are a few issues I would like to ask for help in addressing though.

    During initial startup, I measured the heater voltage at the pins at 6.5 VAC without tubes. At the end of the startup with tubes and the light bulb limiter removed I measured again and got 29.1 VAC. What? This makes no logical sense to me. I measured multiple times to make sure I was measuring at the right pins. Is this a problem?

    I also did the tube bias calculation via Rob's calculator and got 19.6 ma, 6.3 W, and 114.5% dissipation. My "680 ohm" cathode resistor actually measured at 670 ohms. I assume that what I need to do is replace it with a larger value resistor to get the dissipation below 100%, correct?

    Below are Rob's and my measured values, mine incorporate the extra filter cap he added after his readings...

    Rob, Matt
    Wall voltage 125.3 v, 120 v
    transformer output 293-0-293, 286-0-286

    B+ NA, 387 v
    B+1 399 v, 375 v
    B+2 304 v, 261 v

    V1 pin 1 150 v, 127.3 v
    V1 pin 3 2.3 v, 2.0 v
    V1 pin 6 146 v, 125 v
    V1 pin 8 2.4 v, 1.8 v

    V2 pins 1 & 6 390 v, 334 v
    V2 pins 3 & 8 11.75 v, 13.81 v
     
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  2. King Fan

    King Fan Friend of Leo's

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    Where were you measuring?

    For those not familiar with this amp (many of us I bet) here's a 'typical' layout... I know these little guys have a quite a few possible variations.

    Champ_Micro_Layout copy.png
     
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  3. Lavochkin

    Lavochkin TDPRI Member

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    Ah, yes, I was measuring wrong- between ground and the pins rather than between the pins themselves. Remeasured and got 6.5 v. Thank you for prodding me to think in the right direction to figure out what I did wrong!
     
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  4. BobSmith

    BobSmith Tele-Meister

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    Sounds right. You have your heater circuit elevated by where you placed the heater center tap.

    Yours is not referenced to ground like the original. Elevated (and closer to voltages in the tubes) is supposed to reduce noise. (But I’m not sure it’s that big of a deal with a Champ).
     
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  5. panacea

    panacea TDPRI Member

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    Nice looking build! I was thinking of building the Micro Champ, Just to try to get decent tones without cranking the amp and disturbing the family. Can you tell us your impression of the micro? Does it work with hum bucker guitars? I built a Monotone Champ that works great with Fenders, bridge pups only on Gibsons...Mike IMG_1179.JPG
     
  6. Lavochkin

    Lavochkin TDPRI Member

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    Thank you! I am a beginning guitarist, and so may not be the best at evaluating it for you, but I'll do my best. The only other amp I've played is my Blackstar Fly 3, a solid state practice amp with a 3" speaker. My Champ Micro with an 8" speaker (which does not have a cabinet yet) is both louder and sounds better than the Blackstar. As for humbuckers, I have an HSS Squier Strat. Since I just finished the amp, I haven't played it much on the bridge humbucker, but it does sound better than on the Blackstar. On Rob's website page for the Champ Micro (https://robrobinette.com/Champ_Micro.htm) it is mentioned that reducing the size of the V1 cathode bypass cap from 25uf to 1uf is good for humbuckers. I hope this helps.
     
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  7. robrob

    robrob Poster Extraordinaire Ad Free Member

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    The amp looks good, congrats Lavochkin.
     
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  8. 82LesPaul

    82LesPaul TDPRI Member

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    Very nice looking build. Congrats. Unfortunately these builds are like a popular potato chip brand..."I bet you can't just have one."
     
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  9. Lavochkin

    Lavochkin TDPRI Member

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    Thank you! By the way, re my first post in this thread, should I be worried about the high plate dissipation?
     
  10. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Champ's run hot but the power tubes usually take it just fine. I like to use the stock cathode resistor values even though the bias measures hot. If you start to burn up power tubes faster than you'd like you could bump the cathode resistor up by about 10%.
     
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  11. Lavochkin

    Lavochkin TDPRI Member

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    Thank you!
     
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