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Champ Micro buzz/hum

Discussion in 'Shock Brother's DIY Amps' started by Jtmart, Dec 4, 2020.

  1. Jtmart

    Jtmart TDPRI Member

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    Greetings! I’ve been on TDPRI for a couple years but new to this forum. First of all, big thanks to Rob Robinette for his website and the wealth of information he’s provided.

    I just finished starting up my Champ Micro build, I’ve only played through it a short time but pretty sure I’ve found my low watt amp. I love the 5E3 I built a few years ago but it’s just WAY too loud for home use. I’d been looking for a nice 1 wattish amp kit but it seems no one is still selling one and then I came across Rob’s site and with what he provides there is no need to purchase a kit. I found his explanations and info very sufficient to source and build your own amp.

    The only issue I have is a persistent buzz that is not bad unless the preamp volume is close to maxed out, then it’s pretty irritating. I thought I would let someone else take a look and see if they spot something. I’ve moved wires around with a probe but it doesn’t seem to affect the buzz at all, very constant and can be heard even if both volumes are on zero, although it’s very slight. Master volume doesn’t seem to affect it.

    My house power is not that great, much is ungrounded but even the sections that are grounded still produce the buzz.

    Yes, I miscounted my layout so there is empty space on the board.

    Thanks 0F8FD31C-2CEA-4168-91AA-7B637B8FE68A.jpeg for any help!
     
  2. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    I don't see the Heater Center Tap. If the power transformer does not have one, an Artificial CT should be created.
    On Rob's site it shows the CT elevated which can make things even more quiet.

    Nice looking build BTW.
     
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  3. screefer

    screefer Tele-Holic Silver Supporter

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    Rob's micro champ calls for Hammond 270ax with 6.3v. CT. Which tranny did you use?
     
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  4. Jtmart

    Jtmart TDPRI Member

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    Thanks for the replies. Yes, I purchased the 270AX from Mouser but when it arrived there was not a CT for the heater wires.

    So, I guess the solution then would be to create the artificial CT as shown on the schematic?

    D25F13D7-261C-4C3C-BF07-A107519BBF57.jpeg
     

    Attached Files:

  5. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Yes, create an artificial center tap with two 100 ohm 1/2 watt resistors. More than likely that will solve your noise issue.
     
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  6. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    You can solder the 100ohm resistors right on the V2 tube socket for a neat installation. Doing so will also elevate the CT for quieter operation.

    Solder one resistor between V2 pin3 and pin4. Solder the other resistor between V2 pin8 and pin9.
     
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  7. Jtmart

    Jtmart TDPRI Member

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    Thanks a million guys, I created the artificial CT and now it’s quiet as a mouse. I wish I would have waited a few minutes because I started to come off the V2 socket and then thought maybe I shouldn’t so it took a few more minutes but it made a huge difference and I couldn’t be happier with this amp.

    Thanks for the project Rob!

    Now that I’m satisfied I’ll build a very nice cabinet tomorrow!
     
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  8. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Good to hear!
    Let us know what speaker you are using and how loud your Champ Micro is.
     
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  9. Jtmart

    Jtmart TDPRI Member

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    I’ve been testing it through the speaker in my 5E3 cab, which is a 12” Tone Tubby Low Watt. With both volumes I get lovely cleans, throw some reverb on it and Wow, sweet jazz tones! You could still talk over it but why?

    From there on up it gets significantly dirtier and louder, at max my wife tells me to turn it down but it gets really nasty (in a good way) by then. Tons of low end all the up and down.

    The cab I’m going to build will house a 10” speaker, which I’m hoping will tighten up the bottom just a touch. I bought a Jensen MOD 10/35 to try first, an inexpensive speaker I can replace if I hate it but I’ve used the 6” & 8” with 386 Ruby amps and really liked the results.

    Once I get it all put together and played through a bit I’ll make another post of the finished build and report on the 10” speaker in a smaller cab, which will be a fancy little 14” black walnut and gold grill cloth eye catcher.

    After getting rid of the buzz and a little fiddling around I’m confident this will be my go to amp for indoors.

    Thanks for the help!
     
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  10. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Congrats on getting the amp debugged and running right. If the MOD speaker doesn't satisfy I recommend the Weber Sig Alnico in 10 or 12 inch. They are an incredible value in a great sounding vintage speaker.
     
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