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Spare (Blues Jr) Parts Build

Discussion in 'Shock Brother's DIY Amps' started by mcabe, Jun 16, 2015.

  1. mcabe

    mcabe Tele-Meister

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    The signal cable from the input jack to V1 is already shielded. Which other runs do well with shielding?
     
  2. dankilling

    dankilling Tele-Afflicted Silver Supporter

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    Pretty much anywhere the signal path travels- volume, tone, etc... can all be shielded cable.
     
  3. mcabe

    mcabe Tele-Meister

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    Since the treble control sends the buzz to ground, the buzz must be picked up before the tone stack, right? That leaves the volume control and the wires around V1.

    There is a long run from the volume control to the grid of the second amplification stage, but if the buzz scales with the volume control, it must be entering before the pot, right?

    So shield the wire running to the volume pot?
     
  4. dankilling

    dankilling Tele-Afflicted Silver Supporter

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    Yep. Play a little 'follow the leader' and you'll knock it out. I'd love to hear a demo of this one eventually!
     
  5. mcabe

    mcabe Tele-Meister

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    I isolated the noise to the first amplification stage.

    • Grounding the input to the tube (pin 2) has no effect on the buzz.
    • Grounding the output (pin 1) takes out the buzz.
    • I replaced all the signal wires coming from pin 1 with shielded cable, but the buzz persisted.
    • I disconnected all the signal wires and put a capacitor directly from pin 1 to pin 7, but still have buzz.
    • Tried 2 different tubes, but still buzz.

    I think this rules out problems with grounding and shielding, and means the buzz is coming from the B+ power supply, or the 6V filament supply. Here is a sound clip. You can hear the buzz kick in when I flip the standby switch and gets louder when I turn up the volume. Then you can hear it disappearing when I turn the treble down.

    https://soundcloud.com/mcabe-1

    What does this sound like to you?
     

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  6. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Have you moved the amp to another part of the house or even to another location to see if it affects the buzz?

    That buzz sounds like a buzz I had when I had a 120v pilot lamp too close to the input jack so it does sound like power line noise to me.
     
  7. mcabe

    mcabe Tele-Meister

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    Yeah, moved to different rooms and still have buzz. I put the buzz into a free oscilloscope program I found (http://www.zeitnitz.eu/scope_en), and compared the buzz to a 60hz sawtooth wave. There is definitely a lot of other noise here too, but the peaks seem to line up pretty well with 60 hz. So I think 60hz is the primary culprit?
     

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  8. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Yes, that is a 60Hz buzz spike which means it's not coming from the rectifier or DC power supply which would be 120Hz.
     
  9. mcabe

    mcabe Tele-Meister

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    So that means AC is the problem, which leaves the heaters, right? Just realized that my artificial center tap is using a pair of 100K resistors, not 100 ohm resistors! Think that might be the problem?
     
  10. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Yes, that may be issue but the heaters usually pass a 60Hz hum, not a sharp buzz like you have. The 120v from the wall to the power switch, fuse and on to the PT may be the issue but let's see what happens with the right resistors.
     
  11. mcabe

    mcabe Tele-Meister

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    Replacing the artificial center tap with 100 ohm resistors completely knocked out the buzz! Apparently the difference between 100,000 and 100 ohms was significant... stupid units/orders of magnitude.

    Here is a clip of how the amp sounds now: https://soundcloud.com/mcabe-1/amp-buzz-2. It is much, much quieter. There is audible transformer buzz (not coming from the speaker) when the volume is turned down. With the volumes dimes and treble all the way up, you hear the white noise/hiss. Turning down the treble, you can hear what sounds to me like the power transformer hum amplified.

    I think this is all within reason and as expected, but please chime in if you hear something different.
     
    Last edited: Jul 10, 2015
  12. mcabe

    mcabe Tele-Meister

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    FWIW, here are scope images of the transformer hum, white noise and amplified hum. The red line is a 120hz sine wave. Amplitudes here are all pretty small.
     

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  13. dankilling

    dankilling Tele-Afflicted Silver Supporter

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    Awesome! Now lets hear some sound clips that aren't of hiss, hum, or buzz ;)
     
  14. mcabe

    mcabe Tele-Meister

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    Thanks! I'll get a recording of an actual amplified guitar signal soon. Going to try a few different speakers out first. I don't have the best recording setup, so an SM57 and little m-audio interface will have to suffice. Full disclosure, my guitar playing - like my amp building - is pretty mediocre.
     
  15. mcabe

    mcabe Tele-Meister

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    FWIW, the dragonfly preamp is essentially an exact copy of the bright channel of an AC30 top boost. It is a pretty gainy amp. Haven't had a chance to crank it up yet, but I'll report back when I can. Until then here is a clip of a beautiful looking dragonfly.

    https://www.youtube.com/watch?t=239&v=TIpIwMRPnPI
     
  16. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Congrats mcabe, your hum and hiss recording does sound pretty normal as best I can tell. Looking forward to some sound clips.
     
  17. mcabe

    mcabe Tele-Meister

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    Got a chance to crank the amp up this weekend. The preamp is biased very hot. Not much headroom, but it is very warm and sweet sounding. Nice overdrive tones on tap as well. With the 16 ohm VHT chromeback speaker, it is surprisingly loud for a 4 watt amp. You could easily hear it from the backyard, when I was playing in the basement. (Definitely need some soundproofing down there....)

    I have 4 speakers and 3 amps, so I am playing musical chairs with them now. I think I'll end up with the private jack in this amp, if it can handle the 8 ohm load. Planning on recording each speaker in the dragonfly with guitar played back through my ditto looper. Then I'll post results, and pick the best speaker for the dragonfly. Here are the candidate speakers: (private jack, cannabis rex, fender special design - all 8 ohm, and VHT Chromeback - 16 ohm).
     

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  18. mcabe

    mcabe Tele-Meister

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    Just reviving this thread to post an updated schematic for the Dragonfly in case anyone is thinking of building one. The updated schematic, courtesy of Doug Hammond, has higher B+ voltage and a cut control. The higher voltage really brings out the voxy chime in this amp, and the cut helps tame the highs. Highly recommended if you are looking for a low watt vox top boost/ matchless type amp. There is a thread over on ax84 that discussed the mod further.

    http://www.aronnelson.com/gallery/main.php/v/DougH/amp/Dragonfly+Amp/DragonflyNew.gif.html
     
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