5f2a/Ga5 build - extremely loud buzz (grounding??)

Discussion in 'Shock Brother's DIY Amps' started by sonicsamurai, Feb 9, 2019.

  1. sonicsamurai

    sonicsamurai Tele-Meister

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    i've decided it's been a while since i'e built an amp so i've dusted off a few bits and bobs, bought new caps, resistors, wire, tubes etc and built me a new one.

    I have used the schematics for a 5f2a and the mid 2000s gibson Ga5 reissue, so cathode and anode values on the preamp are ga5, its an el84 output like the ga5, tone control and power section a la 5f2 with the 4th electrolytic cap, only minor changes i made really were a 220 el84 reistor instead of 270 and an additional 25/25 cap on the second preamp triode.

    the schematic for the ga5 came with voltages and i pretty much match those bang on. i've checked all my connections, every wire has space and everything is grounded separately, i've double and triple checked against the schematics, i've near enough wired and rewired the whole thing over the last few days. I can get sound out of the amp with the volume at around 0.5 to 1, after that huge buzzing kicks in and i cannot work out why for the life of me. i thought it might be a grounding issue but i've tried several different things and rewired with no effect.

    this is the gibson Ga5 schematic http://www.blueguitar.org/new/schem/misc_amp/gibson_ga5_schem.pdf

    i'm sure you're all familiar with a 5f2a ;)

    will post pics in a sec...you can probably tell that i've been going over everything a few times and getting a bit frustrated with it...gonna put that soldering iron down for a bit now...

    any thoughts/ advice/ help really appreciated. :)
     
    Last edited: Feb 9, 2019
  2. sonicsamurai

    sonicsamurai Tele-Meister

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    IMG_20190209_141752.jpg IMG_20190209_141824.jpg IMG_20190209_141809.jpg IMG_20190209_141800.jpg IMG_20190209_141853.jpg IMG_20190209_141903.jpg IMG_20190209_141740.jpg IMG_20190209_141839.jpg
     
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  3. clintj

    clintj Friend of Leo's

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    Strongly volume dependent says it's in the early preamp stages.

    Looks like you have a heater center tap used, which should rule that out. Should.

    So, a few questions to gather info:

    Where are your V1 cathode grounds tied to?
    Does plugging in a guitar change things?
    Have you visually verified the input shorting jack is shorting when not in use?

    And please put a rubber grommet where those speaker wires will pass through at some point. Chassis holes are savage on insulation.

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  4. sonicsamurai

    sonicsamurai Tele-Meister

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    That was my thinking as well

    Both cathodes (pin 3 and 8) are grounded to the same lug as shown below, worth separating them? Nothing else is grounded to that point.

    [​IMG]



    this is with a guitar plugged in, as stated i can hear the guitar coming through the speaker with the volume really low before the loud buzz kicks in as soon as the volume reaches a certain (very low) level

    don't know what you mean by this. please explain

    The plan was to wire it up as a jack/socket, this arrangement was just for testing purposes.

    Thanks for your help
     
  5. clintj

    clintj Friend of Leo's

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    Your preamp has a vast ground loop in it, for starters. Current is coming from node C to the 12AX7, and wants to return there. As laid out, that node's current goes back via the chassis in two different spots. One is any grid current via the pots to chassis, the other is through the cathodes to chassis. The chassis should not carry current intentionally. It's a poor conductor and stray current can cause strange interactions and noise.

    Have you read Merlin's article on grounding by chance? Most of us use a two point system on smaller tweeds, with the preamp side separated from the power amp side or some variation of that.

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  6. Snfoilhat

    Snfoilhat Tele-Holic

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    Almost everything I have learned about building amps came from one of these sources.

    Here's one of Robrob's excellent layouts that shows a good way to approach the layout of wiring. It is a the 5F1 Champ and not the 5F2 Princeton, but shows the principles well, especially with respect to turning a schematic or vintage layout (ground appears to be any generic chassis connection) to a well functioning amp (ground is carefully wired for minimal noise). https://robrobinette.com/images/Guitar/HowAmpsWork/5F1_Layout.pdf

    Here is the Merlin Blencowe/Valvewizard article on grounding that clintj mentioned. The entire Valvewizard site is excellent and presents the major parts of amps in brief but useful sketches with example circuits http://www.valvewizard.co.uk/Grounding.html

    Good luck!

    Edit: to say that one thing that gets lost in these troubleshootings the helpers leaping directly to ideals and best practices (and all the study it takes to master those) and not first getting people to an intermediate step where the amp is working OK.

    I'm guessing that your volume and tone controls need a dedicated ground wire not to any potentiometer case (I might solder to a pot case when wiring a guitar, but not when building an amp). This ground wire should run from either the volume or tone back to the terminal on the circuit board that has your cathode grounds. I think that would be in keeping with the Valvewizard paper.
     
    Last edited: Feb 9, 2019
  7. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Your soldering iron needs to be hotter or *something* - most of those connections don't look very good. When it's hot it should slow nice and smooth and be shiny, but a lot of your look dull and rough.

    Have you identified the frequency of the hum/buzz? It would be helpful to know if it's 50Hz or 100Hz as that would tell you if it's a ground issue or a power supply issue.
     
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  8. sonicsamurai

    sonicsamurai Tele-Meister

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    That diagram was helpful thankyou I'm working my way through the document - quite a lot to take in. In the meantime i have doubke checked and it seems to do it regardless of whether the guitar is plugged in. Ive also wired as below so all grounds go to 2 points 1 for preamp stuff, and one for ecerything else and kept both pretty far apart. This seems to have given me a little more volume, but still really loud buzz kicks in and lose any guitar sound after that point. IMG_20190209_225513.jpg IMG_20190209_225506.jpg
     
  9. clintj

    clintj Friend of Leo's

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    Ground issues and lead dress are typically a root cause of most noise issues in builds that don't use an established layout that we here have found that works well. Rectifier hash noise in the reservoir circuit and transformer noise carried in the heater wiring play hell with trying to quiet down an amp. That's probably why I go that way first when trying to dig into a noise issue; I want to rule out a known troublemaker before trying to weed out something more specific.

    +1 on knowing the frequency in question. Mains frequency (50 or 60Hz), twice that (100 or 120Hz), or is it just a nasty buzzsaw hum that's hard to pin down to one of the first two? I have a frequency analyzer app (Audiotool for Android) on my phone that's awesome for seeing what frequencies are in a sound.

    Another thing just occured to me. Your pics show the chassis open end up. Do you maybe have a fluorescent light overhead or another common source of outside noise like a dimmer controlled light? Does laying a metal cookie sheet or flipping the chassis open end down change anything for you?

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  10. dan40

    dan40 Tele-Afflicted Silver Supporter

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    Since the 5f2 circuit employs nfb, have you tried swapping the OT primary wires? One quick way to see if this will be necessary is to simply unsolder the nfb wire from the output jack to the 22k resistor on the board. Just unsolder it at the jack and see if the noise is now gone.
     
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  11. sonicsamurai

    sonicsamurai Tele-Meister

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    This has no nfb like the ga5
     
  12. bigguy12321

    bigguy12321 Tele-Meister

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    It appears that you have got signal wires touching heater wires on both sockets. That would be no bueno if it is so.

    BTW I like your homemade bridge rectifier. Looks like I may have to copy something else from someone on TDPRI.

    a
     
  13. sonicsamurai

    sonicsamurai Tele-Meister

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    I get across all frequencies no spike the yellow line Screenshot_20190212-204304.png
     
    Last edited: Feb 12, 2019
  14. sonicsamurai

    sonicsamurai Tele-Meister

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    i've just tried this as the last desperate act lol and boom...works....simple as swapping 2 wires and i've been tearing my hair out!!
     
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  15. dan40

    dan40 Tele-Afflicted Silver Supporter

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    Glad you got it sorted out!
     
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