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Uh oh: 5F2A has a hum from tone pot -- strangeness ensues

Discussion in 'Shock Brother's DIY Amps' started by RustySterling, Oct 9, 2018.

  1. RustySterling

    RustySterling Tele-Meister

    238
    Jun 4, 2018
    Carmichael, CA
    First off, the tube set is Sovtek 5751, EH 6V6 and GE 5Y3. Here are the readings I did yesterday:

    Plate to cathode: 344.5dc (all readings dc unless otherwise noted)
    Across Heaters: 6.86ac

    V1: P1 147.1, P2 0, P3 1.597, P4/5 3.440ac, P6 142.8, P7 0, P8 1.501, P9 3.448ac

    V2: P1 0, P2 3.446ac, P3 368, P4 297.1, P5 4.88, P6 0, P7 3.443ac, P8 22.18

    V3: P1 0, P2 380.6, P3/4 345.8ac, P5/6 346.4ac, P7 0, P8 380

    Here's the deal, I had another problem that I worked on -- some power drop (not a lot but enough to notice) and fuzzy sound when playing after about 15 minutes. It came up consistently. I pushed things around with chopsticks and found an unsoldered joint at the tone pot. AH HA! Alas, the "ah ha moment" was exactly just a moment. Problem continued. So I pushed about with chopsticks more and found nothing else. I took readings yesterday with different rectifiers. Decided on an old GE 5Y3 that gave me plate-to-cathode voltage of about 344. I'm happy with that.

    This morning I turned on the amp and let her warm up a good bit. I again chop sticked around and nothing stood out. Plugged in my guitar and played hard for about an hour and the old problem doesn't present itself. I must have a bad solder joint somewhere and probably solidified it a bit chop sticking. So I'll have to do some reflows.

    Now the problem I noted above. And before I start, I installed star washers on all jacks and pots during my build.

    Had the amp on and well warmed up, guitar plugged in volume down (both amp and git). I turned up the tone pot first and at about 2-1/2 this medium loud hum started (remember, volume is at 0). Seems odd to me. I turned up the volume on the guitar and then the amp and nothing from the guitar coming through amp until its volume is about 2-1/2, then I not only softly hear the guitar but the hum also disappears. So I unplugged the guitar cable from the amp. I turned down volume and tone to 0. Started turning up tone and there's the hum in the same place and consistent all the way through the rest of the sweep of the tone pot. I leave that pot up with the hum, turn up the volume pot and at about 2-1/2 I hear a click-like static (not quite a pop and more than a sizzle), almost like a switch being thrown -- and viola the hum stops.

    I chop sticked around both pots' leads, capacitors and joints. Moving the ground wire from lug 1 back to the #1 input ground lessened the hum (volume is 0 at this point) but it didn't disappear. Played around with other leads and no changes, no noises. Turned up the volume, hum disappears at usual place, and I chop stick around more and still nothing unusual.

    Now I'm thinking this might be related to the previous problem and it's a ground loop from the tone pot to another short somewhere. And that could go back to what I suspect is a bad or weak solder joint somewhere. So I guess that I'll need to focus on the chain for the tone pot and volume pot.

    BTW, she's pretty much dead quiet at playing volumes, no hum and little hiss. There is no hum from my Tele pups unless I'm standing inches away from the amp. And the sound with her 2x8s in a Tweed Deluxe sized cabinet is awesome, when she sounds right. Thanks for letting me vent. Writing is a part of my thinking process.
     
  2. dan40

    dan40 Tele-Holic

    933
    Aug 19, 2015
    Richmond Va
    Any chance you may have let a small wire clipping fall into the pot when building the amp?
     
    RustySterling likes this.
  3. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Also, check your ground on that pot--both snug to the chassis and, if applicable, grounded to the bus bar.
     
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  4. Tapatio

    Tapatio Tele-Meister

    129
    Jun 13, 2016
    New Jersey, USA
    Or even... pull the pot out entirely and put in a new one. If the problem persists, then continue hunting...
     
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  5. mgwhit

    mgwhit Tele-Holic

    840
    Aug 1, 2008
    Louisville, KY
    Do you have another 500pF silver mica cap you can try? Those things are notoriously finicky these days.
     
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  6. RustySterling

    RustySterling Tele-Meister

    238
    Jun 4, 2018
    Carmichael, CA
    There always could be a chance of that but I only remember a couple of little snips that got away and retrieved with my handy magnetic tool I've had since my days of rebuilding old VW engines. Yikes! We can certainly collect a lot of tools along the way.

    Plus one to you Prof. And my post above was wrong about which ground lead I referred. It was the lead back to the jack ground from the middle lug of the volume and installed as per the Mojotone layout. Trimmed that puppy and soldered at a closer point on the buss and no more hum from tone and no more staticy-click with the volume.

    Thanks for that advice. I certainly saw that as a worst case scenario. With the new ground things are all good.

    Read above and you'll see it wasn't the mica cap but that is a good call and I wondered about that myself.
     
  7. RustySterling

    RustySterling Tele-Meister

    238
    Jun 4, 2018
    Carmichael, CA
    The other problem is back. After some lengthy and hard playing at Volume 8, Tone 8, that slight volume drop and slight fuzz tone reappeared. If I turn up the volume on the guitar the fuzz goes up, same if I turn up at the amp. So perhaps in an output stage.

    I'm going to try another power tube and check my solder joints and possibly reflow on the tube socket. I know things are wired properly; been over it quite a lot these last couple of days checking and double checking and taking numerous DMM readings.

    Once I sort out my fuzz tone problem than I'm going to look at a couple other things.

    I'm thinking about adding a grid resistor on the power tube socket Pin 5, I believe it is. Read about it many times in other 5F2A and 5E3 build threads. But don't remember any explanation about it. Would someone please explain to me why this could be a good or bad idea.

    Another thing I'm planning for later is getting rid of the under the board single wire from the volume pot middle lug to pin 7 of V1. I'll get a shielded cable and run it over the top of the board. Not sure what gauge but I am pretty sure guitar cable is too big. I know to ground the shield on the tube side and shrink wrap the unattached shield on the pot side.
     
    Last edited: Oct 10, 2018
  8. Snfoilhat

    Snfoilhat Tele-Holic

    Age:
    37
    645
    Apr 8, 2016
    Oakland, CA
    P5. 4.88 VDC on the power tube's control grid at idle. I don't think that's right. Shouldn't it be 0?

    Edit: it's too late where I live to be troubleshooting amps. Sorry to interrupt! I'm going to bed.
     
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  9. King Fan

    King Fan Friend of Leo's

    Jan 1, 2013
    Salt Lake City
    I know you had pics in the build thread, but the eagle eyes here often do well with closeups of sockets and the pots and...

    The fact it goes wonky after extended play suggests heat — or just maybe vibration — so I’d triple check socket solder, clean pins, tighten socket springs, and def swap out both tubes one at a time.

    I had one of these intermittent fails after long playing — mine turned out to be a dying tube but there are many possible suspects. Only good thing is you get to play your amp a lot. “Sorry honey. Still troubleshooting.”
     
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  10. RustySterling

    RustySterling Tele-Meister

    238
    Jun 4, 2018
    Carmichael, CA
    Oh yeah, that's my story and I'm sticking to it. :):lol:
     
  11. RustySterling

    RustySterling Tele-Meister

    238
    Jun 4, 2018
    Carmichael, CA
    Reflowed everything on the tube sockets and a couple of iffy spots I spied on the board. Played hard about an hour and problem didn't crop up.

    But I know a sure fire way for the problem to reappear or another problem to pop up: Put the back cover on. Works every time. :rolleyes:

    You want pics. I got pics. This is after the reflow.
     

    Attached Files:

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  12. bigguy12321

    bigguy12321 Tele-Meister

    318
    Jul 13, 2012
    Traverse City Mi.
    @RustySterling Congrats on getting it going... again...

    It may be nit picking but in the pics of the 6V6 and 12AX7 tubes there are a couple/few VERY SMALL dollops of solder on the chassis near the back crease. I've found those in small hard to get to places where they didn't belong and had them drop onto components. You might want to scrape them off while vacuuming to get them out of the area. Possible shorts in the making.

    And of course this gives you more amp time!

    Hope this helps.

    a
     
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  13. RustySterling

    RustySterling Tele-Meister

    238
    Jun 4, 2018
    Carmichael, CA
    @bigguy12321 Thanks. I'll take care of that. Appreciate the advice.
     
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