5F1 Kit Build - no sound

Discussion in 'Shock Brother's DIY Amps' started by brackp, Feb 14, 2019.

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  1. brackp

    brackp TDPRI Member

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    Hi,

    This is my first time build so no surprises somethings's gone wrong :)
    I've got power going to all tubes, I have voltage readings (at home) and I can post them later but I think they're pretty much ok.

    I get no sound whatsoever. I've tried a bump test with a wooden stick and nothing coming through the speaker. I've tested the speaker in another amp and it's working fine.

    I tried switching the primary wires on the output transformer and one way (Blue to B+) I get a pretty quiet squeak from the amp, nothing through the speaker as far as I can tell. Red to B+ I get nothing at all.

    I suspect it's a grounding issue somewhere. I'll post pics of the jacks later on. They should be grounded directly to the chassis automatically right? I've seen schematics where they are explicitly ground to a bus.

    Can someone tell me how I can test the output transformer and the in/out jacks for ground?
    Sorry about the lack of pics/details. I'll try get some up later on tonight
     
  2. Tom Kamphuys

    Tom Kamphuys Tele-Meister

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  3. boredguy6060

    boredguy6060 Friend of Leo's

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    What happens when you probe pins 1&2 of the 12AX7, do you hear popping type noise?
    What did pin 3 measure?
     
  4. brackp

    brackp TDPRI Member

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    No popping when I poke anywhere at all. From memory pin 3 on the 12AX7 was 1.6V
     
  5. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    We'll need a voltage chart of all voltages at all tube pins. That should get us started.
     
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  6. Whatizitman

    Whatizitman Friend of Leo's

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    Maybe make a signal tracer? Plenty of guides on the interwebz. It doesn't have to be elaborate. I made one just like this, and have used it for pedal circuits. I'm not sure if it needs to be different for higher voltages. Someone far more knowledgeable than me could answer that. I used a broken guitar cable, alligator clip and wire for ground, resistor, and cap - all stuff I had on hand. I probe with one lead of the cap to trace, strum a guitar on a stand for signal. Hope that helps.

    [​IMG]
     
  7. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    That's all you need - as long as the cap is 500-600V you're good for amp work.
     
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  8. sds1

    sds1 Tele-Afflicted

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    My listen probe shorted a signal node to b+ the other day by accident. A 600v cap in the probe device is a good idea. :cool:
     
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  9. brackp

    brackp TDPRI Member

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    I'll get the voltage readings up in a few hours when I get home.

    As luck would have it I actually have 'spares' of every single component in the kit :) Well, a friend is building the same amp with me and he's a little bit behind so in the interests of science I can grab a cap from his kit for the probe. Would one of the coupling caps do the trick?
     
  10. sds1

    sds1 Tele-Afflicted

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    One of the 0.02uF's? Are they 600v? For this exercise yes those are fine. For longterm use these caps will filter too much low end.
     
  11. Whatizitman

    Whatizitman Friend of Leo's

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    This is good to know. My cap is a mylar 100V. Not gonna touch my amps with it until I get a higher voltage cap. Glad I hadn't tried yet. That would've been bad. :eek:
     
  12. sds1

    sds1 Tele-Afflicted

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    Well as long as you don't probe anything with high voltage on it, no worries.

    Are the coupling caps for this amp only 100v?
     
  13. Whatizitman

    Whatizitman Friend of Leo's

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    The cap I have on my probe is 100v. I haven't had to trace any signal in the amp I've been working on. Only for pedals so far. My amp has has 400v coupling caps. I took one out that was used as a death cap. Maybe I could use that one to probe with? .01uf/400v. Or is the rating still too low to be safe?

    Sorry, OP, if I'm getting too far off topic.
     
  14. King Fan

    King Fan Friend of Leo's

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    Hang in there, brackp. You'll get this. A signal trace is a great idea. Even while you're planning that, a simple voltage chart and some pics often give the pros here a good clue. Well-lit closeups with special attention to the sockets, board, power sector, and jacks are helpful.
     
  15. sds1

    sds1 Tele-Afflicted

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    Only if you don't put the probe on anything over 400VDC.

    All of your signal, plates, cathodes... a 400VDC rated device is fine. But you want to cover your bases in case of mistakes like I made.

    A 630VDC or 1000VDC rated cap is going to be the most universally safe cap to put in there.
     
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  16. harleytech

    harleytech TDPRI Member

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    Make sure you have good grounds going to the input jacks and speaker jack...
     
  17. brackp

    brackp TDPRI Member

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    This is one I'm a bit unsure of, I have wires soldered to the ground points on the jack. That ground point on the jack is connected directly to the chassis. That's a 'good' ground right?

    I'll post pics later on also. Pretty sure my soldering will be torn apart by you lot, go easy!
     
  18. brackp

    brackp TDPRI Member

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    Hi,

    I re-checked the voltages and here we go:

    5Y3

    2. 327V DC
    4. 287V DC (A/C not giving a reading) - I was definitely reading 5V AC last night...not sure what's happening here! (Edit - OK the DC is coming through Pin 8 I guess. Still not sure why I can't get A/C reading)
    6. 287V DC (A/C not giving a reading)
    8. 327V DC

    I'm guessing I've got some serious issues in the 5Y3 on pin 4 & 6!

    6V6

    2. 3.2V AC
    3. 307V DC
    4. 291V DC
    5. 0
    7. 3.2AC

    12AX7

    1. 181V DC
    2. 0
    3. 1.3 DC
    4/5 (jumped) 3.2 AC
    6. 255V DC
    7. 0
    8. 3.5V DC
    9. 3.2V AC

    I also measured a few points on this layout: https://robrobinette.com/images/Guitar/HowAmpsWork/5F1_Layout.pdf

    B+1: 320V DC
    B+2: 307V DC
    B+3: 291V DC

    At the bottom of R7 - 260V DC
    At the bottom of R6 - 180V DC

    On other part that bothered me a little bit is that I measured 3V DC on the top (looking at the layout) of the coupling capacitor C1. Thought there should be no DC voltage there.

    So a few things here, the yellow wires from the output transformer are giving me 287V instead of 5V AC.

    Pics to follow
     
  19. brackp

    brackp TDPRI Member

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    20190214_185159.jpg 20190214_185151.jpg 20190214_185147.jpg 20190214_185123.jpg 20190214_185115.jpg 20190214_185110.jpg 20190214_185104.jpg
     
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  20. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Voltages look fine I believe. The 3V on the cap that goes to the volume control is odd. Maybe that cap is bad? Do you have another cap you can put there?

    I'd swap that cap, recheck that voltage, and then start signal tracing. Feed some signal into the amp, then "trace" around by probing points. When the audio dies, you've found the location of the issue.
     
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