Troubleshooting noise when bypass cap removed, and design improvements

Discussion in 'Shock Brother's DIY Amps' started by andicniko, May 3, 2021.

  1. andicniko

    andicniko TDPRI Member

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    I've just completed my first amp build and could use a little help:
    1. I need to troubleshoot a hum/hiss when a bypass cap is removed from the circuit (i.e. using a switch, intended to reduce gain); and
    2. This is prototype, and I'm looking to check some possible design improvements with people who actually known what they're doing before doing a final version.
    Any help is much appreciated!

    The amp:
    It's deluxe micro using Rob Robinette's design, with a couple of simple mods (passive fx loop, switched bypass caps). Apart from the above hum/hiss when switching out one of the bypass caps, the amp sounds amazing.

    The suspect part of the circuit:
    I have put two bypass caps on switches so they can be removed to reduce gain (using this layout for labelling, the V1B and V2 bypass caps are on switches). This was mostly as an experiment to see what switching them out would do to the sound. Re moving V1B from the circuit works as intended, less gain and no noise. Removing V2 from the circuit introduces a noticeable hum/hiss, and it's unaffected by any other variables.

    I have pictured the part of the circuit that switches out the V2 bypass cap.

    Question 1: Do you know what the noise could be and how I could fix it? Could it be:
    • the V2 cathode resistor got fried when soldering?
    • the long runs of wire to/from the switch be introducing noise (note the noise didn't change when I forced the wires into different routes)?
    • that the V2 bypass cap can't be removed, or not without other changes?
    Design improvements:
    Full disclosure, this is my first amp build and was intended to be a prototype before building the final version. You are guaranteed to see issues! While the above is the only issue I can hear, please do tell me what else can be improved for the final version.

    I have a few specific ideas myself...

    Question 2: Are these worthy design improvements, and can you suggest any others?
    • shortening and running more wires against the chassis with conductive copper tape - my theory is this will shield longer runs and reduce potential for noise;
    • using eyelets instead of turrets - turrets look good but man are they hard to solder without melting everything else;
    • no more screw terminals - I did it in case components were faulty or I made a mistake, but I'm unsure if these actually have any downsides; and
    • using a solid state rectifier - while I love the look of a tube, they are expensive and I suspect this amp is not big enough to get any sag out of the rectifier tube.
     

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    Last edited: May 4, 2021
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  2. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    First of all: very nice post. Most of us can learn from that. Structured, links to information we need. Great!

    I have trouble seeing what is going on at v1a as the black thingy is blocking the view. And what is that black thingy?
     
  3. printer2

    printer2 Poster Extraordinaire

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    Question 1: Do you know what the noise could be and how I could fix it? Could it be:
    • the V2 cathode resistor got fried when soldering?
    • check with ohm meter
    • the long runs of wire to/from the switch be introducing noise (note the noise didn't change when I forced the wires into different routes)?
    • twist wires together
    • that the V2 bypass cap can't be removed, or not without other changes?
    • try flipping tghe capacitor leads around.


    • shortening and running more wires against the chassis with conductive copper tape - my theory is this will shield longer runs and reduce potential for noise;
    • I would just use regular wire
    • using eyelets instead of turrets - turrets look good but man are they hard to solder without melting everything else;
    • whichever you like. make sure turret is clean, wipe with alcohol. make a solder bridge between the turret and the iron. us flux, soldering gun is handy
    • no more screw terminals - I did it in case components were faulty or I made a mistake, but I'm unsure if these actually have any downsides; and
    • may need to be tightened over time
    • using a solid state rectifier - while I love the look of a tube, they are expensive and I suspect this amp is not big enough to get any sag out of the rectifier tube.
    • go with your tastes
     
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  4. andicniko

    andicniko TDPRI Member

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    The black thing is a tone cap, connected to the wires with a screw terminal, and then wrapped in some self amalgamating tape. For the final version I'll definitely solder these in.

    I'll add a picture of the board without the obstruction this evening.

    EDIT: I've added a picture of the board with fewer obstructions.
     
    Last edited: May 4, 2021
  5. dan40

    dan40 Friend of Leo's

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    A fully bypassed cathode resistor on the first stage is pretty common to cut down on noise created by the tube. Yours is not on the first stage though so I doubt that's the issue. Have you tried replacing that preamp tube with a different one to see if that tube may be noisy?
     
  6. andicniko

    andicniko TDPRI Member

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    Alas changing the tube didn't make a difference.

    If I was to describe the hum, it sounds like the mains hum gets sharper (like adding treble frequencies and volume to the much duller/quieter hum that's already there).
     
  7. andicniko

    andicniko TDPRI Member

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    OK does it help narrow anything down that:
    1. the hum's volume is constant, regardless of what I do with the preamp or master volume (or any switch/pot for that matter); and
    2. The hum is the same in character as the one my guitar introduces when I'm not touching the strings (although the guitar hum is much louder). BUT the amp hum occurs regardless of whether a guitar is plugged in?
     
  8. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    Could you explain this? I have never heard of it before.
     
  9. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    Wait a second, V2 is the power tube. As far as I now it is unusual to omit that bypass capacitor.

    The voltage at the power tubes is not that well filtered. Can that have any effect?

    Attached document might interest some, or not...
     

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  10. dan40

    dan40 Friend of Leo's

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    It has been said that a fully bypassed cathode, say 10uf or more, will help to cut down on heater to cathode leakage caused by a noisy tube. I have read that this is the reason that Fender chose to use a very large bypass cap value (250uf) on it's early Bassman circuit. Check out reply #10 from this thread over on the Hoffman forum. HBP gives a much better explanation than I could hope to.

    https://el34world.com/Forum/index.php?topic=17133.0
     
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  11. andicniko

    andicniko TDPRI Member

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    Thanks the doc, I'm trying to wrap my mind around what each component does in an amp and this all helps.

    I got the idea for removing the cap here, but I may have misinterpreted something. There is a paragraph that says if the amp "is still too loud for your practice sessions you can reduce the power output by deleting the 25uF 25V power tube bypass capacitor".

    Does a cap and resistor in this arrangement filter certain frequencies? And if so, could what I'm hearing simply be noise that was there anyway, but was being filtered out until I remove the cap?
     
  12. sds1

    sds1 Tele-Afflicted

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    You nailed it here I think!!
     
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  13. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    You could try heater elevation or a hum dinger.

    Attached is some more info about hum.
     

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  14. guitar_paul1

    guitar_paul1 Tele-Holic

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    Have you looked the noise with a scope?
    Sometimes ultrasonic oscillation presents itself as noise.
     
  15. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    What if we drop the idea of dropping the power tube bypass resistor? I see where in your link (his page about the 5E3Micro) @robrob says you can do so, but if it's making your amp noisy, why bother? As you point out, he does say, "If the Deluxe Micro is still too loud for your practice sessions...." But is it?
     
  16. andicniko

    andicniko TDPRI Member

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    I Ihink this is the most practical way forward for me. It was a fun experiment, but the preamp and master volume pots give me enough control if I'm honest.

    Thanks everybody for your help. I've certainly learned more about hum!

    For what it's worth, I think it is related to the heaters. I don't have an oscilloscope (or much knowledge...) but I can affect the hum by disconnecting the 6.3v line while in use. So a heater elevation or humdinger sounds like a great idea if I try removing bypass caps in the next amp (sadly no room in this one).

    Now to find a use that panel hole and switch...
     
  17. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Good for you. FWIW, you may already have heater elevation if you built to the layout you linked. Note heater CT tied to output tube cathode...

    35EF0BF5-541F-44B8-AEC7-5FA933AB15C7.jpeg

    Now we could test the heater theory. Can you get a 6V battery in NZ? Running the heater circuit (beyond the lamp) on the battery *as a test* will kill all heater hum. But if all your voltages are in the right range, and your amp is now quiet, it may be simpler to call it good. :)

    if you’re set on using that switch hole, a switchable V1 bypass value or a switchable V1 coupling cap ('humbucker' mod) might work. IIRC Rob describes variants of both on his 5e3 mods page.

    But be aware that wiring to a switch can also be a source of noise.
     
    Last edited: May 6, 2021
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