Once again... ideas to solve 120Hz hum

Discussion in 'Shock Brother's DIY Amps' started by andrewRneumann, Jun 30, 2020.

  1. andrewRneumann

    andrewRneumann Tele-Meister

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    5856AC4E-D86B-4D68-96DA-3946391EF6BD.png

    Hey this free app for my iPhone should keep anyone from asking if I’m sure it’s 120 and not 60 Hz. It’s 120 and interestingly, there’s a second harmonic thrown in there at 240 Hz just for fun. Anybody want to explain the harmonic hum?

    This is a single-ended (imagine that with 120 Hz hum) running a pair of 6V6’s in parallel. I’ve got solid state diodes feeding 66uF for a reservoir capacitor. I clipped in an additional 80uF for a total of 146uF and although the hum decreased, not as much as I thought it would. In the Champ copy I built, just going to 40uF seemed more than enough. Why would this beast require so much more filtering? Does filter size vary exponentially with number of power tubes?

    So, is something else going on here? I have seen other posts recommending to break in the power tubes. I certainly will do that, but I think I’m going to have to engineer a fix.

    Will a choke be more effective than a huge reservoir capacitor?

    Might I have to put in an RC filter before the plate supply to sacrifice some power for better filtering?

    Sorry for all the questions... last time I solved this by just adding more capacitance, but this time I’m not so sure that will work.
     
  2. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    How about ground loops?

    Can you show --- schematic - layout - ground scheme - pictures of this beast?
     
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  3. gusfinley

    gusfinley Tele-Afflicted

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    got humdingers?
     
  4. 53Strat

    53Strat TDPRI Member

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    OK, Say you remove the 120Hz component. What is all that other crap going on there?

    I'd be putting the app to work on the power outlet and looking at what is there to begin with.
     
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  5. tweedy_woodpecker

    tweedy_woodpecker Tele-Meister

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    Adding a choke or dropping resistor with the proper dissipation values before the plates will definitely help. I had the same problem in a PSE amp and no amount of other filtering helped as much as in my case moving the choke from between the screens and plates to before the supply if the plates. Just be sure the choke or dropping resistor can handle the current draw of the whole amp.

    And yes, you will loose some voltage but most of the time people a trying to get rid of some voltage in their amps anyhow.
     
  6. Bitsleftover

    Bitsleftover Tele-Meister

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  7. tweedy_woodpecker

    tweedy_woodpecker Tele-Meister

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    Yes, elevated heater supply is also a good idea.
     
  8. andrewRneumann

    andrewRneumann Tele-Meister

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    I’d love to do that, but this is just a free app that uses the onboard microphone. I should have explained that. What’s depicted is what’s coming out of the speaker and all the other ambient noise in my shop. I wouldn’t call it precise...
     
  9. andrewRneumann

    andrewRneumann Tele-Meister

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    I did a little experimentation with clipping in capacitance at the different supply filters. The only place it made any difference was the plate supply. (But as I said earlier, not a big enough difference to satisfy me.) Additional screen supply capacitance made no difference. My master volume is feeding the power tube grids, so when I have the master volume down to zero and I’m hearing a good amount of hum, I’m pretty confident this has to be power amp.

    But based on your experience with a PSE amp and similarity of problem, I think I’m going to have to look at installing a choke before the plate supply.
     
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  10. mgwhit

    mgwhit Tele-Holic

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    My understanding was that (at least in the US) 120 Hz hum was most likely from the filament wiring than the main B+ line, which should be rectified to 60 Hz. I think you're looking in the wrong place.
     
  11. andrewRneumann

    andrewRneumann Tele-Meister

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    Unless I made a mistake somewhere, the only place the circuit (not counting safety ground) touches the chassis is a ground wire from the input jack to a screw about 1” away. My input jack and speaker jack are both isolated.

    I’m working on a legible schematic. This was a home brewed design so there has been a bit of experimentation and the schematic doesn’t always catch up...
     
  12. VintageSG

    VintageSG Friend of Leo's Ad Free Member

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    The harmonic is likely caused simply by the fact it's an amplifier. The hum is on the signal lines, it gets amplified. Single ended amps generate lovely harmonics. You have another around 480Hz too.

    What diodes are you using, and in what configuration? Sometimes, it can be of benefit to use clamping capacitors in parallel with each diode. Not all 1N4001 are created equal. Some are more equal than others.

    Can you inject a 1KHz signal and then subtract it from your 'scope trace at each signal point to see where the noise is coming from?, it may be induced rather than introduced, or is it after the first node?
     
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  13. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    I work like you do. So many here on TDPRI have taken the time to carefully blueprint every little thing. I just slap things together by comparison. I have taken to documenting the repairs and mods I have done on the amps I repair. I have had too many *word of mouth* requests to duplicate "My friend's amp" that I modded 2 years ago. The documentation is more of just notes to myself rather than a drawn schematic/layout.

    Anyway, filtering is not always the cause of 120 cycle hum. There are several other places to look. So it would be best to post something for us to get our collective eyes on.
    That is exactly where you will find this noise.

    :lol::lol::lol:
     
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  14. FenderLover

    FenderLover Poster Extraordinaire

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    That's exactly backwards. The filaments are not rectified and are at 60Hz. The B+ is rectified and is at 120Hz.

    I'm sure you meant 1N4007, but try the UF4007. They have less switching noise and are popular with HiFi. And me.
     
  15. VintageSG

    VintageSG Friend of Leo's Ad Free Member

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    Quite right, my bad.
     
  16. rangercaster

    rangercaster Poster Extraordinaire

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    Get another amp ...

    If that doesn't help..
    .
    Buy a different house...
     
  17. Old Deaf Roadie

    Old Deaf Roadie Tele-Afflicted Silver Supporter

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    One thing I learned from chasing submarines, which is entirely based on discreet frequencies & their harmonics, is that it is entirely possible for 60 Hz the be the root frequency, and it is also possible you have a minor hotspot at 40 Hz. Guess where the 1st harmonic of 60 Hz & the 2nd harmonic of 40 Hz lands? You guessed it: 120 Hz.
     
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  18. andrewRneumann

    andrewRneumann Tele-Meister

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    I elevated the heaters to power tube cathode voltage (+17VDC). This did not change/improve the hum with the master volume at 0. Still hunting...
     
  19. Bitsleftover

    Bitsleftover Tele-Meister

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    I did that first and it didn’t work. I had to set up a voltage divider off my B+ to give about 40-45v to get the noise to go away. But then it was like flipping a switch.
    My problem wasn’t with master volume on zero though. That is strange.
    For the sake of tacking in two resistors, I’d give it a try.
    Good luck. It can be infuriating can’t it.


    Sent from my iPhone using Tapatalk
     
  20. andrewRneumann

    andrewRneumann Tele-Meister

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    I’m working on generating an accurate schematic. They are Vishay 1N4007GP-E3/54. I have the standard two rectifier diodes feeding full wave to a standby switch, and then a third diode after the switch to keep DC off the switch.

    This got me thinking to bypass the standby switch and third diode just as a test to rule out anything from that setup.

    Thanks for your patience everyone.
     
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