Triode 6g15 Build Hum

Discussion in 'Shock Brother's DIY Amps' started by amazingalf, Apr 9, 2020.

  1. amazingalf

    amazingalf TDPRI Member

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    Hi everyone, I've been lurking around here for a few builds but I'm finally stumped (or my luck ran out).

    I built a Fender 6g15 clone from Triode. Sounds pretty good, but overall I'm getting more hum than I would like. Sizzle might actually be a better word. I'm fairly certain this is a ground issue. I have the reverb unit plugged into a mojotone f51 build I made which is very quiet.

    Things I think could be potential problems (or nothing):

    • The PT is inside the chassis
    • I have a ground wire running parallel to two of the filter caps
    • There is a third wire between the rca cables that is not connected to anything

    The hum is increased as the tone is turned up.
    The hum is higher pitch, almost sizzly with the footswitch plugged in and in the on position
    The hum is louder if I touch the footswitch with my hand
    The hum is the same with the tank inside and outside the enclosure

    I've read many of the hum threads here but I think I need a little push in a specific direction to fix the problem.

    I've uploaded a 40 second sound clip walking thru what makes the hum louder and quieter here: https://www.dropbox.com/s/txevjocweqq9oa2/reverb sounds.mp3?dl=0

    I've also added some gut shots to this post.

    Any help is greatly appreciated! I'm happy to provide more photos or anything else that would help diagnose the problem.

    The triode wiring diagram is here: https://neren.paragonrels.com/publi...fb811a-d52b-446f-8fc8-9e40c910ee10&Report=Yes
     

    Attached Files:

  2. Snfoilhat

    Snfoilhat Tele-Afflicted

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    Sounds a little like wifi or power supply noise from nearby electronics

    --not to the exclusion of other causes and their potential fixes
     
  3. amazingalf

    amazingalf TDPRI Member

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    Just tried the unit in a different room with the same result.

    The fact that touching the footswitch makes this noise louder makes me lean more towards some kind of ground issue.

    I'm wondering if this sizzly noise I'm hearing is 120 hz hum...

    Thanks for the suggestion tho!
     
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  4. tubeswell

    tubeswell Friend of Leo's

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    Grounding suggestion for reduced ground loop hum

    The 'low current' returns are green.

    The 'high current' returns are red.

    Its important to separate the returns in clusters this way - in order to minimise the 'micro-wobble' (present in the hi-current returns) from interfering with the low current returns and the low-current returns (pre-amp) filter cap. Otherwise, any AC microrises in ground potential from the high current returns can get reverse-shunted into the B+ rail for the pre-amp (from the pre-amp filter cap riding on a high(er) AC ground return current) - and voila! you have ground hum appearing in the signal.

    All the phone and RCA jacks sleeves are isolated from the chassis and connected separately to the signal ground buss.

    The only connection between the signal ground and the chassis is via the hum loop blocker.
     

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    Last edited: Apr 10, 2020
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  5. amazingalf

    amazingalf TDPRI Member

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    Thanks for the reply @tubeswell ! I'm going to sit down and begin documenting the differences between my current build and the grounding suggestion schematic.

    At first blush the rectifier portion appears to have an additional diode as well as some caps involved. Would you advise I alter my circuit to more closely resemble this? Do you happen to have a wiring diagram or gut shot of this schematic in action? I'm curious to see how the two separated returns look.

    I also found Valve Wizard's Grounding document which I will look at as well to better my understanding. That is located here for those interested: http://www.valvewizard.co.uk/Grounding.pdf
     
    Last edited: Apr 10, 2020
  6. tubeswell

    tubeswell Friend of Leo's

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    The schematic I uploaded is literally the ‘wiring diagram’.

    The signal ground buss is a bit of thick gauge mild steel or copper wire, with the hum loop blocker at one end. The other ‘floating’ end is where all the high-current ground returns get attached. The red and green lines are literally bits of wire. Obviously you want to keep each length of wire as short as possible, whilst also maintaining the high vs low current clusters

    The caps on the rectifier diodes are high-voltage snubbing caps to help suppress diode-switching noise (because its a power supply for a reverb circuit - and reverb recovery stage is particularly sensitive to S:N). 1N-series diodes typically exhibit a (small) switching transient voltage as they go from ‘on’ to ‘off’, which shows up as a voltage spike that gets inducted back through the PT to other windings (heater winding), and thence shows up as ‘hum’ at the cathodes (because it is ‘riding on top of’ the heater a/c). You don’t hear this in normal preamp gain stages, but in reverb recovery stages, any extraneous noise floor occupies a bigger portion of the signal that gets amplified. Alternatively, you could use UF switching diodes, as these have a much quicker time period between on/off.
     
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  7. amazingalf

    amazingalf TDPRI Member

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    Thanks for the information! I think I have an understanding of how to proceed forward. My plan is to build the signal ground buss with the hum loop blocker. I will also replace the jacks with insulated ones.

    Seems like insulated RCA jacks are a little tougher to come by. Currently looking at these: https://www.parts-express.com/parts-express-chassis-mount-rca-jack-pair--091-1120

    I plan on also replacing the diodes with UF switching ones. I believe that these will do the trick: https://www.digikey.com/product-detail/en/diodes-incorporated/UF1007-T/UF1007DICT-ND/248647

    Does the composition of the capacitor used in the hum loop blocker matter?

    I'll report back, hopefully with a nice quiet 6g15 clone! Thanks again!
     
  8. Mike_LA

    Mike_LA Tele-Afflicted

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    I've built 3 triode 6G15s. The last one had a nasty humm. I started removing ground points and it went away. Had it double grounded somehow to make a loop. Best of luck
     
  9. tubeswell

    tubeswell Friend of Leo's

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    Any old sturdy film cap about 0.1uF
     
  10. BobSmith

    BobSmith Tele-Meister

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    I can’t link to your schematic. Is hum present with reverb footswitch unplugged?
     
  11. amazingalf

    amazingalf TDPRI Member

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    Hi @BobSmith ,

    This link should do the trick!
    http://site.triodestore.com/6G15ReverbLayoutV2.pdf

    I believe that the schematic is identical to the original 6g15 expect for two things:
    • The AC Cord ground (which is essentially just slapped onto the chassis)
    • The .047 uf cap on the Triode replaces the .05 in the original

    The hum is not nearly as sizzly or loud when the footswitch is unplugged.
     
    Last edited: Apr 11, 2020
  12. amazingalf

    amazingalf TDPRI Member

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    Last edited: Apr 11, 2020
  13. Snfoilhat

    Snfoilhat Tele-Afflicted

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    Fender-style reverb footswitches bring an electrical lead connected directly to the grid of the amp's (or reverb unit's) reverb recovery gain stage way out across your floor or looped in the back of a combo cabinet and terminating at the footswitch. Open or closed, shielded or not, it picks up some degree of radio frequency noise.

    The easiest solution is for everyone whose playing doesn't necessitate footswitching to just leave them out. But I'm eager to see if the kind of ground-loop reducing measures some people add to the 6G15 also helps mitigate this. Good luck!
     
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  14. amazingalf

    amazingalf TDPRI Member

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    Based on the PT 47609 power transformer what would be the best move for rectification?

    It seems like following the diagram on the surfguitar 101 site with the addition of insulated jacks and a ground loop blocker would be the best bet.
     
    Last edited: Apr 11, 2020
  15. BobSmith

    BobSmith Tele-Meister

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    Agreed. I think that’s what’s happening here. Fender used some odd cable for their classic foot switches (twin reverb, deluxe reverb) where the reverb is shielded, but the vibrato is not...weird.
     
  16. BobSmith

    BobSmith Tele-Meister

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    I’m in the process of tidying up a Revibe which basically is the 6g15 plus vibrato. I employed the ground lift circuit tubeswell refers to. I placed a switch so I could go back and forth between traditional and lift ground patterns.

    With everything on, it’s hard to detect a difference but when you turn off the reverb unit, you can see the difference thru the amp by (using my switch or) unplugging the reverb unit from the wall (breaks ground connection and any possible loops).

    I can definitely tell the lift circuit is quieter. But I think YMMV depending on where you are using it.


    One subtle detail on Tubeswell’s diagram is missing that the shorting pin and jack sleeve are connected. That essentially completes a path to chassis ground thru the instrument cable shield to the neighboring amp. But after looking closer into this, that seems to be by design. The voltage drop across the resistor in the lift circuit will limit the signal traveling thru the instrument cable, and hence not eliminate a ground loops, but rather minimize it as much as possible.
     
    Last edited: Apr 11, 2020
  17. amazingalf

    amazingalf TDPRI Member

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    So after looking at several schematics and diagrams, it seems like I should be able to utilize jbennett's schematic to convert this to full wave rectification, in combination with the grounding schematic by tubeswell. I will also need to isolate all jacks including the RCA ones. This leaves me with a couple questions:
    • Should B3 be grounded to the signal portion of the bus bar, separate from B1 and B2?
    • I should change my current wiring to have the live wire connect to the switch and neutral to the fuse (like jbennetts, not like the triode wiring diagram)
    jbennetts Schematic: https://s3.amazonaws.com/sg101.forum.photos/kfzY5bPpS8SGeG2_QTCbgQ.jpg
    @tubeswell would you be able to let me know if my logic is correct here? Is there anything I missed or you would suggest? Much appreciated!
     
  18. tubeswell

    tubeswell Friend of Leo's

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    Yes the preamp filter cap ground should be kept with the preamp ground returns cluster at the loop-blocker end of the buss. (This is the cap that supplies the preamp, and you want to keep all the preamp ground returns together). The other 2 filter caps (which together with the choke) form part of the CLC filter, should be clustered at the other end of the signal ground buss (with the ‘high-current’ ground returns)

    For the mains fuse, the active (phase) mains wire should go straight to the tip terminal of the fuse holder, and the ring terminal of the fuse holder should go to the mains switch. The other pole of the mains switch goes to one end of the PT primary. The mains neutral wire goes to the other end of the PT primary (you can secure this connection with an insulted screw terminal - saves having to muck around with trying to solder two bare wire ends together and insulate this connection)

    The mains earth wire requires its own separate dedicated chassis connection. Crimp a ring terminal tightly to the bare wire strands before any solder is applied. This will ensure good mechanical connection if the solder melts. Then flow solder into the crimped connection for good electrical connection. Then bolt the ring terminal tightly to the chassis with a locking nut (nylok nut or similar). The mains earth has to be secured to the chassis - this is essential.
     
    Last edited: Apr 11, 2020
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  19. amazingalf

    amazingalf TDPRI Member

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    Awesome! I will report back with the results!
     
  20. amazingalf

    amazingalf TDPRI Member

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    So my 6A diodes came in, and it looks like I made the mistake of ordering ones rated for 1KV (they're quite large). For the hum loop blocker can I still utilize these? Is it possible to use a 1n4007 instead on the 6A?

    If they need replacing what voltage rating would I want to aim for?
     
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