Grounding question on 5f11 Vibrolux

Discussion in 'Amp Tech Center' started by unezrider, Apr 4, 2020.

  1. unezrider

    unezrider Tele-Meister

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    Hello friend,
    I'm finally getting around to building my Vibrolux, & have a couple of questions regarding some of the grounds. I'm mainly following the current 5f11 wiring diagram from Mojotone.
    They have the green from the cord & the CT from the PT grounded together. I know I can just solder them to the side of the chassis. But would it work the same if I put a solder lug under one of the screws for the PT & soldered them to that? Seems the would be easier, as I recall soldering to the chassis side was a bit of a pain with those thicker wires on my last amp build.
    The two resistors coming off the pilot light would also be ground to the chassis, correct?
    Also I've noticed other builders using a bus wire across the pots to a ground. This would be unnecessary since this diagram has a ground coming off of each section, right?
    Thanks for the help, its appreciated
    Solder_Lugs_for_Amps.jpg.png Tweed-Vibrolux-WD---Bias-Pot.gif
     
    Last edited: Apr 4, 2020
  2. dan40

    dan40 Tele-Afflicted

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    The safety ground from the cord should never be grounded with anything else. It needs it's own dedicated ground lug, bolted to the chassis with a lockwasher and nut to ensure a solid connection. The PT's centertaps can then be grounded using a solder lug to the PT bolt, or better yet, drill a new hole and use a ground lug to ground them directly to the chassis. I try to avoid using the PT mounting bolts unless absolutely necessary.

    You will need a ground buss to connect each of the preamp grounds to. Ideally this should terminate at the input end of the chassis. The PT grounds, main filter and screen caps can ground at the PT end of the chassis. I would avoid using a buss mounted to the pots and instead, mount a buss directly along the top edge of the board to connect those preamp grounds and preamp filter cap.
     
  3. King Fan

    King Fan Poster Extraordinaire

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    ^^^ what he said. Exactly what he said.

    Mojo's a good outfit, and some of their layouts are OK, but you need to ignore or improve their AC power wiring, and in this case you need to improve on their ground scheme. That up there is "come back with us now to the distant day of bobby socks and poodle skirts..."

    Our wise friend Rob shows a nice solution here for both AC wiring and ground bus on his 5e3. You can steal the AC wiring (note *black* to fuse, then switch) and adapt the ground scheme to yours. And those two 100R resistors off Mojo's lamp would be perfect to anchor to his Power Amp Ground -- your PT presumably doesn't have a heater center tap? Finally, the HV center tap wiring shown here will automatically upgrade your amp to First Class...

    upload_2020-4-4_19-20-7.png
     
  4. dan40

    dan40 Tele-Afflicted

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    As King Fan pointed out, Rob's 5e3 layout will work great for your grounding and ac power scheme. Notice how he splits the three main filter caps...the first two are the main and screen caps which ground to the PT end of the chassis and the third cap (preamp cap) gets added to the preamp ground buss which attached to the input jacks which are already grounded to the chassis. The red/yellow centertap from the PT gets moved directly to the negative lead of the main cap (for extra hum rejection) and then a single wire leads from there to the solder lug grounded to the chassis. This layout uses a second green/yellow centertap which you can ignore unless your PT actually has one. The PT that came with the kit most likely does not have the green/yellow CT and that is the reason that you will need the two 100 ohm resistors from the pilot light to ground.

    To install the grounding buss, you could use insulated standoffs to secure it in several spots on the chassis or simply solder it to the required turrets and bend it out away from the edge of the board just slightly.
     
  5. unezrider

    unezrider Tele-Meister

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    Thank you, fellas.
    If I were to use that AC wiring, would I just cap off the white wire? & would I still take my 0v from the PT to the switch & my 120v to the fuse?
    I'll probably be back to clarify the grounding. Also, is this other way more reliable?
    Here's the tranny I'm working with, so you know.
    Also including an older mojo layout I had printed up a while back when I first became interested in doing this project.
    IMG_3990.jpg oldmojo5f11.jpg
     
  6. unezrider

    unezrider Tele-Meister

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    a note: it says grn/blk, but it is a grn/yel

    so since I do have a center tap on the filaments I could just ground that, lose the 100 ohm resistors, & wire the filaments as shown?
     
    Last edited: Apr 5, 2020
  7. dan40

    dan40 Tele-Afflicted

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    You will have a black (hot) and a white (neutral wire) coming from your power cord. The black must run to the fuse first, then on to the power switch. On the other side of your power switch, connect the black/brown pair from your PT. The white wire in the power cord will connect directly to the white and brown/white pair from your PT. The remaining black/white lead from the PT will be insulated with heatshrink on the end and tucked away safely. The reason for this way of wiring the AC side is the current electrical code standards. It is much safer than the older way that was used when these amps were first built. Mojotone really needs to update their layouts to reflect the new codes and stop using these older styles.

    Since your PT does have a green/yellow CT for the filament, you can remove the two 100 ohm resistors and simply connect the CT directly to ground.
     
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  8. unezrider

    unezrider Tele-Meister

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    Thanks, Dan
    & thanks for explaining the 'why'. Pretty sure I wired the AC on my 5f2a very similarly.

    I made a drawing, minus the filaments going to the lamp, & then down to the 6v6s.
    & for clarities sake, I would ground both CTs? & can they be ground together, or should I keep them separate? Or is this where I take the HV CT & connect it to the negative end of the first filter cap like in Rob's 5e3, then ground the grn/yel CT?

    acwiring.jpg
     
    Last edited: Apr 5, 2020
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  9. dan40

    dan40 Tele-Afflicted

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    That looks good. You can ground the two centertaps together on a ground lug or, for even quieter performance, ground the red/yellow CT directly on the main filter cap's negative lead. Either way are electrically the same but the second way is extra insurance against grounding hum. You can install a solder lug on the PT bolt for all of your power grounds at that end of the chassis but I prefer to drill an extra hole and mount the solder lug exactly where I want it to keep the wiring layout nice and neat. On the far end of your chassis will be the preamp ground point which helps to keep the high current power grounds away from the lower current preamp grounds. You can terminate this buss directly at the sleeve of the input jack because the Switchcraft jacks are naturally grounded to the chassis when installed. Some folks prefer to drill and install a second ground lug near the input jacks and run the preamp buss directly to that. You can then solder a short wire from the input jack sleeve directly to this ground lug for extra insurance in case the jack ever becomes loose.
     
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  10. schmee

    schmee Doctor of Teleocity Silver Supporter

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    Yes just ground it to a PT bolt. Some say they dont like that, but I have done it a dozen times without issues, and have seen it in amps from the factory. I would separate the CT and Green ground though. Although I can't give data why, sometimes these locations matter in amps.
     
  11. unezrider

    unezrider Tele-Meister

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    Thanks a bunch, guys. Feel I'm learning a bit here. I'm guessing if I follow this layout as we have it so far, I don't need to be concerned about the warning sticker Mercury put on the filament CT? Turns out it is grn/blk, not grn/yel. If that even matters.
    Also, if I bring the red/yellow CT to the negative of the first filter cap, would the wire I have coming off to ground need to be from that same eyelet, or could I have it from the far left top one, as highlighted in my drawing?
    Also, would I connect all of those eyelets, as in drawing, together? But not connect to the last cap? This way everything on the PT side is together, if I'm understanding this properly.

    warn.jpg grndq.jpg
     
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  12. dan40

    dan40 Tele-Afflicted

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    There are certain instances when grounding the filament centertap could cause problem in certain circuits. Amps with DC filament supplies and channel switching power supplies could cause problems depending on how the circuit is set up. Your circuit will be fine with a grounded CT but it may be best if you stick to the Mojo layout and use the two 100 ohm resistors to create an artificial CT. Just be sure to heatshrink the end of the CT and tuck it away.
     
  13. Lowerleftcoast

    Lowerleftcoast Tele-Holic

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    Off topic:
    Since I saw the layout diagram you have above, I wanted to make sure you did not run the other wires from the PT the way they have on the diagram. The red, yellow, and green wires are all AC. They are twisted to help reduce hum. The green heater wire is usually twisted between each socket. Here is a picture for reference. DSCN1426[1].JPG
     
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  14. unezrider

    unezrider Tele-Meister

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    Thanks, Dan. I decided to email Mercury & ask them, & see what they say too. We'll see if anyone is working in that department right now...
    Assuming I take the Mojo route, I can still send the red/yellow to the first cap, & lead that off to ground, correct? & would that ground lead have to come from that same eyelet, or any along that far left row?

    & no worries LowerLeftCoast. I haven't started that part yet. waiting till I'm 100% confident on my layout. Also, I've built a 5f2-a before & got pretty good (more or less) at the winding thing. Here's the PT from that build.

    5f2apt.jpg
     
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  15. King Fan

    King Fan Poster Extraordinaire

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    What part(s) of the Mojo route are you planning to take? I suspect you mean about using an artificial center tap (2 x 100R) instead of grounding the heater CT? As Dan says, that'd work. The simple rule is 'one or the other, but not both.'

    I hope you don't mean taking the Mojo route on the AC power winding, or the grounds! But your drawings suggest you've got that now.

    Well, you could; as long as the HV CT red/yel goes straight to the first filter cap negative to soak up the ripple return current, 'ground is ground.' OTOH if you have a couple inches of extra black wire, you can let the power amp ground bus connect the bias ground back to the reservoir cap and run the black wire from there. This might be 1% more 'explicit' -- I'm no expert, but I have the impression the bias circuit doesn't handle much current.

    Good idea about asking Mercury. They're said to have excellent customer support. For as much as that PT musta cost you, you seriously deserve something less cryptic than that warning, and for that matter those wiring instructions, and that green/yellow wire they ID'd as green/black...
     
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  16. Lowerleftcoast

    Lowerleftcoast Tele-Holic

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    This PT can be used on several amp models. I bet Mercury will tell you the warning sticker is for the end user that is replacing a PT. The original circuit may have had an artificial CT. Rather than tell the end user to change the circuit over to a hard wire CT, they tell them to continue with the artificial and not use the hard wire CT.

    For this unezrider build, I would use the CT and not go the resistors route. OMMV.
     
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  17. unezrider

    unezrider Tele-Meister

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    Thanks, guys
    I did hear back from Mercury. & he told me exactly what you guys have said. & that I would be fine using the CT as long as I don't use the resistors.
    & yes, Kingfan, referring to the resistor part. I'm definitely going to wire the AC like you guys have suggested, & how the drawing shows.
    In regards to the preamp grounds, would there be any difference in using a bus wire, like in Rob's 5e3 layout, than using a green wire as in the 2nd Mojo layout (under the PT schematic)? & does it matter if I start it on the 2nd or 3rd input jack?
    I also noticed the original Fender layout (& the first Mojo I posted) has the resistors or lugs, depending, ground directly to the back of the pots. Am I better off avoiding that & running a line down to a bus wire as in Rob's?
    Thanks again, fellas. This has been very informative.
     
  18. Tele Slacker

    Tele Slacker Tele-Meister Silver Supporter

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    dano... I’m always seeking to better understand.... why? What is the problem or issue with mounting to the PT bolts? Not challenging... only wanting to better understand.
     
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  19. King Fan

    King Fan Poster Extraordinaire

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    Good question. Dan knows more'n me, but he's been doing a lot of the good work here, so let me do something besides +1 his posts. :)

    The mechanically and electrically separate safety ground no doubt comes from the idea you don't want *anything* interfering with the solidity and permanence of the safety ground. The circuit grounds help your amp work. The safety ground keeps you working and your house standing. It's also UL and EU code and so more than just 'best practice' -- it's 'correct practice.'

    At this point someone might ask why not a separate PT bolt? The problem with PT bolts, as I understand, is that many experienced amp techs here say it's common to see loose PT bolts. Mass + vibration = trouble. I imagine that's why Dan also said if you 'must' use a PT bolt for your power amp ground, double or triple your security. Even on a *separate bolt*, many of us use star washer + crimp-on ring terminal + keps bolt + thread locker.

     
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  20. Lowerleftcoast

    Lowerleftcoast Tele-Holic

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    In today's society, when the house burns down or the end user is shocked, the legal term you will hear is negligence.
     
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