Weber 5G15/Fender 6G15 reverb grounding

[Greg_L]

I'm about to embark on building a Weber 5G15 reverb kit.






I've been reading about the potential for ground loop hum problems with these units using modern 3-prong power cords. The solution seems to be to use a buss bar grounding scheme with a small ground lift circuit based around a 5w/15ohm resistor and some diodes in parallel.


http://www.valvewizard.co.uk/Grounding.pdf

My question is....is all this really a problem if the reverb and the amp in use are on the same wall outlet? All of my amps and effects are on the same breaker at the house. When I play live, I plug everything I use into the same power strip going into a single outlet. I've never encountered ground loop problems before.

[Enzo]

I don't know if it is a problem or not, remember you are fighting POTENTIAL hum. However, it is often precisely because two things are plugged into the same grounded circuit that you get a loop. If a small difference in chassis potential exists, when you connect the chassis together with say a shielded cord, a current will flow between them. In fact one "cure" for a ground loop is to use a ground lift on one of the power cords.

[Greg_L]

I don't know if it is a problem or not, remember you are fighting POTENTIAL hum. However, it is often precisely because two things are plugged into the same grounded circuit that you get a loop. If a small difference in chassis potential exists, when you connect the chassis together with say a shielded cord, a current will flow between them. In fact one "cure" for a ground loop is to use a ground lift on one of the power cords.

That's kind of what this "remedy" is supposed to achieve. It lifts the signal ground, but doesn't eliminate it. I'm all in favor of the buss bar grounding, but I'm wondering if this lift circuit is even worth it. I suppose it's simple enough to just try it out and see.

I admit I don't know crap about this stuff, it's been my understanding that multiple devices plugged into the same ground circuit is a good thing. It's when multiple devices in use at the same time take different paths back to ground that you get a ground loop. Do I have this wrong?

[Enzo]

Well, do not assume there is ONE thing that causes ground loops. Is it good to have a system with all components sharing an earth? Mostly yes. SOmetimes the mixer is back here and the power amps are up there and no way to do that. But ground loops can occur in BOTH those scenarios.

[Greg_L]

Well, do not assume there is ONE thing that causes ground loops. Is it good to have a system with all components sharing an earth? Mostly yes. SOmetimes the mixer is back here and the power amps are up there and no way to do that. But ground loops can occur in BOTH those scenarios.

I think I can put this "lift" on a switch. Normal grounding one way, 16 ohm/diode/cap lift the other, in case there's hum.

My plan is to use this reverb unit into a variety of amps, but they'll pretty much always be plugged into the same power outlet. I don't want to be amplifying noise though.

[SoulFetish]

In fact one "cure" for a ground loop is to use a ground lift on one of the power cords.

I respect Enzo, and his knowledge and experience. But I really don't favor this approach at all for any piece of equipment with the power cable (IEC, or permanently attached) carrying mains power right to the chassis. Proper safety regulations require proper earth grounding on any part of the chassis which can be contacted. If a fault condition occurred in that piece of equipment, the chassis connection to the earth conductor(if done properly) is designed to handle the fault current for tripping the breaker. Without that circuit, the only other path to earth is through the users instrument cable shield conductor. This is not a low impedance path to earth, and is not an appropriate alternative.
There are circuits which can be used in the power supply grounding which are equally effective in breaking a problematic ground loop. But, if someone did not feel comfortable, or have adequate training to work on the equipment, the another way to break the loop would be to use a good high quality in-line audio transformer.

[Enzo]

I was not suggesting it as a course of action, I was pointing out that plugging two pieces of gear into the same outlet strip is no guarantee against ground loops. And in fact lifting the chassis ground will sometimes break them. I agree, you should do it another way, but it does serve to point out the gear itself is making the loop, rather than the coincident earth connection. That is why I put "cure" in quotation marks.

In fact, 50 years ago, I bought a brand new Crown DC150A power amp, a companion piece to my Crown IC150 stereo preamp. The preamp had a row of 120v outlets on the rear for the other stuff. The power amp CAME with a ground lift adaptor for the plug, and suggested its use to plug the power amp into the outlets on the preamp.

[Greg_L]

I was not suggesting it as a course of action, I was pointing out that plugging two pieces of gear into the same outlet strip is no guarantee against ground loops. And in fact lifting the chassis ground will sometimes break them. I agree, you should do it another way, but it does serve to point out the gear itself is making the loop, rather than the coincident earth connection. That is why I put "cure" in quotation marks.

In fact, 50 years ago, I bought a brand new Crown DC150A power amp, a companion piece to my Crown IC150 stereo preamp. The preamp had a row of 120v outlets on the rear for the other stuff. The power amp CAME with a ground lift adaptor for the plug, and suggested its use to plug the power amp into the outlets on the preamp.

I knew what you meant, and totally lifting the ground was never on my mind. I understand it can be useful as a test, but not the best for a permanent fix.

I'm also way ahead of myself. I haven't even built this thing yet.

[soundmasterg]

You could buy or build a pedal that properly isolates the grounds from each other such as this one > https://megatonemusic.com/products/k...switcher-a-b-y . I have one of these and I can use two amps together that both have a 3 wire grounding scheme without any noise penalties. I plan to use it when I get my Weber Revibe built too.

Greg

[olddawg]

This “fear of grounding” thing amuses me sometimes.. and I’m all for safety and adhering to the new regulations. But I play clubs. Most of the time it’s a crap shoot that an outlet is even grounded at all (and not reverse wired) no matter how shiny new the receptacle is. Sometimes I have time to check it with an analyzer in my kit. I’ve been using a wireless for ever. I do have a volt meter and the analyzer plugged into the power strip on my pedal board.

[Greg_L]

You could buy or build a pedal that properly isolates the grounds from each other such as this one > https://megatonemusic.com/products/k...switcher-a-b-y . I have one of these and I can use two amps together that both have a 3 wire grounding scheme without any noise penalties. I plan to use it when I get my Weber Revibe built too.

Greg

That's pretty cool, and kind of expensive. Interesting. Thanks for the tip.

This “fear of grounding” thing amuses me sometimes.. and I’m all for safety and adhering to the new regulations. But I play clubs. Most of the time it’s a crap shoot that an outlet is even grounded at all (and not reverse wired) no matter how shiny new the receptacle is. Sometimes I have time to check it with an analyzer in my kit. I’ve been using a wireless for ever. I do have a volt meter and the analyzer plugged into the power strip on my pedal board.

I play some dumps too. I play in three bands. Most of my gigs are small/medium clubs with house sound systems that work, but seem haphazardly slapped together. There's one place that I play frequently with a little hand written sign over one of the stage outlets - "No amps on this outlet". Whatever that means. Most of them have a network of questionable power strips linked together.

My worry with this particular project is not electrical shock. It's noise. I'd rather get shocked than have ground loop hum.

[Mick Bailey]

I built a standalone reverb to my own design and used an elevated signal ground as a precaution. However, 1N4007 diodes are not the best choice as in a worst-case fault situation they aren't rated high enough for earth current flow. I used 6A4 the same as Fender uses: 6A, 400v. R.G recommends a 25A bridge for a similar application, plus an R.F. bypass cap. I did temporarily clip-lead across the circuit and it really does make a difference to hum levels.

[Greg_L]

Update on this thing....it's finished. Sounds clips in here VVVV too.
https://music-electronics-forum.com/...ad.php?t=50291

It works great, it's had a little time on it, everything seems fine.

I went back in today just to do some voltage checks now that it's settled in and I know it works, and I noticed my voltages are a little high...about 15-20% on the B+ rail. I know guitar amps aren't that picky, but I'm curious if this elevated voltage is still in the okay range. I'm getting about 123 VAC from the wall, so naturally that will make everything higher compared to the early 60s Fender schematic, but I'm just curious. Should I drop the preamp's B+ node a little further?

[Enzo]

it is working and everything is fine?

Then stop fixing it.

[Greg_L]

it is working and everything is fine?

Then stop fixing it.

Lol fair enough.