Grounding: solder or screw?

[gtrrich]

Hey all. Happy Thanksgiving.

I am building my first amp, a Brown Princeton based amp. Is it better to solder ground connections to the chassis, or is it just as good to use screws/nuts to make the ground connections. So far, I haven't been able to solder to the chassis. I'm sure my soldering iron is not designed for that. Thanks for your opinion!

[defaced]

Scroll down till you find R.G.'s post.
http://music-electronics-forum.com/t15138/

[tubeswell]

I solder the ground wire to a solder lug, which I attach to a chassis screw/PT bolt, held down with a steel nut, and then a lock nut on top.

[txstrat]

I usually use a bolt and a star washer over the solder lug, to keep the nut tight and improve the connection. Sometimes I put some super glue on the tightened nut to keep it from loosening (to ease myself).

[rudutch]

While I realize this is electronics, not electrical...
For electrical installations, soldering is unacceptable for power grounding. There is normally some very solid logic for the NEC's rules..

My $.02 is NOT to solder a power ground


NEC 250.8 [2005]
–"Grounding conductor and
bonding jumpers shall be
connected by exothermic welding,
listed pressure connectors, listed
clamps, or other listed means.
Connection devices or fittings that
depend solely upon solder shall
not be used. Sheet metal screws
shall not be used to connect
grounding conductors or
connections devices to
enclosures."

[gtrrich]

Thanks everyone for the replies. I can see the importance of the AC power ground lug, and will most likely use a screw/nut/lockwasher for that. I am also wondering about the grounds in the preamp, such as cathode resistors, etc. I notice on my mid 70's Fender Champ, there are a lot of soldered grounds coming from the board, and the preamp tubes. (See pic attached) Do they have to be soldered or is it just as good to use screws or a ground lug in those areas?

FWIW, I'm not sure if the soldering in my Champ is factory work. It almost looks like someone has changed some things in there.

Thanks!

[defaced]

The ground lug from the green wire of your AC power cord to the chassis is there to save your life if something goes wrong. Since you don't want it to fail, that gets crimped into a fitting then machine screw/star washer/nut like R.G. describes in the link I posted.

The ground of the amp circuit to the chassis is often done through the input jack and is really just a reference point. I would say that doesn't require the crimp fitting/screw/star washer/nut deal, but I could be convinced otherwise.

The internal circuit grounds, like the cathode and the volume pot and all that fun stuff, just need a reference, you won't die if they lose their connectivity, so soldering those is fine.

[R.G.]

I solder the ground wire to a solder lug, which I attach to a chassis screw/PT bolt, held down with a steel nut, and then a lock nut on top.

I'll reproduce it here, because I think it's important. Notice that I do not guarantee that this is safe in all cases, only that I've seen it done this way, and I know of reasons why eliminating any of these items may not be safe - which is different!

Modern safety practice says to crimp a ring terminal onto the green wire and bolt this down to a dedicated bolt/nut in a specific way.
- strip the green wire but do not tin it with solder; solder may cold-flow under pressure of a crimp, loosening the crimp and causing a high-resistance connection of wire to ring terminal
- crimp the ring terminal onto the green wire; you may now also solder after the crimp is holding onto the wire, as solder will not penetrate the gas-tight high pressure connection areas that the crimp forms. The solder is a kind of backup, not the primary connection.
- abrade an area around the bolt hole down to bare metal
- insert the bolt
- place a toothed washer over the bolt
- place the ring terminal at the end of the wire over the toothed washer
- place a second toothed washer over the ring terminal
- thread a nut down on the bolt
- tighten the nut to compress but not flatten the toothed washers.

This process gives the toothed washers enough pressure to bite into the chassis, ring terminal, and nut. The bite on the bottom washer causes multiple gas-tight high pressure connections for low impedance between the ring terminal and chassis. In the case of an AC power fault in the amp wiring or power transformer, these connections will have to carry tens to hundreds of amperes until the fuse can clear. The bite on the top toothed washer prevents the nut from loosening.

In particular, the bolt must be dedicated to the AC power ground, not used for another purpose, such as mounting a transformer.

While I realize this is electronics, not electrical...
For electrical installations, soldering is unacceptable for power grounding.

Yep. That's because solder is so close to its melt-and-flow point at room temperature. It's so close that it cold-flows, doing plastic deformation under even minor pressure. This is why it is critical not to rely on solder for mechanical holding - you almost can't hold solder. Any pressure and it slowly flows like very stiff gelatin. What you can do is to first mechanically swage or crimp connections to make electrical connections which do not rely on solder for strength. After that, if you solder the connection, which would have been fine on its own, you just enhance the conductivity that was already there, and also seal out atmospheric gasses even further. This is no real help to the safety aspects ( I don't think... ) but you can if you want, I guess.

The ground lug from the green wire of your AC power cord to the chassis is there to save your life if something goes wrong. Since you don't want it to fail, that gets crimped into a fitting then machine screw/star washer/nut like R.G. describes in the link I posted.

The ground lug from the green wire of your AC power cord to the chassis is there to save your life if something goes wrong. Since you don't want it to fail, that gets crimped into a fitting then machine screw/star washer/nut like R.G. describes in the link I posted.

The ground of the amp circuit to the chassis is often done through the input jack and is really just a reference point. I would say that doesn't require the crimp fitting/screw/star washer/nut deal, but I could be convinced otherwise.

The internal circuit grounds, like the cathode and the volume pot and all that fun stuff, just need a reference, you won't die if they lose their connectivity, so soldering those is fine.

Yes - what you said! It's the AC safety ground that's important to affix solidly to the chassis for safety reasons.

[Bruce / Mission Amps]

Thanks everyone for the replies. I can see the importance of the AC power ground lug, and will most likely use a screw/nut/lockwasher for that. I am also wondering about the grounds in the preamp, such as cathode resistors, etc. I notice on my mid 70's Fender Champ, there are a lot of soldered grounds coming from the board, and the preamp tubes. (See pic attached) Do they have to be soldered or is it just as good to use screws or a ground lug in those areas?

FWIW, I'm not sure if the soldering in my Champ is factory work. It almost looks like someone has changed some things in there.

Thanks!

It looks like most all of the wiring, soldering and parts are fairly original to me.
The negative feedback lift/defeat/elimination switch and wiring is not stock and it looks like the power tube cathode bypass cap and resistor are not stock.
Also, those cathode resistor bypass caps, the silver bypass E-caps and the yellow Sprague E-cap for the power tube, are probably old enough to experiment with replacements now.

[Diablo]

Can someone please explain what's wrong with using the transformer mounting bolt as the AC cord ground mount? I use the crimped eyelet method and a stainless steel locknut (nylock) to hold it in place. So far, it hasn't killed me yet.....but I'd like to keep it that way...

[R.G.]

Can someone please explain what's wrong with using the transformer mounting bolt as the AC cord ground mount? I use the crimped eyelet method and a stainless steel locknut (nylock) to hold it in place. So far, it hasn't killed me yet.....but I'd like to keep it that way...

That actually used to be accepted by the safety geeks, but no longer.

The problem - I think, just speculating here - is that sometimes the transformer mounting foot is covered in varnish or paint, and may not make solid contact. Even if it's bare metal, it's flat surface doesn't necessarily bite into the chassis metal like a toothed washer would, so there are not all those gas tight high pressure metal to metal contacts. The whole point of the grounding wire for third wire ground is to contact the safety ground wire to the metal chassis firmly and reliably. Again, just speculating. It is possible that they thought that people were more likely to loosen the bolt for other purposes and forget to tighten it again. Again, just a guess.

I do know that the bolt used for grounding must not be used for any other purpose. That is stated explicitly in the standards as I remember. If you're doing crimped ring terminals, I'd recommend a separate toothed washer on each side of a flat ring terminal. The bottom one bites into both chassis and ring terminal, the top one keeps the nut from loosening. But I don't know if that's technically correct, or if a safety lab would pass it. I think they would, but there are no guarantees, and it may well depend on whether the tester liked the burrito he had for lunch.

Stranger things have happened. Testers express machismo by having you make changes, not by telling you that you did it right. Seriously. No safety tester wants to be remembered as a softie that you could get anything past.

There is real safety, which only God knows, and then there is conformance to safety standards, which testers interpret. Those are different.

[dtjacob]

One other thing.......external tooth star washers are FAR FAR better than the internal tooth ones. A little harder to find, but WELL WORTH IT.

[Wakculloch]

The not using transformer mounting bolts thing is perfectly logical if you consider the effects of vibration, dropping, being in the back of a van for extended journeys etc.
These things will have hardly any effect on a bolt holding a wire, but a transformer, due to its weight, will amplify the effects of stress on the mounting points.
If anything is going to shake loose it will be the bolts holding the heaviest.