Steel chassis

[jeff]

 I'm building a new amp and usally I use an aluminum chassis. This time I bought a steel chassis. I had to scrape the paint for ground connections.

Is there anything I should do to treat the bare metal so it will still conduct for my grounds but won't be raw steel and rust out?
Should I just sandpaper it clean and not worry?

[phsyconoodler]

Spray some lithium grease on the bare spot.A small amount will keep it from corroding. Or wax.Or a dab of paint.Get creative.

[supro66]

You buy LUGS from this site
bottom of the page

http://www.hoffmanamps.com/MyStore/perlshop.cgi?action=template&thispage=BoardBuilding&ORDER_ID=445134200

drill a hole mount the lug 3 up to 6 lugs
do not solder to steel NEVER

[jeff]

Spray some lithium grease on the bare spot.A small amount will keep it from corroding. Or wax.Or a dab of paint.Get creative.

Does lithium grease conduct electricity? I don't want to use wax or paint. I need to ground my pots which is why I scraped the paint off in the first place.

[softwarejanitor]

You buy LUGS from this site
bottom of the page

http://www.hoffmanamps.com/MyStore/perlshop.cgi?action=template&thispage=BoardBuilding&ORDER_ID=445134200

drill a hole mount the lug 3 up to 6 lugs
do not solder to steel NEVER

I've got a bunch of old Motorola tube amps from the late 1950s and they soldered to the steel chassis in them from the factory.  These amps are over 50 years old and they still work great.  Other than being dusty they usually come out of the old record player consoles looking practically brand new.

[supro66]

You buy LUGS from this site
bottom of the page

http://www.hoffmanamps.com/MyStore/perlshop.cgi?action=template&thispage=BoardBuilding&ORDER_ID=445134200

drill a hole mount the lug 3 up to 6 lugs
do not solder to steel NEVER

I've got a bunch of old Motorola tube amps from the late 1950s and they soldered to the steel chassis in them from the factory.  These amps are over 50 years old and they still work great.  Other than being dusty they usually come out of the old record player consoles looking practically brand new.

They have the right equipment back then
today for the DIY builder lugs are safer no rework later
They had big irons keep in a fire

[phsyconoodler]

I don't think he wanted to solder to the chassis,but you still have to get the paint off it to make a good ground.
 The idea is to just remove enough paint to make a good ground.Why don't you want to use paint to cover it?It would solve any corrosion problems you may think you have and identify it as a connection to the chassis as well.
 

[supro66]

Here is some good reading

http://www.instructables.com/answers/soldering-steel/

[jojokeo]

I don't like working with or using metal chassis' anymore but if/when I do my suggestion is this: solder your pots together as in Doug's grounding scheme. Forget about lugs, taking paint off for those because guaranteed over time these connections will be spotty and depend on a mechanical connection - this is always "bad form" to rely on for grounding purposes. When you drill metal it becomes hot, this breaks down greatly or totally eliminates any protection that was once there and enhances the corrosion process to begin at those areas. If there are testimonial old-age chassis' that exhibit no corrosion after decades, I can assure you that all holes were made or drilled prior to corrosion protection methods that were done afterwards "to stand the test of time".

Also - you don't need many different areas to become your grounds and/or chassis grounds & often only one is all you ultimately need (*think if you made your amp enclosure out of PVC like Doug's Sch-40 amp as an example)

"My" note on grounding - I only do this to pots for shielding purposes (for them & shielded wire points) and not for circuit ground. I keep circuit ground separate from chassis/shielded wire grounds - prevents more chances of possible ground loops.

[softwarejanitor]

I dunno about any special prep or equipment being used when they soldered on these old Motorola chassis...  It looks like the points where they did the soldering to were just stamped out when the chassis were fabbed and the soldering work looks like normal hand work with an electric iron since it is mostly component leads which wouldn't take a huge amount of heat from a big fire heated iron.

At any rate, I've soldered to these points on these chassis w/o any issues using just a small electric iron, including some to extra points which hadn't previously been used (the exact layout of HS-xxx series chassis vary a little from model to model).  So far no signs of any new corrosion, not that I'm exposing them to moisture or anything of course.

Here is a pic of the under side of a 1958 Motorola HS-621 chassis with some of the more obvious places they've soldered to the chassis marked...

[jeff]

 I don't like grounding to the back of pots for my signal grounds. I run a wire from the lug to ground. I just thought to shield the internal of the pot, the pot has to make contact with the grounded chassis. Instead I should have run a ground buss from pot to pot but this makes it hard to replace pots.

I have three other spots I removed the paint to ground to the chassis: one for the 100ohm heater one for the speaker jack and one for all other grounds. I crimp then solder an eyelet to the wires I want to ground and bolt to the chassis.

[stingray_65]

Just my .02

Unless you live in a particularly humid area or are very coastal, the chassis will not rust.

Using the amp  will quickly dispel any moisture inside the chassis, it gets HOT in there!

All of the above is good advice even though some of it contradicts each other.

Don't solder to the chassis, reasoning: It is a DIFFERENT skill than electrical soldering and in general many techs do not know how to do it properly.

Do solder to the chassis, reasoning: excellent connections with little to no chance of corrosion, EASY to learn skill. Like your soldering skills, first time  you might make it stick, but you got better by doing it.

Solder the back of pots or run a buss wire, again arguments go both ways neither is wrong.

Many of us here on this forum LOVE to go the extra mile and do those things that were cost prohibitive for major amp builders to do in production. Lugs for example. It's cheaper to solder a bunch of wires to a spot on the chassis than it is to pay for a lug and several terminals for the wires to go over the lug and then a nut and a washer to boot. Scraping a spot on the chassis to ensure a good connection when manufacturers rely on the clean bore of the hole to contact the pots mounting bushing. Leo added a brass plate, until he found a cheaper way.

Point being, There is  lot of ways to skin a cat, and the counter point why skin a cat if you don't have to.

My approach is to make a good connection where ever possible, as you can see there are a lot of ways to do that, use the one that works for you and if it doesn't pan out, try it a different way.

I by no means mean to confuse you, I want to encourage you to know your skills and use them and learn a few others on the way.

If you've posted here and own a soldering iron, this will probably not be your last amp.

each one will be better in some ways than the last. LOL Lord help you if you build the perfect amp. you'd have no reason to build the next one and would be so swamped answering emails and post on how you did every little detail you wouldn't have time to play and enjoy it!

Let us know which route you took and how it turned out!

Ray
 

[jojokeo]

Nicely put Ray  (it's so easy to get all wrapped up in the minutia)

[jeff]

 I'll get some pics soon. Can't find my camera battery charger right now.