Guitar Amp Wiring Notes

[R.G.]

Good tips. I'll get that one. Yep, I used heading1.

Looking good!!!

In your TOC, the first line shows as 'Guitar Amp Wiring Notes." That is because if you check the font of "Guitar Amp Wiring Notes" on the cover page, it is probably tagged as "Heading 1." If you drag across those words and change the font to "Normal" and then change the font size, that will remove it from the TOC. Don't forget to F9 the TOC when you do this.

[SoulFetish]

R.G. I love that you have taken on this project. When I first began building amps, and eventually repairing them, it was not easy to find reliable sources of information that explained how to connect mains wiring correctly and ensure that the chassis was properly bonded to earth ground. I was able to find some copies of UL/IEC safety standards which were helpful in that they provided the criterea that was needed for safety complience, but NO instruction on how to do it.
To add to the confustion, boutiqe amp builders, some diy sites, and a lot of amp techs simply perpetuate really bad practices that are outdated at best and at worst can be dangerous. The one that gets me is that almost every amp that comes in which has been converted from a 2-wire cord to 3-wire is grounded by a mechanical connection via a transformer bolt, which is probably the worst place to do it because they inherently vibrate and the bolts can and do become loosened (which you touch on).
My preference is crimping an open barrel type ring terminal and bolting in to the chassis the way Rod Elliot illustrates it here:





Oh I did want to mention that there is a mistake on page 9/42 where you reference the illustration figures. I think you meant figure 9 is incorrect, not figure 8. see screenshot below

[SoulFetish]

Also, in illustration figure 14, you describe how to connect the live to the fuse holder terminals. There was a conversation about this not too long ago where it appeared that you stated the live conductor from the line cord is to be connected to the side "ring" terminal, not the tip or end terminal. I had always done it the way you show in the fig. 14 illustration.
I think a few of us may have misunderstood what you meant in your post here:
https://music-electronics-forum.com/...465#post743465

could you clearify?

[R.G.]

Thanks for weighing in. I really appreciate the widened perspective from the people participating here.

I saw Rod's pic on the safety ground bolt at one point. I don't agree with that scheme, based on the advice of two safety certification engineers over the years. I readily admit that standards change and creep, and that standards may be different in Rod's country. Here's the reasoning as presented to me, for international certification by a testing lab in the USA.

Toothed washers make gas-tight high pressure joints with the metal they contact on their teeth. These points are such high pressure that they exclude oxygen, so they don't corrode on those points.the idea from the advisors I had was that having a toothed washer between just chassis metal and the ring terminal meant a direct path between the terminal and chassis with only two interfaces, both with the gas-tight pressure points dug into the metal.

In Rod's scheme, there is a flat washer between the chassis and toothed washer, and a nut between the toothed washer and the ring terminal. Flat washers are meant to spread pressure out, as are nuts, and have not as much tendency to make the fabled gas-tight high pressure points, although it's arguable that they might. The ring terminal's path to chassis is through the nut, then toothed washer, then flat washer, then chassis. There is an argument that the path from ring terminal to nut, to the bolt/screw body down to the chassis could include high-pressure points; but it's less direct.

The advice from my mentors would be that this stack-up would corrode over time, failing special metallurgy chassis, washer, and nut, so simplifying the electrical path with only a toothed washer between ring terminal and chassis is more likely to keep contact over time. I have submitted several pieces of equipment for certification by UL in the USA, all using the chassis => toothed washer => ring terminal scheme, and those passed certification. The caveat is that it's been some years since my last submission for certification, and certification schemes do drift. My scheme was thought to be safe enough back then, though. I did get the grumbling comment from the inspectors that they really preferred a welded-in threaded stud, though. On my first certification, I suggested soldering the ground wire to chassis; bad idea, as solder creeps and embrittles.

Thanks for catching the figure reference mistake. I'll fix that.

I have a long standing groan about placement of figures and such in text processors. Book-styling documents in Word or Libre-Office with cross references and figure references is great compared to old-style publishing preparation, but always includes an irritating (to me at least) requirement for several passes of re-formatting pages to get figures and references on the intended pages along with the intended text. The programs obediently and automatically move the figures along with their anchored text, repaginating as you go. Any text deletion or insertion tends to flow the figures across page boundaries in unexpected ways, requiring more passes to re-tinker where the figures are in reference to their primary referencing text. This starts at the first text insertion/deletion, and propagates through the rest of the pages. Yeah, you can anchor the figure to the page and have the text flow past, but that has its own set of issues. Probably this just means I am a perpetual beginner in using figures in text processors.

On the fuse holders thing: I went off and did some more current research on what other people think about fuseholder wiring, as I clearly (!) come down on different sides of the issue when I re-think it. I believe that the reality is that you have to choose who to protect, the external user or the tech working inside the amp. Panel-mount fuse holders of the kind we're thinking about have an internal ring by the panel and a spring-loaded end contact, to set the stage. When you stick the fuse in the innermost fuse terminal touches the spring loaded end, and the fuse-end under the removable cap touches the inner ring, which in turn has the terminal nearest the panel.

As you remove the cap and fuse, all modern fuse holders bring the fuse out with the cap. This is to keep the external user from contacting a perhaps-live fuse end. Pulling the fuse out, it breaks contact with the ring b the panel first, then later opens the end contact, and even later, just before it leaves entirely the innermost fuse end can touch the fuseholder's near-panel ring before it clears.

To the tech working inside the chassis, the end terminal is easy to touch, so if it's live, it's a hazard to the tech. It's always a hazard to the tech if the fuse is not blown, but if it's blown, having the line wire soldered to the panel-end terminal is safer to the tech. To the external user, having the line wire connected to the end terminal of the fuse holder is safer, as it clears first, and once, so that removing a not-blown fuse is less hazardous because the last fuse-end doesn't become live touching that last internal ring in the fuse holder. It's conflicted.

Fuseholders do seem to be evolving. There is now a batch of "finger-safe" fuse holders on the market that try to avoid the issues with which fuse terminal clears the wiring first. The type of fuse holder that captures the fuse in the cap was a step in this direction. So - yeah, I need to re-think that all. The right answer is to either use a fuseholder that is not the panel mount style, like my favorite IEC power entry module with a fuse drawer, or to always disconnect the line cord before messing with the fuse. I have some thinking I need to do.

Edit: source updated for next draft.

[SoulFetish]

That’s a good catch on Rod’s flat washer inclusion, I actually use a tooth washer/nut/tooth washer/ring terminal/nylock to maintain a gas tight connection from ring terminal to chassis (theoretically). My reasoning is that the first nut lifts the wire/terminal and minimizes the ring terminal and strain relief from potentially getting torqued out and damaged during install. It certainly felt like there was a higher pressure connection than is typically achieved going ring terminal to chassis because it prevents the stud from rotating during termination.
You make good arguments for terminating ring terminal direct to chassis via tooth washer, so Im going to read through your post again. After all, there’s no good reason not to follow best practices.

[Tom Phillips]

...I did get the grumbling comment from the inspectors that they really preferred a welded-in threaded stud, though. On my first certification, I suggested soldering the ground wire to chassis; bad idea, as solder creeps and embrittles...

Does this mean that it was the inspectors that said believe solder creeps and embrittles. I have not experienced that, even on equipment that was manufactured 70+ years ago.
I can understand why they would prefer the stud though. They probably feel that the stud method is more likely to be properly assembled than to expect modern assemblers to do a proper soldered chassis connection.

[R.G.]

Does this mean that it was the inspectors that said believe solder creeps and embrittles. I have not experienced that, even on equipment that was manufactured 70+ years ago.
I can understand why they would prefer the stud though. They probably feel that the stud method is more likely to be properly assembled than to expect modern assemblers to do a proper soldered chassis connection.

The best I remember the exchange, I had been fixing one thing after another for about a week, and when they saw the final version with a through-bolt, they relented and said they guessed that would pass, but allowed as how a welded stud would be better. I was - well, not in the best mood and said OK, well we can just specify it to be soldered to the chassis. They looked a little surprised, and one of them said don't do that - solder creeps under pressure and gets brittle when soldered to zinc coated surfaces like the chassis material. I just said OKFINE and left. I later looked it up. Solder does in fact creep when it's stressed, and there are some conditions where underlying metal ... yadayadayada. I stewed in my juices for several days. In any case, I was glad not to have to specify soldering with a 0.21GW soldering tool to get a good joint on the chassis, so the bolt did it. The bolt works, got certified, and a drill is better than sourcing a soldering iron that will make a reliable joint on a metal chassis.

[Helmholtz]

Solder definitely creeps and eventually cracks when stressed by forces.
I've had some professional experience with cracked solder joints due to creep with fully potted electronics.
The failures were analyzed by our company material scientists and they identified solder creep (by thermal expansion/contraction of the potting material) as the root cause.

[Tom Phillips]

... well, not in the best mood and said OK, well we can just specify it to be soldered to the chassis. They looked a little surprised, and one of them said don't do that - solder creeps under pressure and gets brittle when soldered to zinc coated surfaces like the chassis material. I just said OKFINE and left. I later looked it up. Solder does in fact creep when it's stressed, and there are some conditions where underlying metal ... yadayadayada....

Solder definitely creeps and eventually cracks when stressed by forces.
I've had some professional experience with cracked solder joints due to creep with fully potted electronics.
The failures were analyzed by our company material scientists and they identified solder creep (by thermal expansion/contraction of the potting material) as the root cause.

Very interesting. Anyway, for fun, attached are two photos.
One shows wires as originally soldered to the chassis of a 1954 Fender Deluxe Amp chassis as taken in 2025 and one that shows how I solder the earth ground to a steel chassis. I submit that solder creep or embrittlement is not an issue in these cases.

Cheers,
Tom

[tedmich]

Solder definitely creeps and eventually cracks when stressed by forces.
I've had some professional experience with cracked solder joints due to creep with fully potted electronics.
The failures were analyzed by our company material scientists and they identified solder creep (by thermal expansion/contraction of the potting material) as the root cause.

yikes, pretty much the opposite of the desired effect of potting! Where these hard potted epoxy type compounds?
I've heard softer silicone or polyurethane is supposed to be easier on solder joints

[Helmholtz]

yikes, pretty much the opposite of the desired effect of potting! Where these hard potted epoxy type compounds?
I've heard softer silicone or polyurethane is supposed to be easier on solder joints

Over the years we tried many different potting materials, like different epoxies, polyurethanes, silicones and even asphalt (tar).
Silicone has huge thermal expansion and could only be used in thin layers like a conformal coating.
Otherwise it liked to crack ferrite cores.
Best results we obtained with some special polyurethane compound and asphalt (which has other issues).
In the end all kinds of full potting increased the failure rate more or less.

[R.G.]

Very interesting. Anyway, for fun, attached are two photos.
One shows wires as originally soldered to the chassis of a 1954 Fender Deluxe Amp chassis as taken in 2025 and one that shows how I solder the earth ground to a steel chassis. I submit that solder creep or embrittlement is not an issue in these cases.

You do a very nice job of soldering wires to big pieces of metal! And on top of that you leave plenty of length in the wires leading up to the solder points. Nice work indeed.

Your comment on the inspectors wanting it to be properly assembled is highly likely to be a driving factor in their preferences. They are interested not only in the details of the sample they look at, but also the repeatability. I understand that manufacturing processes get inspected for repeatability and resistance to drift as well.

The world of electrical safety inspection is a bit strange. They really, really don't want to say "this way is safe"; they will say "we can't find anything in our current standards to say that this way is not safe".

[R.G.]

New expanded draft is up.

https://www.dropbox.com/scl/fi/8fscw...=iivpmhc7&dl=0