Monday, May 20, 2013

Doing a 3-wire mains cable conversion on a vintage Silvertone 1481 guitar amp

Today’s patient is a lovely old Silvertone 1481 tube amp. As with many old amps, it still has its original 2-wire mains cable. Not only is the lack of an earth/ground wire risky, but there is an additional danger in the form of what is commonly known as a “death cap”. “What’s a death cap?” I hear you say. Well, in an effort to combat hum, a capacitor was wired into these old amps between the live/hot wire and the chassis. The problem is that if this capacitor fails in any way, the chassis starts becoming live too. Of course it wasn’t originally called this (it was probably called a filtering capacitor or something), but it soon earned its name.

This particular amp behaves itself if you plug the mains plug in in one direction, but in the other direction, you get quite a shock if you touch the chassis, the power switch, or even any metal parts on your guitar.

Anyway, there’s no question that this amp should get a 3-wire conversion. It’s a fairly straightforward job, doesn’t affect vintage value, and could save your life.

This particular amp is missing the back cover, so I’ll skip the step where we remove it (a simple case of unscrewing the four screws that hold it on). I'll also skip the step where I unplugged the amp. Just to be clear, THE AMP SHOULD BE UNPLUGGED.

Next step is to remove the metal enclosure from the wooden case. To do this we remove four screws.

One here:

One here:

And then the same two screws at the other end.

Note that the two wires that run from the metal enclosure to the speaker are somewhat fragile. There’s just enough slack to let you lift out the metal enclosure very gently and rest it on the wooden supports that the back cover would normally be screwed onto, but do take care if you ever do this.

Now we need to open up the metal enclosure. To do this we need to remove six screws.

One here:

One here:

And then similarly-placed screws halfway along and at the other end of the chassis.

The metal cover can then be slid out (careful, its edges are kinda sharp).

Considering its age, the amp is in incredible shape inside. Just a little bit of dust towards the ends.

If you ever do this job, be aware that there are some potentially big voltages inside if the capacitors have not discharged, so do not touch anything in here until you are sure there is no danger. I checked all of the capacitors with my multimeter first before continuing.

OK, let me try to explain what’s going on here. Firstly a link to the schematic for this amp:

We are only interested in the two mains wires (live/hot and neutral) coming in through the bottom of the chassis, the power switch, fuse and the “death cap” as mentioned previously.

I’ve drawn what’s going on there below, as well as what I’m going to do to make it safer (as with all images here, click for a larger version):

As you can see from the left-hand image, one of the mains wires goes to the fuse, then on to the transformer. The other mains wire goes to the power switch, and then onto both the transformer and to the “death cap” which then joins to the chassis. Since the mains plug has two connectors, both of which are the same size and shape, the plug can be plugged in either way, so you never know which will be connected to the fuse and which will be connected to the switch/death cap.

So step one is to remove the death cap (remember I’ve already checked that this is safe to touch).

OK, that’s it out. Amazing to think this could quite literally kill you:

Now we remove both wires of the mains cable (from the switch and the fuse):

We also remove the wire from the other end of the fuse to the transformer, since we are going to wire the switch and the fuse in series, rather than have them on separate wires.

OK, that’s everything out. Now it’s time to put some new wiring in there.

Firstly, we install a new cable grommet/gland. The old one is too small to fit the new 3-wire cable through, so I put a new (and better) one in.

We join the black live/hot wire to the end connector of the fuse.

Then the other connector of the fuse is connected to the switch with a short wire. I'm talking about the wire to the bottom-right of the photo that's totally blurry (stupid auto-focus).

The white neutral wire is soldered to the remaining transformer wire (which has been stripped and prepped), and heatshrink is applied over the connection.

The green earth/ground cable is soldered to a ring terminal, then fastened to one of the bolts holding the transformer to the chassis. You’ll notice the addition of a locking washer in there for added security.

Here’s how it all looks after that:

The cover is then slid back on (again, remembering to watch the fingers on those sharp bits):

Screws are re-screwed-in:

And the metal chassis is again screwed into its original location in the wooden case:

A new wooden back cover is fabricated and installed.

The amp is then fully tested (and works great). No hum either, even without the death cap.


WA6DZS said...

Hi Stu- Why the Line (black) wire to the back lug of the fuseholder?

stu said...

Hey Phil, the reasoning there is that it's the live wire's very first point of contact. If the fuse blows for any reason then absolutely everything from there on is safe. You could theoretically put the fuse after the switch, or even on the neutral side, but this seemed like the safest possible option. If you're specifically asking why it's connected to the back lug and not the front lug, then I figured that the internal connection to the back lug was the hardest to touch, so again this seemed like the safest option. Stu