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:
http://www.freeinfosociety.com/electronics/schematics/audio/silvertone1481.pdf

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 grommet. 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.

Fixing an 80’s Tokai Z-II Overdrive Pedal with Volume Level Issues

Here’s one for the books. This pedal had been exhibiting strange behaviour where it would work fine for ages, then suddenly the volume would drop to about 50% or so. It would stay this way for a while and then jump back up to 100%. Repeat ad infinitum.

I wouldn’t really have jumped to a “dodgy jack” diagnosis, but decided to spray a little contact cleaner in there anyway, since it was cheap and easy to do so.


It didn’t fix the problem.

The next thing I decided to try was to spray some contact cleaner in the pots (after that, it was going to be a case of checking the electronic components), but they could only be accessed by opening the box.

So I removed the back cover:



Moved the insulating sheet to the side:


Lifted the board, and immediately saw the problem. You see it?


Let me show you a close-up:


Yeah, you see it now, right? The jack on the right? The one that's supposed to be in one piece? Well here’s what it looked like:


I think we can quite safely call that a “dodgy jack”.

I can hardly understand how the pedal managed to even “almost” work fine. I can see how the ground connection was fine, but the positive contact must’ve leaned against the tip of the jack plug with just enough force to make an electrical connection. Sometimes this was so weak that the volume actually dropped.

I didn’t have one the exact same, but a standard jack fitted perfectly, so in it went:


The pedal was put back together, tested, and confirmed to be working fine:

Building a Modified EA Tremolo Pedal on Veroboard

Good morning, everyone. Who wants to build a tremolo pedal today? Well I know I do.

The original EA Tremolo was published in “Electronics Australia”, an Australian (you don’t say!) electronics magazine, in November 1968 and later proved extremely popular among the DIY community. Since then, several modifications/improvements have been made.

Here’s the one I’ve decided to try:


A description of the circuit can be found here: http://www.home-wrecker.com/eatremolo.html

Helpfully, a Veroboard layout is already available. This one was laid out by Andrew Carrell, who asks that it not be reposted, so instead I will simply provide a link:

http://www.aronnelson.com/gallery/main.php/v/DRAGONFLY-LAYOUTS_0/album19/album145/EA_TREMOLO_VERO.gif.html

As you may be aware, I’m a bit of a fan of Veroboard circuits, since they can be quite easy to build (assuming you choose a fairly basic circuit) and don’t require you to buy or make a specific circuit board for each project (here’s one I did earlier, for example: http://diystrat.blogspot.com/2011/05/building-quick-brian-may-style-treble.html).

I decided to try a few new techniques this time around, which I’ll mention as we go along.

Let’s start with the Veroboard itself. We need a 26 x 9 grid (26 holes x 9 strips), so we cut that out first.


Then we need to cut the tracks in 17 specified places, as shown in Andrew Carrell’s layout. Previously I’ve used a number of different tools to do this, all of which have been a little more time-consuming than I would like, so this time I decided just to drill holes through the board at those specific points:



I was hoping this would help avoid having to check each track to make sure it wasn’t touching the next one near the break, but I found I had to do just as much tidying up as with previous tools. Also, if you drill too many holes close to each other, you’re going to weaken the board (not really a problem in this case, but worth thinking about in future), so I don’t think I’ll be using this method again.

I'd previously laid out all of the components I’m going to be using on a piece of paper. I haven’t really ever done this before, but found it a great way to stay organised and reduce errors.


Note that due to a lack of local availability (story of my life) I had to substitute all three of the transistors for alternatives, as follows:

BS170 - > 2N7000 (please ignore the 2N3904 written on the piece of paper)
J201 -> 2N5457
2N5088 -> 2N5089

The components are soldered to the board:


And the wires are added:


Now let’s deal with the enclosure.

I decided to just mark the holes on top of the plastic protector that came on the box.
These are the hole sizes I needed (careful – you might require a different size):

Pots – 7mm
Audio jacks – 9mm
DC-in jack – 12.5mm (I drilled to 12mm then used a taper to enlarge it just a little more)
LEDs – 8mm
Switch – 12mm


Then I punch dents to centre the holes:


The holes are drilled using a stepping drill bit (of course standard drill bits will work too):


And here are the finished holes (I had to remove the back cover to finish the switch hole, since the stepping drill bit was hitting off the back cover):


You’ll notice that I haven’t drilled the LED hole(s) yet, as I wanted to check on a few things before committing to a position.

OK, now we put everything we can in place. You may notice that I’ve not put the DC-in jack in (I decided to move its position and you’ll see it making an appearance later in this post).


I’m laying everything out to get an idea how long I need to make the wires. Some are cut a bit shorter and one or two might even be replaced with a longer wire if necessary.

NOTE: If you need a hand wiring up the switch, jacks, DC-in, etc., have a look here: http://diystrat.blogspot.com/2009/06/buhow-to-wire-up-stomp-box-effects.html

Once all the wires are cut and soldered in place, we have this (note the new DC-in jack position and the LEDs):


And here’s how it looks from the top. I haven’t decided how I’m going to finish the enclosure, so it’ll stay like this for the time being.


So what about the hole left by the original DC-in jack position? Well, it turns out I had an old coin lying around (a Spanish one peseta coin) that fitted exactly in the hole. Here it is:



A couple of things I should point out before letting you hear the pedal. Firstly, in the Veroboard layout, there is a note that says: “Vol 2 goes to output”. This is basically the output of the CIRCUIT and should go to the appropriate pin on the stomp switch, not straight to the output jack.

Secondly, the value of the 100k trim pot (TR1) is critical and it must be adjusted slowly and carefully while using the pedal in order to find the sweet spot (my pedal had NO OUTPUT AT ALL until I found the right position, at which point the output was suddenly perfect). Alternatively, you can measure the voltage at Q1 drain, and adjust until it reads between 4.5v and 5v.

There's nothing else to add now, except that here is a demo. You will notice that the yellow pulsing LED (which indicates the speed/rate) is always on once the jack in plugged in. Apart from letting you know the speed/rate, this also serves as a good reminder that you are always using the battery, even though the effect isn't switched on. The steady green LED indicates that the pedal is switched on. The pots are, from left to right, Volume, Speed/Rate and Depth.

Enjoy!