Tuesday, May 28, 2013

The Volumizer (II)

I don’t often build more than one of the same pedal. In fact, I think the only one I’ve ever built twice was the Minimixer (http://diystrat.blogspot.com/2008/08/two-channel-mini-mixer.html), but I don’t think that even counts as a pedal.

Christian over at Black Sunshine Media was so enamoured with the Volumizer, however, that I offered to build one for him too. I also took the opportunity to do a better blog post about it, since at the time of doing the original one, I kind of skimmed over some important points.

Firstly, some background. The Volumizer is my clone of an MXR Micro Amp, the schematic of which can be found at General Guitar Gadgets. There is basically no difference between my circuit and the one shown there, apart from the layout and possibly a couple of very slightly different parts (due to a lack of local availability). This pedal is a great little pre-amp, which boosts volume and nothing else. It’s nice and clean and only provides distortion if you use it to drive something else (which would be what actually distorts, not the pedal itself). Christian’s found it to be extremely useful as a pre-amp stage before going into his mixer, which was a use that I hadn’t even considered when first making it.

Anyway, this time around I’ve taken the time to draw up a veroboard layout in Illustrator (you’re welcome). The layout is the same as last time, with the exception of how I wired the switch. There’s really no significant difference in how it’s wired up; it’s just more in line with the standard way I wire up switches these days. Here it is (as usual, click to enlarge):

Also, this time around I managed to source a more standard-sized enclosure, since I really had to cram the other one into the slightly smaller box.

OK, let’s build this thing.

First we need some veroboard cut out to 15 x 8 (you may want to make it a little longer to allow for more capacitor space – explained later).

Then we cut the tracks as noted by the red X’s on the verobard layout. Just remember that you’re cutting these on the back, and the veroboard layout is as seen from the front.

Here’s mine after cutting the tracks.

Incidentally, if you’ve been following my posts, you’ll have noticed that I’ve been trying different methods for cutting veroboard tracks. I finally came up with a solution that I’m happy with. This is a pin vice with a 3mm drill bit. It works really well for cutting the veroboard tracks. This is going to be my go-to tool from now on.

Here it is in action:

All right, the veroboard’s ready to receive components, but before I do that. I’m going to give it a scrub with a steel brush, since it’s a bit tarnished (a brass brush, sandpaper, etc, would do this job just as well). I’m doing this because it will make it much easier for the solder to take hold.

You can hopefully see which bit I've scrubbed so far on the right of this image:

Here are the parts soldered in place. You may notice an additional capacitor sitting at a funny angle to the top right. That’s because I couldn’t source a 15µF one, so used a 10µF and a 4.7µF one in parallel, giving me 14.7µF (close enough). That’s why you might want to leave a bit of extra space when you initially cut out the veroboard (or you can just cram it in there like I did). Also to the lower left, you might notice that I have two resistors soldered together,. These are a 2MΩ and a 200KΩ resistor soldered in series to make the total value 2.2MΩ.

Now we deal with the box. I mark the holes on the plastic wrapping (which saves a bit of cleanup), and stamp the centres with a hole punch. After that, I drill the holes with a stepping drill bit. If I feel that the drill bit is hitting the back of the enclosure, then I wait till that’s off before finishing the hole.

The DC-in jack hole is a tad wider than 12mm, so I drill to 12mm and then ream it out just a little more.

I do a test fitting with all the parts before taking this any further. I was still waiting for some new 9-pin stomp switches to arrive when I took this, so had to settle for a 6-pin one in its place temporarily.

Once I’m happy with the layout, I remove everything and then paint the box. This one’s getting flat black with no decals.

Wait, what the…? I thought I took some photos at this stage. I guess I just got so excited I forgot.

OK, hold on...

Picture this: That box I just drilled holes in is sitting on a newspaper. To the right, I am spraying black paint out of a rattle can. Said black paint is landing on the enclosure and turning it... black. OK, now imagine I took a photo of that and you’re looking at that photo right now.

All right, now that the box is painted, as shown above [ahem], I place all the hardware back
inside and wire it up (you’ll see the 9-pin switches had arrived by this stage – hooray!) You will also note, no doubt, that I have also put some insulation in there so that the circuit board does not touch the metal enclosure, causing a short.

I do a quick sound comparison with the original Volumizer and confirm that they sound the same (they do).

And we’re done. Here’s a nice pic:

Oh wait, that’s upside down. How about this?

Enjoy, Christian.

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.