Monday, 21 February 2011

Tube glow

Everything is in the right place. Or at least I hope so, because when I turn this thing on I don't want it to explode or electrocute me.

We need to turn the circuit on in stages, so we can identify where a potential problem might lie. Or at least that's the theory.

First up we plug it in with no tubes at all. This exercises just the first part of the circuit. I was pretty edgy plugging this thing in to the mains. I think it's because the chassis is made of metal, and metal conducts electricity. All 110 volts of it.

So I gingerly plugged it in and then nervously turned the volume knob to switch the amp on. The power lamp glowed a satisfying red, the fuse didn't blow, nothing went bang or started to smoke. So far so good.

Next up I had to insert the 5Y3 tube and repeat the exercise. Again nothing went pop.

No it's the turn of the 6V6 tube to get warmed up. I also had to connect the speaker at this point, as the load it provides is important to prevent the amp burning out. Still no smoke. And no ominous sounds from the speaker.

Lastly I put the final 12AX7 tube in and inspect once more. Still nothing to suggest we had any problems. Could it be that I got it right first time?

So I turn off the amp, plug in the guitar, and turn it on again.



Damn. Looks like there's a hard to find gremlin lurking in there after all. How on earth am I going to find it?

But then, about 10 seconds after turning it on, the sound of the guitar starts to build in the speaker. Everything is fine after all, it just takes a little while for the circuit to warm up!

The tubes glow with a very satisfying orange, and this thing can sing! The sound is really warm and fat, and makes my '66 Jaguar sound awesome. Even my cheapo guitar sounds good.

It's loud, too. I'm not sure my neighbours really appreciate the riffing as I take it through its paces.

I haven't put the back on the amp just yet. It's much more fun to be able to see the guts of the thing. Just got to be careful not to put any fingers inside!

Monday, 14 February 2011

Mounting the chassis in the cabinet

Now I can put the finished chassis into the cabinet, and see how it looks.

It looks good!

Can't wait to fire this thing up and plug in a guitar!

Finishing the chassis

Lots more soldering.

This time we need to attach each of the jump leads to the right components. This normally involves finding the right wire (not always that easy, given that some of them run under the board, and come up out of the same hole as another wire), and then attaching it to a very small pin on the tube sockets.

So far so good. All the joints look pretty clean, and some elementary tests with a multimeter seem to suggest that the right points of the circuit are wired up to their counterparts.

There's definitely a lot going on in there, but it's amazing how neat and tidy it all looks.

The instructions neglected to tell me what to do with the black wire from the output transformer, but I managed to work out that it needed to be grounded to the speaker output jack by looking at the circuit diagram. The old magic is still there!

Virtual center tap

Now for something a little funky.

We need to run some of the wires around the chassis and to some of the important components, but in an effort to reduce some of the "noise" in the amp (stray RF), we're going to twist these wires as tight as we can.

I twirled these stiff green wires together, and then proceeded to solder them to the right pins on the tube sockets.

Once the wires are twisted it's a little difficult to work out which pin on one end is mapping to which pin on the other. Time to mark the ends of one of the pieces of wire with a pen so I can trace it effectively.

Things are starting to take shape.

Mounting the board

Now that I've got all the bits and pieces I need, and some tips from the kit supplier, I've been able to move things along.

The tube sockets were a little tricky to mount. I ended up using a drill to ream the screw holes a little larger. Still a tight fit, but they went on with only a little persuasion.

Then I was able to put the volume control in. And then run the ground bus. This is a bare wire that connects up some of the earth connections on the inputs, and other components. It terminates on the chassis, just south of the power transformer. I hope I get this wiring right, otherwise the chassis is going to be "live" when I plug it in.

Then I squeezed the circuit board into the right place. There's definitely not too much room to move in there. The final piece of the jigsaw is the pilot light.

But. Hang on. The pilot light won't fit.

That last capacitor on the board (yes, the big one) is in the way. Sigh. Time to take the board out again, unsolder that big old capacitor, and then remount it slightly further away from the top of the board. That done, it all fits in quite nicely.

Phew! What's next?

Saturday, 22 January 2011

Completed cabinet

I finished the cabinet today, and it looks pretty nice!

After the five coats of lacquer, I needed to drill a couple of holes which will eventually be used to mount the chassis. I had been wondering about how to find the right place to put the holes, as I only had one chance to get this right, and there wasn't much room for error.

Luckily I found a template online that I could print out and place over the cabinet to indicate where to drill. I was also able to borrow an electric drill from someone at the office.

After a glass of beer I had enough courage to give it a go. The holes went in cleanly, and in exactly the right place. Phew!

I put all the hardware back on (feet and handle), and then added the Marsh logo plate on the front.

Looking good!

Now back to the electricals and the chassis...

Saturday, 15 January 2011

Working on the chassis

One of today's tasks was to start to prepare the chassis. This involves mounting the power transformer and input transformer to the back of the metal box, and then mounting other components such as the valve sockets, inputs, volume control, fuse and fuse-holder to the relevant holes.

This was a pretty straightforward exercise and it hasn't required much soldering so far.

The inputs require a little attention. There's a resistor soldered across the tabs of one of them, and then a jumper runs across to the other.

I've reached a minor hiatus in construction. The nut that holds the volume control in place was missing from my kit (so I'll have to wait until another is shipped out to me). And in addition, the screws that are supposed to hold two of the valve sockets in place just don't seem to want to go through their little holes (I've asked the kit manufacturers for some advice on that one).

So it's back to more lacquering for me.

Second coat of lacquer

I put the second coat of lacquer on the cabinet today. The second coat was a mix of clear shellac, amber shellac, and denatured alcohol. The idea is to start to give the cabinet a nice vintage look, without it getting too orange.

I had been wondering what sort of difference the amber shellac would make. It turns out that it has a very pleasing effect -- you can see the contrast between a "raw" and "cooked" cabinet in this picture. It should look pretty good when it's done!

I think I'll do one more amber-mix coat. And then top it all off with a final coat of clear.

Music while you work

I seem to be listening to a lot of blues while I construct this kit (for example, while lacquering the second coat on the cabinet, I was listening to "From the Cradle" by Mr Clapton).

My beautiful wife asked me a very sage (if a little grammatically clumsy) question: are you only listening to music played on that amp as you build your amp?

Maybe I'm already doing that subconsciously, but I think I'll try to stick to that assertion going forward.


Monday, 10 January 2011

How to mount the chassis in the cabinet

Something I'll have to work out sooner or later is just how to mount the chassis in the cabinet.

The chassis is the metal box that all the electricals are attached to (valve sockets, transformers, circuit board etc). It looks like I need to drill some holes in the cabinet, and then screw the chassis inside with some bolts... but the instructions don't mention anything about how to do this, or where exactly I need to drill the holes.

I still have some time to think about this part, as I have to finish the lacquering of the cabinet, and there's quite a lot more to do on the electricals side before it will all come together.

Sunday, 9 January 2011


Next up was to wire up the jump leads from the edges of the board. These will eventually connect to the tube sockets, input jacks, speaker, etc.

The kit came with cloth covered wire for these jumpers, and as I cut the first piece to the right length I also tried to strip the end so I had some clean wire to solder. The cloth covering wasn't the most forgiving sheath to cut through, and I had little success in getting a nicely stripped end.

I looked on the RadioShack website for a wire stripper tool, and found one that looked reasonable. I planned to buy it the next day.

For some reason I decided to Google for tips on stripping cloth covered wire. I'm glad I did, because it turns out that you don't need to strip it at all! You just pull back the covering to expose the wire underneath. Easy!

I worked through each jumper connection, and the couple of under-board connections. This didn't take too long, but I found that I needed to refer back to the big circuit diagram that came with the kit to make sure that I was putting things in the right place.

Wiring up the first components

While the lacquer was drying, I started to "stuff" the circuit board. This involved locating the various components (resistors and capacitors) and poking their legs through the holes in the circuit board.

This was a pretty straightforward exercise, and proceeded quickly. The only marginally tricky part was to get the capacitors lined up pointing in the right direction (they require the positive end to point to the positive rail on the circuit). Nothing too complex, though.

Once all the components were in the relevant locations, it was time to see how good my soldering skills were after all these years.

I bought a 25W soldering iron from RadioShack. The low power seemed to be a good idea, as I didn't want to cook any of the components, and it might give me a semblance of control over the process.

As I worked from location to location soldering the joints, I was pleasantly surprised to find that I was able to produce a reasonable joint if I didn't rush.

I also found that the tip of the iron needed cleaning regularly, and used a damp Scotch Brite sponge to keep things flowing. The abrasive side was good at keeping the crud off the tip; and the spongy side acted like a classic soldering sponge.

Saturday, 8 January 2011

Shellac-ing the cabinet

First job was to start to lacquer the cabinet.

I could have easily bought a pre-lacquered cabinet, but what's the fun in that?

There are all sorts of forum posts about how to approach the lacquering, and I settled on the simple approach of using one coat of clear shellac to seal the edges and corners, and then a number of coats of amber shellac to give that vintage tone.

The clear shellac was easy to buy locally, and I found a reasonably cheap supplier of the amber online and placed an order.

I also read online that you can clean the brushes in denatured alcohol, and it's also useful to dilute the amber so that the color isn't so glaringly orange. So I put some on my shopping list.

The first coat was pretty easy. I took the cabinet apart (removed the speaker baffle, took off the handle and feet) and laid it all out on some newspaper, and proceeded to apply a coat of clear shellac.

It dries pretty quickly, so I was able to turn it all over and reach the parts I hadn't got to about an hour or so after the first pass.

Selecting a Champ kit

As I surfed around for a new guitar, I decided I wanted a vintage Fender Champ amplifier to go with it. It was high time to have some valves in my life.

As I Google-ed around for some information on which models were available in which years, and which years were considered better than others, I came across a very interesting forum post. Someone had built a Champ from a kit. Now there's an interesting idea!

I've always been a geek, and I'm somewhat proud of that. I used to de-solder pretty much anything and everything when I was a kid, so the idea of building a kit by following a set of instructions was not that daunting, and was definitely pretty exciting.

I channeled my Google activity away from vintage Champs, to vintage Champ kits. There are a number of companies out there that are putting these kits together, and most are clustered around the $500 price point for all the electronics, a speaker, and a chassis and cabinet for it all to live in.

The forums gently pointed me towards Marsh Amps, and this company also happened to offer some of the "options" that I was interested in. So I pulled the trigger on my order, and added an international power transformer (just in case I move back to Europe at some point), and also the Weber alnico speaker (which seems to get a pretty good writeup in the forums).

I also couldn't resist getting the Sprague capacitors. Even though I wasn't quite sure why I needed them.

It also seemed appropriate to get the "Marsh Amps" badge to put on the front. It wasn't right to put a Fender logo on there, and seeing as the good folk at Marsh had gone to effort of putting the kit together, their logo on the front was the right thing to do.

Marsh Amps provided me with the kit. Now all I have to do is build it.