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.