Schematic

While there are still a few things to tweak to get the amp working 100% properly, I went ahead and drew the schematic for this amp, using Adobe Illustrator to keep things nice and tidy. Yes, I did emulate Fender's Vibro Champ schematic while making it, but I also made a few changes along the way. Some capacitors were substituted, etc. In addition, I rearranged a few things compared to the Fender schematic, and now the drawing more closely resembles the actual layout of the chassis. Here it is, the schematic (1st version, anyway) of the Lewis LB110:

The yellow circles denote a change from the original Fender schematic. (click to view full size)

I made the schematic long and thin like that because I intend to transfer it to a plate (like the control panel) and put it on the inside of the back cabinet cover. This will serve a dual purpose: heat shielding & information for anyone attempting to repair or modify the circuits.

If I don't get the tremolo sorted out soon, I am considering taking the amp to an amp tech for assistance. I am certainly not above asking for help!

Test Flight, part 2

After reading lots and lots of stuff about amp repair and troubleshooting, I was fairly certain the issue of volume was due to lack of gain. Either there was too much negative feedback killing my signal, or one of the gain stages wasn't properly connected, or possibly one of the components was shorted/defective.

First, I got the multimeter out and checked continuity on every single connection of the amp. Everything was fine. Next, I began testing all the resistors. If a resister was defective or shorted out/fried, I would be able to see that using the resistance meter (ohms). Most of them checked out almost exactly what their values were supposed to be, which is quite amazing considering most manufacturing tolerances allow for up to +/- 20%. There was one section of the board that I kept coming back to, however. Something just didn't seem right about it. The resistor wasn't measuring correct. I assumed the component was bad, so I removed the original one and replaced it with a fresh one. I tested the amp again, and no change.

One of the FAQ pages I was researching was talking about a feedback wire Fender usually installs on the vibrato tube. It goes to a 2200 ohm resistor and then ground. I didn't remember seeing one, so I examined the schematic for it. It wasn't there, but there was a 2700 ohm resistor. The very same one I had just replaced. Then I saw my embarrassing mistake: On the schematic/layout, there was a dotted line connecting that resistor to one of the coupling capacitors. Usually, a dotted line refers to a wire that is under the circuit board. I checked my board, and I had indeed forgotten that wire! How I missed it during one of the triple and quadruple checks I do not know, but nonetheless I had missed it. 

The dotted line, the one that completes the "Y" shape (and looks remarkably like a Flux Capacitor, thank you Doc Brown!), represents the connection I accidentally omitted.

With all the jumpers already soldered in place, it made no sense to disconnect the turret board and solder this missing wire to the bottom. I simply connected a bare wire between the two turrets.

Although it's not the prettiest soldering job I've ever done, it should hold. My main concern was just to get a wire in there so that I could test the amp again.

With that connection finally made, I plugged everything in and tested it again. Right away I could tell it was fixed. The hum is a little more noticeable now, but still not overly bad at all. My giant 100 watt Fender Quad Reverb is a pretty quiet amp (at idle), and it has more hum than this. I played a few notes with a guitar and WOW! I had lots of volume. It is now every bit as loud as the Vibro Champ I emulated. In fact, there might be a little too much, as now if I turn it all the way up I can hear some unpleasant distortion, probably from the speaker overloading. It is like a harsh clippy sound, not a smooth, desirable overdriven sound. As this test speaker isn't what I plan on keeping in the finished cabinet, I'll just deal with that later.

I tested the tremolo again, and while the sound no longer cuts out completely when you turn up the depth knob, it now gradually dampens the overall volume. Still no tremolo effect.

At least I solved the riddle of the missing gain. I'll do some more research and try to get the tremolo fixed soon.

Test Flight, part 1

There is a lot of excitement and adrenaline flowing as you are about to test an electronic device that you've designed and built from scratch. Also, nerves. As mentioned, I had successfully tested the heater wiring, so I did have some confidence, but now the real test was about to happen. I plugged it in, connected a speaker (as you should always do with any tube amp while it's turned on. Failure to connect a load to the output can and will damage your output transformer), and plugged in an electric guitar. Nervously, I turned it on and began turning up knobs.

The good news: very low hum. Little if any sound came out of the speaker at idle. No smoke, sparks, or static. But no music was coming out, either. All that adrenaline that was zooming through me 3 minutes prior was all gone now.

After getting over my initial hopes of just firing it up and it working perfectly, I began to troubleshoot in my mind. After all, what fun would any of this have been if it just worked like it was supposed to? What is to be learned from that?

I turned all the knobs down to zero, and then began to bring them up one at a time. Beginning with volume, I brought it up to about 75%, and voila! Amplified guitar sounds! Bolstered by this good news, I tried some other knobs. The bass and treble seem to work exactly as designed. The volume, while it does work, is not as loud as it should be. In fact, I would say it's about 25% as loud as my actual Vibro Champ. The tremolo, however, does not work. Not at all. The rate control does nothing (that I can hear) and the depth, once turned more than about 5 percent, cuts out all volume. This must have been the original problem I encountered; I think the depth knob was up a bit on the initial test.

Now the real troubleshooting begins. I fear I have a lot of reading and researching ahead of me. Wish me luck.

Wiring Complete

After wiring the circuit board and heaters, I gained a lot of momentum. I wired all the potentiometers, jacks, and connected all the jumpers from the circuit board to the tube sockets. I used a lot more solder than I had anticipated, but everything went together without too much hassle.

These are just some shots of the rat's nest I have created:

Here you can see the mess I've assembled. I tried to keep things as tidy as possible, and even so things look a little out of control.

A photo of the potentiometers. Notice how the solder tabs all angle inwards. I did it this way to keep wire runs as short as possible, and to make it easier to get a soldering iron in and make the connections.

A close-up of the vibrato tube socket connections.

A bit overwhelming, no?

Here is the AC power cord and power transformer area. Using wire ties keeps things neat. Notice the extra supports on each side of the transformer.

Control Panel

Having your name appear as a logo makes anyone proud.

The Layout

Designing a control panel requires two disciplines: ergonomics and aesthetics. Not only should it look nice, it should perform nicely as well. The controls should be laid out with enough room between them so that adjustments can easily be made with human hands.

While working on my control layout mock-ups, I kept going back to a "V" theme. At first I had all 5 knobs aligned in a "V" configuration, and then I found some knobs at Mouser Electronics that came in various sizes. I thought using three different sizes, with the largest one in the center, might look really unique. Most amps have identical knobs all the way across the front panel, and I used this opportunity to give my design some flair. 

Normally, "volume" or "gain" is the first knob you might see on an amp, left to right. However, with the various sizes of knobs, there is a center emphasis, meaning the centermost (and coincidentally, the largest) knob should hold higher importance. Some guitarists might disagree, but in my mind the volume control is the most important, so I put it in the center. That left two "banks" of controls, one pair to the left of the volume and one pair to the right. I chose to make the bank to the left the "tone" controls (treble and bass) while the bank to the right holds the controls for tremolo (Fender calls them speed and intensity -- I chose to call them rate and depth).

Even though I changed the order of some control potentiometers in this layout, it didn't seem like it was going to cause wiring issues. The order of Fender's Vibro Champ read, left to right, Volume, Treble, Bass, Speed, & Intensity. In my layout, by putting volume in the center, I wanted to make sure I kept things relatively neighboring each other to prevent unnecessarily long wire runs and additional resistance or hum. Logic tells me that bass should come before treble anyway (as it does on most equalizers I've ever used), so essentially I just flipped the controls as Fender had them: Bass, Treble, Volume, then left Rate and Depth as they were. 

I now had a control panel layout that looked nice, made logical sense to me, and would hopefully make sense to anyone else who used the amp.

Control Panel Construction

Being a prototype built in my kitchen, I'm trying to control costs wherever I can. Certainly, I could have a piece of steel bent, chromed, and silkscreened just like Fender does, but that would be prohibitively expensive for only one panel. I also considered skipping the chroming step and just silkscreening my design on metal. Still pretty expensive. I thought of making a vinyl label and adhering it to the chassis, but die-cut vinyl might peel up, and printed vinyl might fade or rub off with prolonged use. The durability might increase if a thin piece of plexiglass or Lexan was placed over it. 

I work at a newspaper, and we use aluminum plates to transfer the ink to the newsprint on the presses. The images from our computers get on the plates through film negatives and emulsion exposure, much like that of combining the photographic and lithographic processes together. Once a plate has been exposed and washed, the images appear on the aluminum plate in dark blue. I asked a few of the press operators how durable this blue emulsion was, and they all told me it would last pretty much forever. I had found a solution, at least for my prototype!

I had been doing my mock-ups of control panel layouts in Adobe Illustrator at full size, so ultimately all I had to do was print that layout to our film negative imager and expose a plate. The result, once placed under plexiglass for further scratch protection, looks quite professional (see photos above). 

After drilling both the plate and plexiglass, it looks like my alignment was off by about 1/16" but not very noticeable unless pointed out. The result is that the labels beneath each knob nearly touch the knob itself. The only spot that is quite noticeable is at the On/Off switch, since the toggle switch is supposed to be centered between the words "on" and "off."  Overall, I decided I could live with those imperfections. Afterall, this project should retain some "hand-made charm."

It Glows!!!

Glowing pilot lights are a good sign!

Heater Wiring 

Wiring all the tubes together to the proper lead from the transformer allows them to each receive some voltage so that they can do their job. This voltage is referred to as the "heater" voltage, since it allows the tubes to get warm and glow. It is recommended that you wire the heater wire before any of the signal wire on a new project. Heater wiring should be routed as neatly as possible and as far away from any signal wiring channels as possible to prevent hum. If the heater wire does indeed have to come in contact with a signal wire, it is usually recommended that it cross perpendicularly rather than running parallel. This way, if there is magnetic field leakage, at least it's only on that one spot as opposed to the length of the wire run.

I was able to tuck the heater wire neatly against the corners of the chassis and make nice straight runs (it is the green wire in the photos).

After I got all the heaters wired up, I was ready for a test. This would be the first time I plug the contraption in to 110 volts. As Morgan Jones says in his book, the first time you test equipment in progress, even the most experienced builders have the same emotion: sheer controlled panic. I was a nervous wreck, but I plugged it into a power strip with an on/off switch in case I needed to quickly disconnect. I also used a few "throw-away" tubes I had lying around so that if something was wrong, I wouldn't damage my nice expensive vintage tubes. 

I plugged it in, took a deep breath, and switched it on. Nothing. No noises, pops, crackles, puffs of smoke, nothing. I looked closely at the tubes, as properly heating tubes will glow and emit heat. Nothing. The pilot light on the front panel was lighted, so I knew something was wired properly. I scratched my head, unplugged it, and took a closer look. Upon looking at the schematics and double-checking that I had wired the heaters to the proper solder tabs on the tube sockets, it dawned on me: I forgot to ground the tube sockets! AC needs + and - (hot and ground) to work, so if it only sees positive, nothing is going to happen until it is grounded. I grounded the appropriate tabs, popped the tubes back in, and plugged the amp in again. Deep breath, switched it on, and finally! Glowing tubes! Small success!

If you look very closely, you can see that the tubes are glowing.