Upping the Amp-erage - Building the Mojotone 5F2-A Tweed Amp Kit

I've been building pedals for several years now, but like any gateway dru... errr... hobby, I soon discovered that there were people who built their own amplifiers as well. Much like with guitar pedals, being able to build your own amp opens up options that may not normally be available because the amplifiers are rare, expensive, or likely both. Even if it is a more common amp style, building one yourself can often be cheaper. It also allows customization and choice of components to create a unique sound. I'd started my first amp project about 1.5 years ago, but sort of got bogged down and intimidated by the scope. I decided it would be advisable to maybe try a slightly more entry-level build to understand the ups and downs of the process a bit better. Mojotone offers a variety of kits with good documentation, wiring diagrams, and schematics. They aren't incredibly cheap, but all of their designs are well-proven. Their easiest kit is the 5F1 Tweed based on the 1950's vintage 5 Watt amplifier. This was a little more basic than I wanted to go as it only has a volume with no tone knob, so I went with the 5F2-A Tweed Princeton, which was first introduced in 1957 (and should pair up really well with my '57 American Vintage 2 Stratocaster). 

The photo above is of the completed chassis with all of the primary guts of the amplifier. Unfortunately, I didn't take as many "in progress" photos as I should have, so it may be good to take a step back and talk about what you get in the kit. 

Kind of like a good vintage fuzz pedal, an early amplifier isn't a terribly complex circuit, but they can be sensitive! Many of the pieces you get for these early amplifier kits are pretty common, basic, and inexpensive such as resistors and capacitors. The kit comes with everything you'll need, but I decided to upgrade the resistors with some higher end ones from Amplified Parts. Most of the cost in the kit itself is in the amp chassis and the two transformers (a power transformer and an output transformer). Good cabinets for the final build are also a higher cost component. Good speakers run a fairly wide range, but are in the upper regime of the cost pareto as well. 

Somewhere in between cost-wise are the tubes - of which this kit has three - a 5Y3, a 6V6, and a 12AX7. Due to supply-line variations, the exact manufacturer of each of the tubes can vary depending on what is available. Two of my tubes were J J Electronic (the 5Y3 and 12AX7) and one was an Electro-harmonix (6V6). Rounding out the old-school nature of the kit and build, there are no PCBs. Instead, most of the circuit is built on eyelet board with a backing insulation panel (and I'm not going to touch on the debate between PCBs versus turret or eyelet board construction in amplifiers here!).

The kit includes a really good wiring diagram which details all of the connections required to build the amp (there's also a schematic with voltages for testing as well). Most of the resistors and capacitors are on the eyelet board, but a few aren't. Many are wired point-to-point directly to the pots or jacks. Then there is the wiring of all of the tube sockets. Most of this was fairly straightforward, though it was a bit tight wiring up the bulb. The only real issue I had was getting the power cord to feed through with the strain relief connector as the cord didn't want to compress well. Ultimately I got it through, and the cord itself is affixed to the side of the enclosure with a cable clamp as well, so it shouldn't move around any.

I tried to keep all of the wiring runs to the tube as short and neat as possible. During the debugging process I learned that higher voltage runs should intersect at 90 degrees so that the voltage doesn't induct to the adjacent wire, so some of the wires were moved around a bit. 

Speaking of debugging. Once I got the whole kit together, I started going through the voltage tests Mojotone recommended for the amp. All of that looked fine, so I went ahead and installed the tubes and moved to the next phase which is where a minor disaster struck. The amp worked, but I had a high pitched squeal and sounded robotic / metallic any time you tried to play anything through it. So I turned it off, went back through all of my connections and couldn't find anything wrong. I shot an email off to Mojotone to see if they had any suggestions, and I also posted on the PedalPCB forum as I knew there were some amp experts there. I want to give a huge shout out to RetiredUnit1, Giorfida, jubal81, phi1, and HamishR - with their help the problem was identified as a positive feedback issue, which is apparently fairly common for these amps per Rob Robinette's Tube Amp site. The fix was simply reversing the red and blue wires coming from the output transformer (Mojotone got back to me Monday morning after I'd fixed it with the same diagnosis and suggestion). With those two wires reversed, the amp suddenly went from a squealing monster to an amazing-sounding amp! 

Rexie is very excited to help get the amp in the cabinet!

With the unit now fully functional, it was time to get everything into the lovely tweed cabinet I ordered with the kit. I did manage to take more photos of the final installation, and as you can see from the photo above I had some help!

The first step is getting the speaker ready. I'd picked up the Anthem speaker when I bought the kit as that was the recommended one for this amp. Once again I was twisting wire together, which was a bit of a pain as the white and black wires were of slightly different gauges. It took some time, but I did manage to get everything squared away. I could have probably gone with a shorter wire, but I erred on the side of caution. The leads had to be hand wired into the jack plug as well and I did add some heat shrink insulation on the positive lead to make sure it stay isolated.

Once the speaker was tightened down, it was time to add the amp chassis to the cabinet. Rexie and Tremor the Astrodon (inside joke) were happy to help here as well as you have to align the chassis to the back plate. Note that the tubes have been removed at this point.

Turns out Rexie's small arms and hands are perfect for inserting tubes!

Once the chassis was aligned, it was time to reinstall the tubes and tube covers. This is the last time when you actually get to see all of the cool parts in the amp itself. Once the back plate is on, all of guts of the amp are invisible (which is probably good, because high-voltage and everything). The wood back plate also impacts the resonance of the cabinet, so having a see-through plate would likely impact the sound quality.

Here are a couple of views of the completed amp. From the top you can see I've added the chicken head knobs. The controls are really quite simple, but the amp does have two channels (low and high). Per the Mojotone site, "the low input is is more tame with a slightly warmer overall sound, while the high input breaks up easier, and is glassier and brighter in nature." I need to play around a bit more with both, but so far I'm absolutely loving the sound of this amp. It blows away the little Fender Champion 20 I'd been using upstairs - but that was never a fair fight to begin with.

From the front the amp looks just like any other tweed amp. The cabinet is really very high quality and getting the cassis and speaker into the cabinet was really very simple. I'm quite excited to have this project finally done. I learned a lot, and will likely continue to build more amps in the future.