Welcome, one and all, to another installment of Mojotone’s ‘Make It Monday’ series. This week, we are diving into a very exciting project; Mojotone’s all new GA5 Style Amplifier Kit. The GA5 Style kit is the newest member of Mojotone’s amp kit lineup, it’s a beginner build, and perhaps the coolest thing about the kit: it’s a true point-to-point build!
In the past, we have put together a number of Mojotone’s amp kits, and while they have all been vintage hand-wired amp kits, they have also all implemented turret boards and/or eyelet boards to route together the smaller electronic components. The GA5 style kit does not implement a component board of any kind, requiring the builder to wire all electronic parts directly to one another. This is going to be a bit different for us, but as long as we take our time and pay attention to the details, we should be just fine. If you are following along at home, here’s a list of the tools and supplies you’ll need…
Before we get started, let’s take a quick inventory of everything in our kit. Watch the video below for a quick look…
I would like to note that my actual workshop is in a transitional phase, so don’t judge me; a man still has to build!
Once inventory has been taken, we are ready to get going. We will start by prepping our chassis. This will involve mounting all of our hardware components (transformers, tube sockets, pots, etc.) to the chassis itself, using the mounting hardware included with each part. For recommendations on the order in which you should mount your hardware, follow along with your instruction manual…
Note: Pay special attention to the way your components are mounted; e.g., your power transformer is mounted from the underside of the chassis and is recessed into the chassis’ insides, whereas your output transformer is mounted completely on the inside of the chassis facing up.
In some amp builds, the outer lugs of of grounding terminal strips need to be wired to the center lug for proper grounding. In this particular amp kit, however, we will not need to do this. Simply mount your ground terminal strips to the appropriate mounting hole and stop there.
It’s time to start getting our power transformer groomed. Follow along with your instruction manual and prep each of the color-coded wires as the manual specifies…
After your wires are grouped/twisted together, and trimmed to the appropriate length for making each connection, you’ll need to strip back about ½” of PVC shielding from each wire. Once your wires are stripped, it’s time to start putting them in their respective places, and making solder connections where possible. Remember, we only want to solder a wire into place if NO OTHER wires need to be soldered into the same connection point. This means our black + black/white wires can be soldered to the power switch, but the yellow wire going into pin 8 on our rectifier socket CANNOT be soldered yet. Reference your manual, take your time, and make the right connections.
Once we've soldered in some, but not all, of our power transformer wires, we'll need to get started on our filament wiring. This will employ the use of the green wire that came with our small parts kit. Hold the wire up and measure from the solder points on the pilot light assembly to the lugs on your preamp tube socket. Now cut the wire to length, and then cut a second piece of the same length and twist the two pieces together. Strip back all the ends of the wires and refer to your instruction manual for their solder locations. One green wire should go into each of the terminals on the pilot light assembly (these can be soldered in right away). On the preamp tube socket side, one green wire should be connected to pin 9, and one wire should be connected through pins 4 and 5, but do not solder these in yet, as we will need to cut more green wire to carry this filament connection over to the power tube socket.
Repeat this process to prep the connections from the preamp tube to the power tube (following along with your manual), and once everything is in place, all of these connections can be soldered in. One more connection can also be made at this time: Trim the black wire from your small parts kit to length so that it may connect the right terminal of your power switch to the AC power inlet terminal closest to the volume pot (refer to wiring diagram for visual).
Moving on to our output transformer, let’s go ahead and locate all of our color-coded lead wires, and make note of their respective connection points on the wiring diagram. Notice that the blue and yellow leads have connection points which include multiple components -- this means we will not be soldering them in right away. In this case, just cut the lead to length, strip the insulation back, insert the lead into its lug and use a set of needle nose pliers to hook it around the lug until we are finally ready to solder. For the red, black, and green leads, we can go ahead and solder because they have single component connections...
Now it’s time to...well...do the rest of the wiring. Your instruction manual provides an excellent step-by-step breakdown if you are looking for an order in which to do all of this wiring. For time’s sake, I won’t be able to outline each step in this article so please spend some time with your wiring diagram and build instructions -- it’s a fun one! But first...
Note: Because this is true point-to-point, capacitors and resistors will be ‘floating’ above other components. Use some of the insulation from wires we have already trimmed to cover the leads of caps and resistors -- this will prevent them from touching one another and shorting out when power is applied. And pay special attention to the orientation of your polarized capacitors (refer to wiring diagram for proper orientation).
Once we have carefully studied our wiring diagrams and completed each connection, our amplifier circuit is theoretically finished; provided our testing goes well. So give yourself a pat on the back, this is a huge milestone.
Now it’s time to begin testing. Again, for time’s sake, I won’t be able to outline these steps individually so follow along with your manual and run through the testing steps. Remember to test AC voltages first without any tubes installed, and then if everything checks out, you can add in your rectifier tube and continue testing, so on and so forth. Let’s meet back here after testing! Oh, and how is my build going, you ask?
Note : In order to complete the final phase of testing, you will need to connect the speaker to the amp’s speaker jack. This means you need to assemble your speaker wiring harness in the process. If you are unfamiliar with this process please refer to our article on CABLE WIRING.
As I said in the video above, all of my preamp plate voltages checked out (this should be around 150V +/- 30V), along with my filament, etc. If your voltages were not right, you'll need to go back and check all of your connection points (in conjunction with your wiring diagram) to make sure all necessary components are included in each connection. Another important thing is to make sure none of your leads are touching somewhere in the amp and causing issues. Verify that solder has been properly applied to all connection points. Once you've double-checked all of these bullet points, you'll want to start at the top of the testing process and see how far you get.
Once testing is successful, all that is left to do is install the speaker (if you have not already done so during the testing phase), and then install the chassis into the cabinet. Mojotone has made this part extremely easy: Using the small horizontal slits in the face of the cabinet, slide the chassis into place. Next, use the two black wood screws from your small parts kit to secure the chassis. That's it! It's time to rock on this bad boy!
Thanks for tuning in once again, and please send any questions or comments about the kit (and any future projects you'd like to see) to [email protected] See you next time!