Mojotone Princeton Reverb Clone Project

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Here is a log of how I assembled the amp. The Mojotone kit does not include any assembly instructions, so you have to plan out all the steps yourself.

Drill chassis mounting holes in cabinet

There are four long screws that are used to mount the chassis in the cabinet, but the cabinet does not come with the holes drilled. I decided to drill these holes first, before installing anything heavy in the chassis. The chassis fits into the cabinet like this:

Finding the right place for the holes is a bit tricky because the front end of the cabinet and chassis is sloped. I used a ruler, the metal straps, and the chassis itself as a guide to mark the drilling location for the holes. I decided that it would be easier to measure from the back of the cabinet (to avoid the slope). I used a 11/64" bit to drill the initial holes and then later used a 3/16" bit to widen them out a bit.

Install tube sockets

Next I installed the 3 large and 4 small tube sockets using 4-40 3/8" and 4-40 1/4" screws, respectively. Make sure pin 1's position matches the wiring diagram. (I forgot to put the retaining springs on the 3 big tube sockets and had to go back and add them later.)

Install power transformer

I installed the power transformer using 4 8-32 nuts, keeping the transformer label pointed towards the back to make it easier to see when the chassis is installed in the cabinet. I also looked at installing a ground terminal on the power transformer screw, but discovered that the screw is too big to fit through the hole on the 3 lug terminal strip.

Install output and reverb transformers

Next, I installed the output and reverb transformers using grommets and 8-32 3/8" screws and nuts. I also went back and installed the retaining springs for the 3 big tube sockets.

Install cap can

The chassis holes for mounting the cap can clamp are the same size as the holes for mounting the tube sockets, so the biggest screw that fits is the 4-40. Unfortunately Mojotone only provides enough 4-40 screws and nuts for the tube sockets, so I ended up going to Home Depot and buying 2 more 4-40 3/8" screws and nuts (the first figure below shows how the next largest size screw will not fit through the hole). Once I had new the screws and nuts I was able to install the cap can.

Install pots

Next I installed the pots in the chassis. One of the tone pots was missing a flat washer, so I contacted Mojotone and they sent me the missing washer.

Install input jacks and pilot light

I installed the instrument jacks and pilot light next (they'll have to be removed when their wiring is installed). I was a bit concerned about the pilot light assembly shorting out on the chassis, so I later added some black electrical tape to further insulate it (see further below).

Install RCA jacks for reverb and footswitch

Next I attempted to install the RCA jacks for the reverb tank and footswitch. Unfortunately, Mojotone included the wrong type of RCA jacks in their parts kit, so I had to get them to send me replacements. The screw threads for the RCA jack should be set up so that you install the nut that holds the jack to the chassis from the outside of the chassis, not the inside. Otherwise you won't be able to get enough of the threads through to get the nut on securely. The first picture below shows the wrong RCA jacks, while the second two pictures show the replacement RCA jacks properly installed.

Populate eyelet boards

Next, I populated the main and bias outlet boards with resistors, capacitors, and diodes.

Part check once eyelet boards are done

Once the eyelet boards are populated, the following resistors and capacitors should be remaining in the parts box:

1 .047 uf orange cap: the "death" cap (unlikely to use this)
2 100 ohm resistors: virtual center tap for heaters (off pilot light)
1 6.8K ohm resistor: basss tone control
1 15K 2W ohm resistor: on the cap can
2 68K ohm resistors: for input jacks
1 100K ohm resistor: speed pot
1 220K ohm resistor: reverb pedal jack on back to ground
1 1M ohm: for input jack circuit

Reduce main eyelet board lead count

Some of the eyelets in the main board were over-stuffed with wires. For example, the eyelet in the top middle of the board had 6 leads going into it. To reduce this, I decided to solder the leads of each cathode bias resistor directly to the leads of its cathode bypass capacitor rather than running it through the eyelet (the cathode bypass capacitors are the black caps seen in the picture).

Solder wires onto main eyelet board

Next I soldered all the needed connection wires to the eyelet board. To do this, I estimated the length of each wire and then soldered it in. When multiple wires used the same route, I used a marker on the white cloth to color code them to ensure that I made the proper connections. I trimmed off the extra leads on the back of the board as I soldered in components.

Add flyover power wires to main eyelet board

I installed all the flyover power wires next. These wires carry DC power from the power supply nodes B, C, and D on the left side of the board to the right side. I used a marker to color code the cloth wires: B=red, C=black, D=blue. The wire marked X in the wiring diagram is also in flyover bundle (I left it white). (In pictures of old Fender amps B=red, D=yellow, X=white and C is not used). Once this is done, only two eyelets should be left unsoldered: these are where the red wires from the output and reverb transformer connect.

Drill and solder bias eyelet board, insulate chassis under pilot light

Next I worked on the bias eyelet board and pilot light. I put the bias eyelet board in place to mark where I needed to drill in it in order to put in the mounting screws (I used an 11/64" bit). I also added tape to the area where the pilot light comes close to the chassis to ensure that it doesn't short out on the chassis. I then soldered all the leads to the bias eyelet board.

Build instrument input circuit

I flipped the input jacks around to the outside of the chassis so that I could use the holes in the chassis as as guide to assemble the input circuit. I used heat shrink tube on the resistor wires to protect the circult. I also added a ground wire to the input circuit to connect the ground on the pots.

Install grounding bus and lug

The diagrams provided by Mojotone leave the details of the amp's grounding scheme up to the builder. A poor choice of grounding scheme may lead to a noisy amplifier (e.g. due to ground loops). After researching grounding, I decided to use the grounding scheme described here: http://el34world.com/charts/grounds.htm. For this grounding scheme, you solder a grounding bus across the back of the pots and connect it to the input circuit (the input circuit is grounded to the chassis). The power supply is grounded to a 3 lug terminal strip I installed near the power transformer. The two end lugs are connected to the center lug, so all three lugs are grounds. The picture below shows the ground bus soldered to the back of the pots. I used the ground wire from a 14-2 romex power cable for the ground bus.

Screw main eyelet board to chassis

Next, I screwed the main eyelet board onto the chassis using 3 6-32 screws and nuts. I had to loosen the output transformer to get one of the screws in. I had to drill holes in the backing board for the screws to go through (maybe I should have done this before installing all the parts).

Solder input and pots to eyelet board

I soldered in the input circuits, the pots, and the ground connections from the main eyelet board to the ground bus on the pots.

Solder preamp tubes, speaker/RCA jacks, reverb/output transformers

Next, I soldered in the 9 pin tube sockets, starting with the first 7025/12AX7 preamp tube. I also soldered the reverb and output transformers. The black (ground) wire from the reverb transformer is not shown on the Mojotone wiring diagram (it goes to the RCA jack shown in the figure).

Solder power tubes and cap can

Here I've soldered in the cap can, and the 6V6 output tube sockets.

Solder power transformer rectifier circuits

Next I soldered in the 5V and high voltage secondary leads of the power transformer (the red/yellow wire is the grounded center tap of the high voltage circuit).

Twist heater wire and install to pilot light

I twisted the green wire together for the 6.3V heater circuit, and then I attached it to the pilot light along with the 2 100 ohm resisters that form the virtual center tap for the heater circuit. Then I soldered the whole assembly to the 6.3V feed from the power transformer and reinstalled the pilot light.

Install heater power wires to all sockets

Next I soldered in the twisted 6.3V heater wires into all the tube sockets. The green wire is twisted and kept above and apart from the other wires to avoid unwanted AC power noise.

Install 3 prong mains power cord

I installed the 3 prong AC mains power cord. The black hot wire goes from the cord to the fuse and then to the power switch. From the power switch it goes into the primary winding of the power transformer and returns through the white neutral wire. The green ground wire from the plug has its own connection to the chassis through one of the screws on the power transformer.

Remove tubes from box and install

I removed the tubes from their boxes and checked their fit in the tube sockets by installing them. I later removed them before powering up the amp for the first time

Install speaker wire and speaker in cabinet

I twisted together the black and white stranded wire and installed the 1/4" jack on the end. I then installed the speaker in the cabinet and put the chassis in the cabinet to verify that everything still fit.

Next steps

The amp is now ready for testing before final assembly!

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This page maintained by Chuck Cranor