Cherry Bomb Build Diary

Here is a build pictorial we did for customer recently. We thought we'd share it so folks could what goes into building a Cherry Bomb. You can follow along with the pictures posted above.

The first thing we do is put the transformers and choke onto the chassis. We also put in the chassis mounted power tube sockets. Obviously we have to route the transformer wires and put in the grommets so the wires don't get cut by the chassis. We also put in the impedance selector and the power on light. If you look at the pictures, we also add some grounding tabs at the feet of the power transformer and you can see there is no powder coating on the chassis there. We'll be running ground wires to those tabs from various places. Those ground tabs don't quite fit on the transformer screws so I have to shave them down a bit... Kind of a pain, but it works. 

We also assemble all the parts for the amp from my inventory and put all the hardware into a magnetics tray so stuff doesn't get lost. Works great! 

Notice as well that we have the boards already populated. We have an expert on staff who is just incredible at doing the PCBs - does the work much better than I ever could, that is for sure. Attention to detail is amazing - we've never had a board with a misplaced part or any functionality issues. I still have 15 sets of boards complete for more amps and because of how quickly and accurately this individual works, I can get 10 new sets in 3 weeks or so - very fast!

Next we start cutting the power transformer leads and twisting them as required to keep them neat and get them set for routing to the power supply (PSU) board. The unused leads are capped and twist tied. This way if someone wants an international voltage, it is no problem to take off the PSU, unsolder some wires and route the new ones. Now we wire in the power and standby switches and the IEC plug.

The next step is to wire in the fuse holders and everything is now complete for the PSU board to be put in. You can also see that we twisted the output transformer wires and installed the EFX loop board. Next we installed and wired in the PSU board and also wired in the output transformer to the impedance selector and speaker outputs.

Next t's time for the main board to go in. You have to be a bit careful when doing this to make sure that the LEDs line up right and don't get trashed when putting in the board. In some pics you can see more detail around the speaker outputs, impedance selector, output transformer - all now connected to the main board.

Now, we finish wiring in the switching jacks. It's a bit complicated, but we have to wire these in to test that the switching is working. We also wired the power supply board to the main board and the main board to the EFX board (you can see the new wires connecting the MIL-STD screw connectors we use on each board - we don't think anyone else is doing this in the amp world...). What's cool about this is once that is all done, we can test the  heater and switching supply voltages to make sure the tubes will heat up and that all the switching works. We use heavy gauge bare wire to link the three switching jacks so it is ROCK solid. You can also see a shot of the wiring of the main board to the EFX board - just 4 wires, but, allows us to do the heater testing. What is cool is that we use these MIL-STD screw connectors everywhere so that servicing the amp is a breeze - to take out the main board, we just unscrew all the flying leads instead of having to unsolder everything.

OK, now to test the switching. We powered up the amp and you can see the big amber main power light is lit and it is a little tough to see, but there are several shots where we have tested the switching and the correct LEDs lit.

Finally, we turn the amp off, pop in the preamp tubes, turn it back on and checked that they all lit up. They actually all lit up as expected, so the DC heaters are working. Measured the AC heaters for the power tubes and the DC heater voltages for the preamp tubes and those are in range. The 6V supply for the LEDs and relays is right as well. The DC heaters and how we did the layout make this one of the quieter higher gain Marshall-style amps you will hear, even cranked!

Next, we wire in the power tube sockets to the main board. This is a really time consuming activity because it is a total of 32 1.5" wires that have to be cut and stripped on both ends (and then tinned) as accurately as possible so it looks good and makes good connections. There isn't a lot of room so you have to be really careful and take your time so as not to burn the board or any other components with the iron. Everything went well and here are a few pics so you can see the job. All solid, tight connections and it looks cool too...

Ok, now on to the the hard stuff! The shielded wiring... Definitely the most time consuming part of building the amp. There are 13 shielded wires that have to be put in the amp... Next we wire in the rest of the pots that don't require shielded wire. We also wired up the footswitch as well, which you can see in the last two pictures in this section. Totally heavy duty case and PCB/aerospace connector wiring.

At this point, we do the burn in. What we do is just bring up the voltage in 5V increments on the variac over a couple hours until we are at full voltage to make sure everything is cool. I don't want to bring the full power on right from the start just in case of an issue. No problems here though. Next I biased up the amp (~38 ma for this amp with a 461.7 V B+). You'll see some pics above when we were doing the bias. Note the speaker cable coming out of the speaker out. It is connected to my dummy load at 16 ohms. Once the power tubes are in, you always have to make sure you have a load or risk power tube and output transformer damage. When you are just playing a finished amp, it usually isn't an issue because the cab is always connected, but when doing a build, you want to really make sure you don't forget when testing! At this point, we test all the voltages to make sure everything is in spec. Everything was perfect in this case.

Now, on to our favorite part! Firing up the amp and testing it out with a guitar for the first time. We alway set the amp to  standard settings for each channel and master and at first keep the overall master on the back pretty low.You can't see it but it is actually on in the second and third pics. We hooked the guitar into our W/D/W rig, plugged in a Music Zoo Nitro Relic Charvel and took it off standby. Checked the PLEX channel first, then the ROD, then the SOLO boost master. The line out was working as verified by the two wet cabs firing and sounding great. All the SATURATION switches and the INTERNAL PLEX JUMP switch work as expected. The footswitch and cable work as well (Note that when the footswitch is hooked up the LOOP, SOLO, and CHANNEL SELECT switches on the front of the amp are disabled. When you disconnect the footswitch, they work).

So... next we need to check out the EFX loop and make sure it works in both Series and Parallel mode. We also do some VERY loud testing to make sure it sounds great cranked. That's it!