Wednesday, April 2, 2014

Mojotone 5E3 Deluxe Kit

This was my first attempt at building a tube amp from a kit. I should state first off that building an amp like this is not easy to do and it is especially difficult to troubleshoot when you have no idea what you are doing. It is a learning process, start to finish. In the end, however, the amp sounds great. I had been thinking about building an amp kit for a couple of years and was debating between B.Y.O.C.'s Tweed Royal, or a 5E3 (Deluxe) or 5F1 (Champ) from Weber, Marsh, or Mojotone. Mojotone put several amps on sale the week after Thanksgiving and I picked up a complete 5E3 kit for $500 plus shipping. When the kit arrived there were a couple backordered parts (including the fiberboard) which arrived separately within a few days, and one part which was mispicked, which Mojotone replaced immediately. The kit components are all high quality--Carbon Comp, Sprague, TAD, Orange Drop, and JJ and EHX tubes. The cabinet is unlacquered tweed and not drilled for the chassis. The only component which I wish had been different was the Jensen C12Q ceramic speaker. I would have opted for a P12Q alnico speaker or perhaps no speaker and ordered a Weber 12a124a separately. The sale price did not allow for substitutions or deletions, however. Mojotone does not provide detailed instructions for their kit. There is a 5-page general instruction pdf available on their website, and the kit ships with a schematic and layout, the later of which was most useful in building the kit. I ended up relying on detailed instructions for other 5E3 kits and, well, figuring it out as I went along.

Soldering components to the board is pretty easy.

I won't bore you with all of the details about building the kit. Mojo's website claims that the kit will take about 5 hours to build. I took about a zillion hours to put this thing together. I spent several evenings and a weekend putting the kit together, and many hours more troubleshooting, researching problems on the web, resoldering, etc. Of course once you build one successfully, the amount of time would go down significantly. What I will do instead is to describe the the various problems that I had to sort out in case you run into the same things.

1. Ground the fiberboard properly. The Mojotone layout schematic does not include any direct instructions on how to internally ground different parts of the circuit. It took me a while to understand that the chassis itself serves as a ground and that there are three ground leads coming to from the assembled fiberboard that need to be soldered or bolted to the chassis, pots or jacks. My first attempt at grounding the circuit was to solder all three ground leads from the fiberboard to one lug which I then attached to one of the bolts from the choke. This did not work and the amp was very noisy. By chance I started prodding leads and the fiberboard with a chopstick and, completely randomly pressed the fiberboard near at the top left, according to Mojo's schematic, and grounded all three leads when one of the soldering points on the back of the fiberboard made contact with the chassis. The amp sounded better when I would do this so I knew that I had to rethink how to ground the leads. My "accidental" realization that there was something wrong with the grounding was only possible because I had not used the backing board that Mojo had supplied with the fiberboard (see #2 for more on this). My solution to the grounding issues was to buy new grounding lugs and install them in two places, first on the same choke bolt  where I had first soldered all three ground leads, and second on one of the mounting screws for V1. I relocated the ground that comes off the top right of the board to this screw. I then relocated the ground that comes off the top middle of the board to the ground lug on the normal circuit volume pot. This seemed to provide a better overall grounding scheme for the circuit and the amp started sounding better.

2. Use the backing board. Mojo's instructions didn't indicate what I should do with the backing board, so when I originally assembled the kit, I didn't use the board at all. After working through the grounding issues, I realized that the backing board was necessary to prevent the soldering points on the back of the fiberboard from shorting out against the chassis. I first "solved" this problem by unbolting the pots and jacks and pulling the top of the fiberboard out just enough to slide the backing board in behind the fiberboard and then pushing both boards back into place. This of course was only a temporary fix, and eventually I drilled the fiberboard, backing board and chassis so that the whole caboodle could be bolted to the chassis. This is a bit harder to do once then entire amp is built, so better to do this first before everything is wired in place.

3. Double-check your soldering. It is worth it to double-check your soldering as you are assembling the amp. When I was unbolting the jacks to squeeze the backing board in, I realized that I failed to solder the #1 jacks to  the #2 jacks such that that two of them fell off when I unbolted them. They had worked when I originally assembled the amp because the resistors leads I used to connect the jacks had made contact with the lugs even though they weren't properly soldered. I resoldered the jacks which fixed the intermittent cut-outs that I had in the #2 inputs.

4. Don't use brute force to seat tubes in tube sockets. In my case I had a lot of trouble seating the 12AY7 tube in the V1 socket. I ended up forcing the tube in force rather than trying to figure out why the tube wasn't seating properly and I bent the tube pins as well as bending and displacing the metal in one of the socket pin holes. After removing the tube, I managed to gently bend the tube pins back to their proper position and to open up the metal in the socket pin hole using a unbent paperclip (one of the most important tools ever invented), but after seating the tube again, I realized that some of the pins fit well, and others not so much. I ended up ordering a new tube socket from Mojo and replacing the original one. I was much more careful installing the tubes after this lesson.

5. Clean up internal lead dress. The amp still didn't sound right and after often reading about how lead dress affects an amp's sound, I decided to rewire most of my connections to the tube sockets, jacks, and pots. It's pretty easy to find pictures of the various ways to dress leads online--pick one and try your best to follow it. Shorter leads are best, and avoid crossing over leads if at all possible. Common practice also seems to be to run leads against the chassis wherever possible. This is more an art than a science. I ended up shortening most leads, especially to the pre-amp tubes, jacks, and pots. In the end, the inside of the amp looked much better, and seemed to be less noisy as well.


6. Double-check your soldering again. After all these problems the amp still didn't sound right, the biggest problem now seemed to be that the amp's volume was really loud with very little distortion, and seemed to reach maximum volume at around 3-4 in both the normal and bright circuits. I decided to check all my soldering again, and this time I found another loose wire, from pin 4 of V3 which connects the power tubes screens to the caps on the left of the fiberboard. While I am only beginning to understand how this amp works electronically, this seemed to have the effect of producing no screen voltage and no attenuation of the power tubes, thus producing full undistorted power at 4 on the volume pots in either circuit. I resoldered this lead and the amp was significantly quieter and, for the first time since I had assembled the kit, the amp would distort with both volume controls at about midway. I was making progress, but I wasn't quite there.

7. Use Voltage charts. I finally decided to compare voltage readings from my amp with what was available online, in particular Doug Hoffman's Valve Data Page. There are several users who have posted voltages from their 5E3 amps to make comparisons easy. I was hesitant to check the voltages in my amp because I didn't want to stick anything conductive in the amp and kill myself, but once I got over my fear of death and got voltage readings from the amp using a cheap Radio Shack multimeter, it became pretty clear that the voltages for V2 were not correct. Everything else, the rectifier, power tubes, and V1 were more or less within tolerances of what different users posted on the Hoffman site. After thinking about this for a while I decided to double check all the cap and resistor values on the fiberboard, especially in the phase inverter section to see if I had made a mistake with the components. After everything checked out O.K. I thought that there might be a problem with the 12AX7 tube, probably due to my own repeated installation and removal of the tube. I decided to order a new 12AX7 tube just to be sure. While I was waiting for the tube to arrive, I realized that I could use a 12AX7 tube from my Fender Champ 600 reissue amp to see if this would give proper voltage readings. When I removed the suspect V2 tube I realized that I had, in fact, installed the 12AY7 tube in the V2 socket, which doesn't provide nearly the amplification factor that a 12AX7 tube would. Which leads to:

8. Install the V1 (12AY7) and V2 (12AX7) tubes in the correct socket. Just double check this as you are installing the tubes. I ended up with two identical 12AX7 tubes since I thought one was bad, and a common substitution for the V1 12AY7 is a 12AX7 which increases preamp gain significantly.

9. Crank it up! In the end, after much trial and error, the amp sounds great. I'm still learning how to use the interactive controls, but the classic Fender tweed sound is amazing. It's also given me the desire to build more amps, and I am considering a 5F1 Champ kit, a blackface Deluxe Reverb kit, and a tube reverb unit kit to give the tweed amps a little twang. The folks at Mojotone provide an excellent kit and are super responsive to issues with parts. They do not provide detailed instructions, but I sure learned a lot as I put together the amp!