6) tube pin configurations

note that the view of the pins shown above is looking into the socket from the same side as the tube mounts. When wiring the sockets, you will be working from the other side (or underside) of the socket, and the wiring will appear the reverse of above. The pin numbers are usually stamped on the underside of the socket.

tube type......class by construction.............service

7025...........high-mu twin triode...............class A amplifier
12AX7..........high-mu twin triode...............class A amplifier
12AT7..........high-frequency twin triode........class A amplifier
6V6............beam power amplifier..............class A amplifier
5AR4(or)GZ34...full wave high vacuum rectifier...full wave rectifier

5) placing the componants

Mount the transformers and tube sockets with the screws provided. I like to place the screws so all of the nuts are outside of the chassis and will be visible for quick inspection by looking inside the cabinet (without removing the chassis). If a nut works loose, I want to know it before it falls off inside the chassis, and the much dreaded "poof" transcends the cabinet.

peripheral components
Secure the main eyelet board and backboard (loosely) with two 1" screws (not provided). Place the pots and jacks and remember to put the black face plates on first. The fuse holder has to mount through the faceplate from the front. Although this is only a preference, I find it easier to wire all peripheral components (jacks, pots, transformers, tube sockets) in to the eyelet board before mounting resistors and caps on the board. Mojo sent a small coil of white cloth wire that was about half (or less) of the required amount. Either get more from Mojo or improvise. I was too anxious to wait for Mojo to supply more, so I used some other wire I had on hand.

tube heaters
First, wire the tube heaters with the heavy green wire. This will allow the the wire to be dressed neatly close to the chassis (under all of the subsequent wiring). This is an attempt at minimizing the likelihood of 60 hz hum and to add to the visual appearance. Quality counts.

After the heater wiring is tucked neatly against the chassis, wire the peripheral components. There are a number of points that require grounding. Attempt ti minimize the length of ground leads, by gathering grounds at convenient points as required.

commentary
Mojo isn't really big on documentation. Other than the layout and (incorrect) schematic, we got diddly. In order to complete the project with a zero guesswork factor, I dug up an old (early '70s) GE tube manual that became integral to our success. The base/pin configs are a must-have, either from old tube manuals on your bookshelf, or by scarfing the net.


resistors & caps
Do nor clip res & cap leads too short, and do not leave them too long. A judgement call. Leave them with enough length to look neat, and to allow for troubleshooting (which may require desoldering, resoldering, desoldering, resoldering, desoldering, resoldering . . . . ). With good luck, this won't be necessary, and your DR will ring out in melodious tones the first time you switch it on. Mine didn't.

little things help a lot:
1. on the schematic, the cathode end of the diode (depicted by the solid bar being pointed to by the triangle) corresponds to the solid bar on one end of the actual diode.
2. when looking down at the eyelet board in the inverted chassis, the white dot on the optoisolator faces up toward you and is on the end of the optoisolator mounted toward the tube sockets.
3. vacuum tube pin configurations - see diagrams in next post

4) Securing the eylet boards to the chassis

Before any components are positioned on (or attached to) the chassis, the main eyelet board, the filter board, and the bias adjustment board (small board near the pilot lamp), along with their associated backboards should be positioned and drilled through the chassis. Five holes in total. The filter board carries the power filter circuitry and is attached to the outside (or underside) of the chassis. The idea here is to drill the holes and remove all of the metal shavings prior to placing any components in the chassis. Drilling up front saves a headache later.

Prior to the build, it is essential to scour the web for as many images of the chassis as it takes to make you comfortable with positioning the boards and the components on the boards. No big thang - there are tons of images out there.

main board:
Two 8-32x1" machine screws work well to secure the main eyelet board to the chassis. Drill holes (centered laterally on the board) about 1 1/4" in from both ends of the board. The 1" screws are longer than the ones that come with the kit. They provide the advantage of allowing the board to be lifted away from the chassis (by backing the nuts almost all of the way off of the screws) while still remaining in place and attached. Lifting the board allows wires to be fed under the board, as will be required later.

Note that the screws should be placed so that the nuts are on the outside of the chassis, opposite the main eyelet board, thus (down the road) enabling a quick visual inspection inside the cabinet to verify that the nuts are still secure. If the nuts are toward the eyelet board inside the chassis, they could eventually work loose and fall into the components on the board, unbeknownst to the guy with the dead BFDR that smells of burnt components.

filter board:
The position of the filter board on the outside of the chassis is defined by the holes pre-drilled for its can cover. When the filter boards' position has been established, a screw hole must be drilled through the sandwich made up of the center of the filter board, the chassis, and main board. This single screw will secure the daughter board, and is sufficient as the daughter board fits neatly under the can cover with no room for lateral movement ie. the can cover holds the board in place. Use an 8-32x1" screw here as well, for the same reasons mentioned above.

bias board:
I'll call this small board the bias board (for lack of a better name). It can be positioned and drilled near the pilot lamp. Two of the shorter 8-32 screws (with the kit) will work fine here, as the behind-the-board wiring can be completed before the board is screwed in place. Again, I prefer to keep the nuts outside the chassis.

That's it - 5 holes completed.

3) Instructions? What instructions?

Mojo provided 11x17" photocopies of the component layout and the schematic, and that's about it, two sheets. A Google search of "ab763" results in tons of renditions that are closer to the Fender originals. The on-line ab763 schematics are in a couple of versions (C-FD and J-FC) and are basically the same.

Interestingly the schematic that comes with the kit is a totally redrawn Mojo version of the ab763 that has a gaping error in it. The "D" supply that powers both preamps shows an incorrect connection to the plate of the normal-side 12AX7 tube. This shouldn't be. A verification of this schematic against the Fender original will set you straight. Also, a couple of resistors don't match between the Mojo schematic and Mojo layout. I called Mojo and noted the "D" supply error. The guy agreed and made the change on the master. It amazes me that after all of the kits they have sold, the error hasn't been picked up. On one hand, Mojo appears to be a very conscientious and reputable company, and on the other hand, they seem like two guys working out of a basement.

Their pitch: By not providing step by step instructions, Mojo say they are limiting the likelihood that the circuitry of any two amps will look and sound the same. Each amp will be built as the builder chooses, each having unique characteristics. Kind of like creating audio art. At absolutely no additional cost to you, they have provided the premium service of allowing you to blindly create a unique sound experience. Is this good marketing or what?

My bitch: The problem arises when the builder has no previous experience with tubes and tube heaters, proper grounding principles, and a sense of proper lead dress. Previous experience has gifted me with a sense for these. Not always the case.

Marsh provides a similar kit and does provide instructions, at least that is what their site says. Marsh may also use Mojo cabinets.

2) First impressions

Got the boxes home and opened them. First the components. Nothing special. A resistor is a resistor is a resistor. Black face plates (front and rear) are very well done. Jensen C12Q speaker. OK. Chassis looks OK - no sharp burrs or corners, and all drilled. My most favorable impression came from the cabinet. It was perfect! Mojo does an amazing job replicating the cabinet and grill. I'm betting that it is as good or better than the original Fender. Only one minor problem is that the handle strap is so stiff I can't get my fingers under it, but I think it will stretch and be OK. Overall this would impress Ed Asner.

Decision point:
I suggest that all of the components be checked off against the packing sheets, to ensure everything is there. If you don't do this, and you get to the end and realize something is missing, you don't know if you chucked it out with the trash, or if you ever had it in the first place. Verifying against the packing sheet up front may mitigate a bad surprise at the end. We received an extra choke and an extra faceplate. Bonus. The components were all there, with a few resistors to boot.