Buy Boards and Parts:
The Millett Hybrid MAXed - Construction

Casework Part 3 - Wiring & Assembly

The following represents a method for MAX casework, but not the method. Some of you may have a drill press or some of you may use Front Panel Express. Still others may have access to a complete machine shop or wood shop. However, the following casework is based on the Hammond case in the MAX BOM and was accomplished with only a few inexpensive tools, along with simple items that can be found in most homes:
  • Step Drill Bit set from Harbor Freight - $10-$20
  • Conduit/Chassis Punch Set from Harbor Freight - $10-$20 or
    Greenlee 1" O.D. Slug Buster Punch - $45 (details on Part 2)
  • 6-32 tap (see details on Part 3)
  • Computer Printer with drawings on this site
  • Rubber Cement, file folder, scissors, paper hole punch
  • Scrap 2x4 or other wood scraps
  • #6x1/2 wood screws
  • Pocket Knife (Leatherman or Swiss Army preferred)
  • Manual Center Punch or Automatic Center Punch from Harbor Freight - $2.50
  • Handheld Power Drill
Tip Jacks
One of the advantages about the Millett MAX are all the adjustment points, allowing you to specifically tune every aspect of the MAX's performance. One of the disadvantages about the Millett MAX are all the adjustment points. Seriously, since the MAX incorporates a complete amp from power supply to tubes to fully discrete output stage, there are a lot of adjustment points. If you adjust your MAX with the top removed and settle on a pair of tubes, there should be no reason for re-adjustment for many months or longer. However, if you change tubes, change locations where the wall outlet voltage changes (there's a lot of variance around 110 - 120VAC), then you will need to readjust the voltage set, the tube bias, and perhaps the DB's bias. Of course, you may be fiddling with different settings to explore the nuances of the effect on sound quality, and you would readjusting these setpoints continuously anyway. In those cases, it makes sense to provide for adjustment from the outside when the enclosure top is on and the amp is completely buttoned up. Of course, in the previous pages we drilled all the holes to make this happen - only wiring up the tip jacks remain.
The tip jacks used are about as cheap as they come - Mouser #530-105-0801-1. There are much better tip jacks available - but these are entirely functional, are totally insulated being plastic, and have a crimp/solder connection. Higher quality tip jacks sometimes used by others have one of those p2p studs that has to be wire wrapped. A good method of attaching is to push the wire far enough into the crimper so that you can crimp down on the insulation at the end, then use a small amount of solder
on the bare wire lead where it appears through the opening in the crimp/solder connector. Careful with the heat, though, these are regular plastic and will melt quickly. If you prefer, just crimp, period.

Make sure you are organized if you use these - they are attached from the inside. Once soldered to the board, they become a permanent attachment. Moreover, the backplate also becomes permanently attached to the board. Because of this, it's a good idea to put in as much slack in the leads as possible. The leads in the photo are about as long as you can get without doubling them over. A few judicious cable ties helps to keep these from getting out of control. Don't worry about the case bezels - they are easily able to slip over the endplate and maneuvered to the back of the endplate, even with all the wires soldered to the board. The center gray jack is the ground, the voltage adjust jack is off to the side away from the others. Then the first two on either side are the tube bias - L/R. Then DB bias - L/R, A and B. Of course, you simply use the L or R tube bias test point as the reference for the DB test points for L or R, respectively. There is no separate adjustment between the A and B sections of each channel's DB. However, the test points are there and they do provide useful information. First, they are an obvious indication of how well you matched transistors. Second, they will indicate which side measures more current, allowing you to accurately set the maximum on each channel. Others have remarked that there's not enough room between the jacks for typical DMM test probes, but all you need do is plug in the COM jack all the way into the requisite tip jack. Then, even if the test point jack is immediately adjacent, only a partial insertion is plenty to make a measurement - and to stay in place.
PCB Center Hole
To digress a bit, please note the bottom stud in the middle of the board: Please do not construct your MAX without utilizing this feature! The MAX is a very large board. It is only supported by the slots on the sides of the enclosure if using the recommended Hammond, or worse - only by the corners if another case type is used. Yet, you will be flexing the center of the board on a regular basis when plugging/unplugging the tubes. Admittedly, drilling a hole in the center of the case and getting it lined up perfectly could be an issue - especially if you are attempting to ensure alignment with all the holes we drilled in the top plate earlier. The MAX board actually has some clearance on the back side or the front side, depending on which volume pot you use. So, the board center is not necessarily the absolute center of the bottom of the case. However, a compromise is shown in the pic above. Simply add enough washers/spacers until the stud rests on the bottom of the case. You may need to tension this fit, though, because the typical board may have a slight dip in the middle already. The downward force of plugging in a tube is typically much greater than the pull when unplugging. So, this method works well to support the center of the board. You can use a nylon nut on the tip, or a threaded nylon spacer. That will prevent scarring the case. Although being on the inside, it may not matter to you anyway - please use something, though. It would be tragic to rip some of the board's traces in the future because of the repeated stresses from replacing tubes with no support in the middle of the board.  
Remaining Backplate Wiring

The remaining backplate wiring is fairly straightforward. At a minimum, you will need a power jack and signal jacks. It's best to use a switch, too. Again, a couple of inches of slack is advised. You do not need as much slack as for the tip jacks, because the remaining wiring is connected to the board through the screwed terminal blocks.
Enough slack should be used to completely remove the backplate, and to be able to move it away and up over the top of the case. While the wires are still attached to the terminal blocks, the top plate can slide out if the backplate assembly is moved down, first.. After that, sliding the backplate assembly up and over the top of the body of the case will allow you to remove the entire board by pulling it out of the case by the frontplate - with all of the wiring still attached.
This is a closeup showing the signal jacks - two audiophile quality, gold-plated RCA jacks with teflon inserts. If you can find the type with the ground ring tabs, soldering is a snap. Simply solder together the ring tabs in the middle as shown. The signal wires themselves are inserted into the soldering tails and soldered. The entire MAX board and case is negative grounded, so contact with the case is not a problem. It should be noted that I was told braiding the wires in this fashion may encourage cross-talk.
I haven't noticed this to be an issue, and braiding certainly keeps the wires organized and tight. However, you may want to choose a more conservative simple twist or let them run free - your choice.
Here we see the power jack and a switch. The MAX includes its own power supply section, except for the primary AC input, typically provided by an inexpensive AC-AC wall transformer ("walwart" as commonly described). The entire board is likewise negatively grounded, per the original Millett design. However, that means that the MAX and its case must be completely isolated from the AC input. Accordingly, an isolated/insulated plastic power input jack is shown here and specified in the MAX BOM.
The power switch is interjected into the AC power input with a simple jumper from one of the power jack leads. The lead from the other switch terminal simply continues the rest of the two wire AC input to the terminal block on the board. NOTE: the production MAX board incorporates the use of a fuse. The fuse (typically a 1A fast-blo) is thought beneficial if high currents (>50ma) are used in biasing the output stage. It should sever to help protect the devices until your build is fully tested. If you refer to the updated Layout or Schematic section of the website, you will see that a 3-terminal block is employed in this position on the production board. Two terminals are traced directly to the board's regulated power supply, just as shown in this photo of the 2nd prototype. The third terminal may be used instead of one of the other terminals and the trace will be routed through the fuse, first. Your choice. An additional benefit is that the 3-terminal block removes the need of any 2-terminal blocks anywhere.  
Frontplate Wiring

In order for the tube holes in the top plate to line up with the tube positions on the board, the board's front edge must be flush with the edge of the Hammond case extrusion. To make this happen when using the Hammond bezels, you need two of the spacers provided for the Neutrik board-mtd. phone jack. These are shown if you look closely in the photo at left. Once these are in place, you may tighten down the jack nut - along with the black ferrule underneath. This will set the board in the right position.
Another item shown is the additional mini-jack. Adding this jack is sort of my own personal practice with any amp. It makes it convenient when listening to smaller phones such as the ubiquitous KSC75. In this case, I soldered the extra leads to the Neutrik jack. This was somewhat difficult due to the plastic framing inbetween, which was at a higher level than the metal contacts underneath. Regardless, the production board incorporates separate pads on the board next to the jack for a terminal block or for use in soldering leads directly - your choice. In both cases, the Neutrik jack operates by breaking the connection to the auxiliary leads when a standard phone jack is plugged in. The mini-jack in this case is my personal preference - a barrel jack with an outside ring nut. This jack is more or less a commodity part and is packaged under many names, including Philmore at Fry's and elsewhere. Mouser has it has #161-7400-EX. Since the mini-jack is mounted with the ring nut on the outside of the panel, it is easily removable from the endplate without removing it from the board. Also shown is a T1-3/4 chrome bezel. It is held with a nut on the backside of the front plate, but the LED is held in place with the rubber insert and is easily removed from the bezel while it is still attached to the front plate. Of course, it is routed to the center LED position on the board. I used a medium-power diffused LED of the same color as the high-powered ones under the tubes (blue in this case). Thus, counting the pot discussed below, the front plate is completely detachable from the board.  
The Alps RK27 is shown. There are pads for a Noble pot, too. In addition, if you are very cost conscious, there is a $2-$3 Alpha pot at Mouser that will fit if you twist and splay out the outer pins. I think the MAX deserves the Alps or Noble pot, however - or something better if you wish. In keeping with making the front edge of the board flush with the body of the Hammond case - as mentioned above - the locating pin on the Alps must be trimmed.
A large knob is best for aesthetics, but your choice, of course. As a side note, Tangent is correct about using a fine hacksaw or razor saw to cut the shaft. (See various forum threads on Alps pot shaft trimming.) You can certainly mask up the amp with a couple of baggies and tape around the shaft - then cut away to your heart's content with a Dremel cutoff wheel. However, after repeatedly cutting this way, the essential problem became clear: the Alps shaft metal is so soft, it fouls the cutting wheel before the cut is complete. This fouling builds up spots of the metal on the wheel, increasing the wheel's thickness at the very time it has the most contact in the cutting slot. The result is almost a guaranteed continuous backlash with almost no control. Instead, clamp your shaft securely to keep it from putting undo torque on the soldered pins, and saw away manually.  
Final Assembly

Typical tools used for assembling the amp are shown at left. One thing not mentioned earlier is that all the Hammond screws positions are tapped for 6-32 screws. The typical 6-32 tap has a cutting surface about 11/16" long. Since some of that length includes smaller starting threads at the tip, and because the Hammond bezels add a bit of extra length, 6-32 x 3/4" screws work the best. I prefer socket head cap screws. These are tough, very seldom need backing washers or lock washers (the head cuts slightly into the metal), and the socket head is knurled.
This lets you treat them as small thumbscrews except for the last turn or so, or in situations where the threads are under stress. This happens in the Hammond at the top screw positions, because the screws make partial contact with the edge of the top plate. No drilling is required - the hole sizes are already perfect for a 6-32 tap. You don't even need a tap handle if you can clamp the tap into a small pin vise or similar handheld tool. You may need a glove for gripping on the final threads, however. Be sure you cut as far as you can - with the tap in all the way to the shoulder. Also, the use of the 3/4" screws helps to keep the threads maintained. Longer screws will end cutting metal on their own and shorter screws have more chance of misalignment - both could work toward stripping. When you assemble and disassemble, always use your fingers first, and check for alignment until you're sure the screws are comfortably moving in the threads. Even in the best scenario, there is are only partial threads throughout due to the slots in the case. Also shown is a 7/16" wrench, good for the pot shaft nut and the Neutrik phone jack. It helps prevent you from scratching the endplate when assembling/disassembling. A smooth pair of needlenose, not serrated, is good for the ring nut on the mini jack, but care is advised - it is easy to scratch the front plate on that one. Also shown is a screwdriver-handled 6-32 allen wrench with a ball end. You can find one of these at many places, including hobby shops. It helps to speed up screwing the screws in when you have a screwdriver handle. At the same time, the ball end preserves the socket shape longer, while the screwdriver handle prevents you from torquing down too much on the soft aluminum Hammond case - just lightly tight works fine.

  MAX Completely Assembled!

file last changed:Sunday, September 7, 2008 7:00:00 AM
Please contact the MAX webmaster for questions about these web pages.