Millett "Starving Student" hybrid amp - Page 238

OK. I'd like to add two to my order, to be shipped with the kit. What's the best way of going about it?

Since this issue started at beezar why not use the "contact us" link right on that site?

Oh. I thought this was an issue highlighted as a result of posts on this thread. Others might be interested?

But yes, I'll go via the beezar site.

Having a little issue. Right after powering up (bumping) I got a snapping sound a little spark. I unhooked the power and seen that there was a little black spot in the mounting hole of the transistor. Apparently it was arcing to the heatsink? I am at a lost I have looked over it a couple of times and found nothing.

Can the back side of the transistors touch each other or share a common contact? I am attaching them to the heatsink with pressure using a brass tube.

I am using pads and thermal grease as well.

I'm pretty sure that if you make contact with ground with the metal tabs on the MOSFETs, you're probably going to get sparks, at a minimum. Be sure they're insulated, in other words.

OK, so the intent was not to show you that the focus of building the SSMH PCB was the tube LEDs - it's not. So, for the next bit of Step-by-Step, we'll focus on the PCB -

1. Gather Your Tools and Organize Your Workspace -
Here's my stuff:


A board to build the PCB on (1 x 10 high-finish pine), soldering iron, brass wool, light, solder, de-soldering braid, smooth-jaw needle-nose pliers, flush cutters, detail scissors, wire strippers, and a Leatherman to catch everything that the previous tools don't. Also note the bending jig - these are very cheap, only a couple of dollars when I bought one - but very valuable in bending the leads on resistors for varying size pads. Also note the BOM. You can essentially build the entire SSMH PCB with only the PCB and the BOM as reference: match the part numbers on the PCB with the part numbers on the BOM - that's it!

2. Install the Resistors on the PCB -
When you populate a PCB, you want to start with the lowest-height parts first, then progress in ever-increasing height to the last parts. You do this because using the wooden board, you're able to turn the board over and mash down with your free hand while holding the soldering iron tip to a solder joint to keep it melted. Remove the soldering iron, let cool - all the while mashing down - and every part will go down flat against the board.

Anyway, it's a safe bet the V-D resistors are the smallest things on the board, so we'll do those first. Here are a few I've pulled out to determine resistance values -


With V-D (Vishay-Dale) resistors - at least the mil-spec kind such as sold by Mouser - there is no resistor color code. Instead, V-D uses an 4-digit scientific notation. The first three digits are a whole number, while the last digit is the base-10 exponent. So, in the pic below, we have:
3302 = 330 x 10^2 or 330 x 100 = 33,000 or 33K ohms
2203 = 220 x 10^3 or 220 x 1000 = 220,000 or 220K ohms
5102 = 510 x 10^2 or 510 x 100 = 51,000 or 51K ohms.


Keep this in mind while interpreting the resistors. Also - MOST IMPORTANT - bend the leads on the resistors so that the exponential rating is facing up when you install them on the PCB. Without the color code, the exponential notation is the only way to determine value on sight. You can use your DMM to determine the resistance of each resistor, but once installed in the PCB, there are many parallel connections and attempting to measure the resistance of a single resistor is somewhat meaningless.

3. Bend the Resistor Leads and Place Them On the PCB -
Here we see me using the bending jig to bend the resistor leads so that they'll fit the pads on the SSMH. All of them fit correctly on slot 4, but there are two exceptions - R16 and R17 (those two are already bent at top in the photo). These are simply bent at the ends of the resistor body - their pads are much shorter and this will allow a perfect fit. Slot 4 works fine for the rest, however. Again, if you don't have a bending jig, then eye ball them as best you can.


4. Solder One Lead Only with the PCB
Here we see the PCB with all of the resistors placed into the pads, and the PCB turned over with the leads sticking straight up. The idea is to solder one of the pads for each resistor, while mashing down on the board with your free hands. (Melt the solder with two hands, but while holding the soldering iron to the joint to keep it melted, use your free hand to apply down-pressure to the board.)
\

One hint: while masing down on the board to apply pressure with a melted solder joint, shift the PCB slightly up or down, left or right, to keep the leads as nearly vertical on the PCB as possible. This keeps the resistors from twisting in their pads, possibly obscuring the exponential rating.

4. Ensure Alignment and Complete Resistor Soldering
Here we see the silkscreen side of the PCB with one pad per resistor soldered. We do this so that we can check the alignment of the parts on the other side. If some are crooked, we can still slightly adjust their position to correct for the effect.


When you look at the other side, inspect each pad for a "wicking" solder joint. The solder should travel all the way through to the other side and climb up slightly on the input side to the airline. If they don't, apply a slight bit of solder on the other side around the lead and the pad - just enough to make it look as if the wicking worked:


In the pic above, note that the right side of leads are not soldered. That's OK - we have more Acts to perform. Note the wicked-through solder on the joints that are acceptable. If you're happy with the resistors's alignment when you get to this point, then flip the board back over and solder all the leads on the other side.

5. Trim All Leads w/Flush Cutter
Here we are with all of the resistor leads trimmed with the flush cutter:


Here's the PCB with all of the smaller resistors installed correctly:


6. Next up is the LED resistor -


The LED resistor is pretty big. It's sized to handle the two tube LEDs at 10ma a piece and 48VDC. So big, in fact, that it won't fit on the same side as the rest of the parts. This is no problem - it fits fine on the other side. Since it's also much shorter than the tube sockets (they also go on the other side of the PCB), it's best to install it now.

Bend the leads sharply on each end as shown. That way, it will fit in the longest distance pads on the PCB. Here we see the resistor placed on the other side with the leads sticking out. Just as with the smaller resistors, press down as you solder one lead, then the other:


Here it is soldered into place. As before, check to make sure you've got wicking, but be very careful about the center hole. We're going to use a standoff there later and it won't take much to come into contact with the lead on the LED resistor from that center hole:

If there's a cancellation, and that a full kit becomes available, i'm a taker !

I did quite of reading about transistor mounting and think the way I have it is OK. So I remounted them and powered it back up and got the same result. I made sure that there was no contact between them.

Can you use any thermal compound? The stuff I have is from a PC. Could that cause problems? I am at a loss at this point.

Yes, you should use a thermal compound, but that should also be isolated from touching both the MOSFET back/tab and the metal of the case.

Pics?

Building the PCB (continued) -

7. Install the tube sockets -
After all of our work to drill the tube sockets and populating the PCB so far, hopefully the glue has cured and we're ready to install the tube sockets. As mentioned before, they go on the other side of the PCB which will be mounted up against the top plate of the custom Hammond case. IOW, the tube sockets will poke through the top of the case while the other parts will hang upside down inside the case.

The tube sockets always come with the pins pretty much mashed into right angles from the sockets. However for proper mounting, they need to be splayed out at an angle. In fact, it works better to make sure the pins are splayed out a little further than the holes in the PCB. This way, you don't have to force the sockets from a "sprung" position to keep them lined up in the holes while soldering them. When the pins are splayed out slightly further than the holes, the act of forcing them into place compresses the socket into place. Shown below is an example of what the sockets may look like without bending the pins, and one with splayed out pins ready to be installed into the board:


Test fit the sockets/pins into the holes as you bend the pins out. It may take a few trys to get them all OK. Notice the alignment, too. If the top surface of the socket is not parallel to the board when trial fitting, then bend the pins out a little more on the high side.

What you want to do is place both sockets into position, turn the PCB over, then solder. With both sockets in place, the PCB is actually supported by the sockets and it makes it a lot easier to solder - simply press down slightly on the board while soldering to make certain they're flush. Here are the sockets in position with the PCB ...


and turned over with the pins sticking out ready to be soldered:


The tube sockets undergo a lot of stress, so we want these solder joints with the pins to be mechanically sound, in addition to being electrically sound. You should fill the holes completely so that there are no gaps in the solder. At the same time, you want to apply enough heat indirectly (to the solder) that there's wicking along the pins on the other side. A good joint will show the solder completely filled, wicked somewhat along the pins on the other side of the PCB, and will probably show a slight concave surface on the side where you apply the solder. The concave surface is the result of gravity pulling the solder down slightly through the hole. You'll make a mess if you try to get a flush surface with the solder - it will continue to run down the pins on the other side. After a few of them, you'll get the feel however. There's nothing wrong with going back and applying a bit more solder.


The best way I've found to solder the pins is shown below. Hold the wedge tip on your soldering iron flush against the pin on the outside. Most of the hole opening will be on the other side of the pin surface. Apply solder to the side of the pin opposite of the soldering iron tip and feed the solder until the hole is filled. Keep the heat applied while mashing down slightly on the board. While still mashing down slightly, remove the iron and let the joint cool - it will take longer than you're used to because of the quantity of solder.


Alternate from the pin on one side of the socket to the other - soldering opposite pins around the circle, so to speak. This is similar to tighteining the bolt pattern on your car's wheel when you change a tire - it keeps the forces symmetrical on the socket and helps to keep it straight. Once you've got two pins soldered on opposite sides of the socket circle, flip the PCB over occasionally to see if the sockets are straight. If the top surface of the socket is crooked (not parallel to the board), apply pressure in the opposite direction while soldering the remaining pins. A slight mis-alignment of the socket will cause the tips of the tubes to be pointing off-centered. Now, some of the glass in the tubes is crooked anyway, but it's still better to start out with a level tube socket surface than not.

Here we see the sockets soldered in place on the pin side of the board. Note the slightly concave surface of the solder at each position. This is what they should look like to get the best mechanical and electrical connection (Note the clear flux blobs around each pin - we'll need to clean that off.):


On the other side, you should be able to see the slight wicking along the pins:


At this point, it's best to clean the PCB on both sides in preparation for installing the rest of the parts. That's because you'll never get all that solder mess cleaned around those socket pins once you solder all the other parts into place.
(Ask me how I know that! )

I just use rubbing alcohol and an old toothbrush. Walmart sells some 97% pure isopropyl alcohol for about $1 a quart, so it's very cheap. It may take you several rinses to get all the flux off, but keep trying. Use a paper towel to blot up the alcohol - it should turn a dirty yellow where you're picking up the dissolved flux. Rinse, then blot dry, and repeat. When dry, dissolved flux will have a white powder appearance. So, if you still have some of that around the joints - you haven't rinsed enough - do it again.

8. Install the Tube LEDs -
Next up is what may be the hardest part of building the SSMH PCB - installing the tube LEDs. Still, it's fairly straightforward and with the help of this tutorial, it should go pretty smooth. I had the most trouble trail-blazing the method of how to do this, so maybe it will become much easier for you guys.

The trick is to get the LED's soldered to the leads without creating such a blob that you can't get the LED into the tube socket center hole. You also need to thread the leads so that they'll curve around the parts that will go in later. Again, there are a couple of little tricks to do this with the least amount of trouble. Hopefully, this will illustrate and explain those tricks for you.

Soldering leads together is always a bit difficult, but with a set of Helping Hands, things go a bit easier. The trick is to tin the wire leads, then clamp the leads and the LEDs in place so that the tinned wire touches the LED lead where you want to solder it. Apply heat and it should stick, allowing you to apply a bit more solder for strength. You'll want to ensure that these joints are carefully done, because you'll put the LED under stress when threading it through the tube socket. Once that's done, though, they'll never undergo stress again because they'll be completely protected by the socket.

Anyway, here's two pairs of leads with the tips stripped and tinned:


Cheat and look forward to the photo showing the leads soldered into place below. You'll want to cut the leads at least this length, plus another inch or so to account for the vertical length through the socket. Do this by placing the wire along the PCB in the shape shown below, and then add the inch or two of extra and cut. Don't worry about getting them exact - we'll trim them to length once we get them threaded through. Be sure you have more length rather than less, though.

Here we see both wire leads soldered to the LED leads:


Important - remember which lead is which! You'll need to trim the LED's leads as shown here, but once you do that, you'll no longer know which one was the longer lead (positive). I always use either black or green for ground, though. So in these pics, the black wire is negative and was soldered to the shorter lead.

Another important point - try to solder the leads on the top, bottom, or inside of the LED leads. If you solder the wire leads to the outside, then the overall diameter of the assembly might be too large to fit inside the tube socket hole!

Here's a closeup of what I mean - soldering the wire leads have not added to the overall outside diameter. This one's slightly out of order because the LED leads have not been trimmed yet. Nip them carefully as close to the solder joint as you can:


Another trick shown here before threading the assembly through the tube socket hole and the PCB hole. Put heat shrink on one joint, only! This is small stuff, but I suppose a very small bit of electrical tape will work, too:


Again, nothing is going to come into contact with these leads once they're soldered into place and the LED is inside the socket. However, there's a good chance that they might be squeezed together. You only need to insulate one, though. That way, you haven't created a blob of tape/heat shrink so large that you can't get it into the tube socket hole.

Here we see the LED's threaded through. You'll want to put the tip of the LED flush with the top surface of the socket. This will provide the best effect for tube lighting. There will be plenty of give in the LED/wire assembly, though, so if the LED sticks up past that a bit, it's OK. If your finger easily pushes them down, then so will the tubes, too.


Finally, we'll flip the board over and position the leads into the proper pads. Remember, ground is negative and black in this case. So the purple wires have gone into the holes marked "+". Those should also correspond to the longest lead that was on the LEDs, too - way back when you first soldered the leads to the LED. (NOTE: Use this photo as an illustration of how to measure the lead length for the leads in the first step above.)


Trim the leads to length shown here, although a bit of slack is not going to hurt. You've got plenty of room with the taller capacitors and such that will go on later. Once you have them trimmed, slightly tin the leads and bend them down into a 90-degree angle. This makes it easier to fit them into the holes. Be careful, though - those pads are small!. Once you have them in, flip the board over and solder them into place. Once again, you should be able to push down on the board slightly to ensure that the tinned leads are pushed through completely and the insulation is flush with the board.

NOTE: I use 22ga Navshipps SPC wire for everything. You might cheat here and use 24ga. That could make it a lot easier to thread through and solder the leads into the pads. 22 ga works fine, though, if you're careful not to make a blob when tinning the ends.

That's so clever!

Nice tutorial overall, Tom.

Thanks!

Let's finish off the PCB!

9. Check the Relative Heights of the Remaining Parts -
Here we see all the rest of the parts, except the MOSFETs, laid out into a row, according to height:


Note the order - this is how we'll finish the rest of the PCB - shortest to tallest, until they're all done. Things get FUN, now.

10. Solder the Terminal Blocks in Place -
As with all that's gone before, place the parts in the proper pads, turn the board over and press down slightly while soldering the leads. In the case of the terminal blocks, the pins have a lot of slack in the holes. (This is by design - it's critical that the pins make good contact with the board and you have no control over wicking.)


I like to solder one of the interior pins, first. The blocks seem to rotate better on one of the center pins. I've soldered one of the ends first, found out it was misaligned, but then lacked enough slack to turn the block straight. So - small caveat - but generally easy enough to solder. Some caution is needed because the blocks are plastic. It's possible to apply enough heat that you can melt them.

Be sure you have the openinings toward the outside of the PCB!

11. Solder the WIMA Film Capacitors In Place -
There's four of them and they're easy enough to place in position. However, the leads are thick and short - they don't stick the parts in by themselves. If you're using the pine board like me, though, there's a trick:

• Hold the PCB with the Wima's in place in one hand.
• Pick up the pine board with the other hand, turning it upside down.
• Place the pine board over the top of the Wima's and PCB.
• While holding the PCB/pine board with both hands like a patty-cake, flip the assembly over so that the pine board is back on the bottom again.
• Proceed with your soldering.

You'll find that the Wima's are so square and so evenly placed on the PCB that the entire PCB is supported in a straight and level position while upside down - nothing could be easier!


Don't forget - like everything else - solder one lead on each Wima while pressing down slightly to ensure that they're flush. Note that there are little tabs on the bottom edge of the Wima's, so "flush" will still mean there's an air gap underneath. Then flip the PCB back over and make sure they're not crooked. Straighten them gently, if necessary. Flip the PCB back over onto the pine board and finish the soldering. Trim the leads when you're done.

Note: I don't trim the leads on anything else but the capacitors - the terminal blocks, volume pot, and headphone jack are left un-trimmed.

12. Solder the Headphone Jack in Place -
Straightforward, but you'll have to press the PCB down and keep things lined up while you solder the first couple of pins. That's because there's no headphone jack on the other side of the PCB to even things out.


Use the same method for soldering the headphone jack as you did with the tube sockets:

• Alternate pins from side to side while soldering. This has two purposes - 1) It's easier to keep things aligned, and 2) Unlike the ceramic of the tube sockets, the headphone jack is plastic. Too much heat and it can melt, although it takes an awful lot.
• The headphone jack also undergoes a lot of stress. Ensure that the pins are completely soldered and pads filled, and try to ensure that there's wicking to the frame on top.
• Solder one of middle pins first, ensure that everything's aligned. Solder the ground pins last, because they'll be the hardest to solder (the soldering iron heat will get soaked up by the ground plane a bit).

13. Solder the Volume Pot Into Place -


The pins and pads are small on the volume pot, so it's much easier to keep the pot aligned while soldering. However, it's short from front to back, so it's pretty easy to get the shaft pointing a bit down or up. This will drive you crazy when you try to set the volume knob in position, so do your best to get the pot shaft parallel to the PCB. Solder a couple of the back pins (leave the ground pins for last), then check the alignment. Press down in back or in front, depending on what's needed to keep the shaft aligned. Also check the side-to-side alignment, too. A little care doing all of this will payoff when you go to set the knob in place. Otherwise, the knob may scrape at different positions in volume travel. You may end up with a large gap to keep it from scraping, which may not look aesthetic.

14. Solder the Electrolytic Capacitors in Place -
This one's easy and fun to do! Note from the first pic up there that all of the electrolytic capacitors we've supplied for you in the kit are all the same height! So, we'll want to place everyone of them on the board at the same time, pick up the pine board, turn it upside down while placing it on top of the caps on the PCB that we're holding in our other hand, and do our reverse patty-cake maneuver. Be sure you have them inserted in the proper pads, though. All the long leads go into the holes marked "+".


As with the Wima's, the electrolytics completely support the PCB in a straight and level position. Solder the leads - as always, solder one of each for all of them, flip the PCB over and ensure that there's no mis-alignment. Finish all of the soldering and then trim the leads. (I don't trim the leads on anything else except the Wima's. The terminal blocks, headphone jack, and pot all stay untouched.)

There is one last tweak we'll do to the PCB before we move on to the case assembly, but I haven't done it for real yet. So we'll need to wait until I get more done. It's soldering a small lead from the "G" pad to the metal body on the volume pot. This will remove any ground hum you might hear when grabbing the volume knob to adjust the sound level. I'm not sure that pot hum is an issue with this pot, but it won't hurt anything and will ensure that the issue never comes up. We'll do that next and then assemble the case!!

Very nice tomb, This should be a seperate thread in my opinion.
Don't want to push things :S, but did you get my pmsg? It's just that I'm looking to order another set of tubes (and parts for the pcb version that I can't find at mouser), but I also still need the 'lug' sockets for those tubes of the p2p build. You once (appreciated alot by me) proposed to sell me an extra pair of a spare set of your lug miniature sockets. I was wondering if you still had those, I'll buy them for a reasonable price ofcourse.

I haven't caught up with all of my PM's, yet, so I'll check and let you know. I've been off the mainland until last night.

As for a separate thread - no, this is the proper place. I don't believe in creating a bunch of separate threads for the same project. This amp has gotten plenty of publicity and everyone knows where to find it. There's also the search engine within the thread. My next task is to get all of this on the website, anyway. So it'll be easy to find in the future, regardless.