NickC wrote:Laimbrane wrote: ........
Um... maybe? How should the transformer primary be switched and fused differently from what's in the schematic?
Death cap should be eliminated, as well as the ground switch. The ground wire from the AC line-in should be securely bolted to the chassis near the entry point .... and nothing else attached with it. Securely, as in "will never, ever, ever come loose."
Both the fuse and the mains power switch should be on the hot leg, not on the neutral leg of the AC power in.
Point is .... a builder MUST know these things
cold before embarking on this journey. There are way too many gotcha's in this build, aside from the considerable safety issues. When one goes to so much trouble and expense, one hopes for a working amp. There are so many things that can go wrong here, if one doesn't have their build-chops down solid. In spite of having schematics, and layouts, diagrams, voltage charts, pictures, etc ........ it just isn't a paint-by-numbers endeavor. For instance, how would you connect the 12V Radio-Shack transformer, once you remove the death-cap and ground switch?
Sorry, don't mean to be a downer. Just trying to keep it real.
Well that makes me feel better... one of the next round of questions was to eliminate the ground switch entirely (I didn't see any need for it). I was assuming that the hot lead would go through the fuse, then the main, and then connect to the two black wires of the transformers. The other black on the 12V R-S and the white from the TF-130 would flow back into the second wire in from the plug (with the ground-wire SOLIDLY attached to the chassis). Out of sheer intellectual curiosity, would that work?
Aside from that, though, given the advice of everyone here, I'm pulling the plug on the project. The class is a five-week winter-term (3 hours per day) Tech Project class that is supposed to be challenging, but when he proposed it in December I assumed that he had more experience in this area than he did, and neither one of us realized the complexity of the task. He's been working on it for all of the five weeks now and actually has it nearly assembled, but this thread was a bit of a wake-up call. As I think back, I can see potential problems: iffy (but passable) soldering to the case in a handful of spots; alternative types of capacitors (e.g. using a glass capacitor instead of an electrolytic one, a few radial instead of axial); using four 390pf capacitors instead of a missing 500pf (one in parallel to three in series gives ~520pf); silver-plated copper wire that may not have been properly spliced (i.e. a nick in the silver-plating could cause corrosion problems), etc.
The money is not a problem, but the safety and success of this amp is. I work in Northern Michigan, and a year ago a father took his son on a canoe trip from the shore of the mainland to an island about 10 miles away, during somewhat rough weather. The DNR officers at the parkshore warned him not to do it. The people at the canoe livery warned him not to do it. He did it anyway. Halfway through the trip, the canoe capsized, and his son died of hypothermia (he survived). Had he listened to the experts and not his own ego, he would still be a father.
I'm not making the same mistake. As one sports commentator said "It's okay to be scared! Be scared, because it's a whole lot better than being dumb."
So we're going to go back to basics - he has the chassis and a shit-ton of parts, so I'm in the process of looking for a simple tube-amp project that he can complete in the 7-or-so hours that we have to work on this. Then, as Sctructo suggested, he can work himself up to the Dumble eventually (since he still has the parts). There's no doubt in my mind that this class has been a success; he went from knowing next-to-nothing about circuitry and electronics to being able to read a schematic, understand what all of the parts do, solder pieces together and to boards, calculate resistance and capacitance in circuits, etc. He's probably propelled himself farther beyond where most of his contemporaries are at this point, and the lesson that we're working past our limitations, and it's okay stop this before we make it worse is a good lesson for ANY teenager, not just one working on electronics. Plus, hopefully, he still gets it built.
So thank you guys. There's a very good chance that you saved a lot of money, and perhaps a life or two. Thank god for the good side of the internet.
