Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
After Eks set me straight on cleaning the contacts involved with the Ampeg’s Echo circuitry, we emerged from his shop into brilliant sunshine. He looked into the thing and shouted “Tin whiskers!”
It turns out the Hammond folks made the outer frame from tin- (or, shudder, cadmium-) plated steel that has grown a dense crop of whiskers on its interior surface. They glittered in the sunlight like a carpet of crushed glass, with the longest ones maybe 3 mm tall!
This view looks nearly parallel to the side of the channel (upward as it mounts on the speaker box), with the steel wall to the bottom of the image. I applied gruesome contrast stretching to make the whiskers more visible:
Ampeg Spring Echo Unit – Tin Whiskers
This is the first time I’d ever seen a tin whisker in person and there’s a bazillion of ’em in there!
If that Ampeg had transistorized components, it’d be dead as a doornail! Fortunately, a tin whisker doesn’t stand a chance in an analog vacuum-tube circuit. The power supply puts 400-ish V into 40 μF caps, providing plenty of energy to vaporize the errant whisker; all you’d hear is a pop.
Mad Phil asked me to fix up his trusty Ampeg B-12-XY (*) bass guitar amp, having recently fired it up and discovered that the power output tube plates glowed red-hot. I’d planned to replace the electrolytic caps, but Eks, who does this sort of thing all the time, suggested that leaky interstage coupling caps can also cause that problem; the leakage wrecks the phase splitter bias and thus kills the drivers.
While poking around in the amp I found that the Echo hardware circuitry doesn’t match the schematic for either the B-12-X or B-12-XY. Mad Phil says that’s probably because he had the factory upgrade his original B-12-X to a B-12-XY for the munificent sum of $25, back in the day. It’s unlikely you’ll ever need this, but here’s what I found:
Ampeg B-12-XY – as-found Echo circuit
The topology resembles the -XY schematic, but with different tube sections and part values.
The Echo unit over there on the left consists of two springs with magnetic transducers on each end, evidently made by the Hammond Organ folks, who should know something about reverb. This is the bottom view, with the unit attached to the board that supports the amp chassis:
Ampeg Spring Echo Unit
The input transducer, just in case you forget to label the ends before you take it apart:
Ampeg Spring Echo – input end
And the output transducer:
Ampeg Spring Echo – output end
Getting the thing off the speaker box posed a bit of a problem. Remove the four big screws holding the chassis to the board, tilt it carefully forward, hold it in place while you remove the six nuts-and-washers from the vibration isolators, then transport the whole disjointed affair to the workbench. Turns out you (well, I) can’t get the RCA plugs out of the Echo unit’s sockets from the top of the board, but the unit’s mounting screws are on the bottom of the board, where you can’t get to them before you remove the board. Of course, the cables leading to the aforementioned RCA plugs tether the chassis to the Echo unit with pretty nearly no slack at all.
With everything apart, I rounded the ends of the RCA plug cutouts enough to get them out from the top the next time around, with the board screwed in place atop the speaker box:
Ampeg Spring Echo unit – top view
After putting the whole thing together with new caps, the Echo circuit didn’t work. I had cleaned the contacts and connectors, but Eks showed me how it’s really done. Apart from the rotted caps, all the other problems came from minor corrosion in switches, connectors, and tube sockets. Now I know better.
You get one chance to throw the snake over the side
The Great Greene grew up in the Midwest, with the type of summer job one might expect of a teen in an area surrounded by grain fields. One summer he found himself standing knee-deep in the wheat pouring into a cart beside a combine harvester, tasked with shoveling grain into the corners to level the load.
In addition to combines, the fields were full of rattlesnakes.
A rattlesnake adopts a characteristic pose when it senses a predator: body coiled, head and tail up, rattle vibrating vigorously. The smaller critters that dine on rattlesnakes (evidently, young rattlesnakes are tasty little pushovers) have figured out, over the course of their long shared evolutionary history, that such a display means this isn’t an immature rattlesnake and they should move along, move along. Raptors pay no attention, having invented the whole death-from-above thing long before we figured out powered flight.
Combines, having not evolved alongside rattlesnakes and being entirely unaware of the threat display, also pay no attention and simply sweep the entire snake into the threshing machinery, where the snake’s characteristic writhing-ball-of-fury reponse to an attack only serves to give the machinery a better grip. The rattlesnake emerges from the combine’s front end as a snakeskin belt surrounded by gibbage.
The combine’s sorters and sieves and transports that separate grain from straw don’t work well on rattlesnake remains, to the extent that much of the snake emerges from the conveyor belt as a damp blob dropped atop the pile of grain in the cart.
In addition to leveling the grain, the Great Greene was responsible for tossing debris over the side. He observed that the machinery downstream of the combine couldn’t do much more than sort out the larger chunks (it’s not like you can wash grain), so if he missed a snake the smaller bits were certain to wind up in your breakfast cereal bowl.
The thermoplastic (who knew?) pads melted right off my long-reach clamps while calibrating those thermocouples, leaving the thermoset (who knew?) clamps behind. I tried a few of the obvious candidates for the job with no success, but (while fiddling around with something else) I came upon an unopened tube of Permatex Ultra Copper copper-loaded silicone gasket glop.
The cured silicone rubber is very flexy, which is sort of what you want in a pad, even if I’m not convinced they’ll stay in place. They seem securely mounted in the recesses of the pad tips; I worked the glop in with a screwdriver tip.
Using the pressure washer to blast the crud off the propane grill has become an early summer ritual around here. I’d reconfigured the extension pipes to reach up the side of the house, so I started by swapping the connectors around to put a shorter pipe at the handpiece. Surprise: those connectors were firmly affixed and a rubber strap wrench on the pipe lacked enough grip.
Rather than wreck that nice chrome plating with a pipe wrench, I clamped two pieces of scrap plywood in the drill press and poked a half-inch hole right down the midline. Add a dab of rosin to improve traction, crunch everything in the bench vise, and spin the connector off.
Well, that’s the way it went for the first connector, with the PTFE joint tape I remember adding last time around.
The connector on the other end was more recalcitrant, perhaps because it still had the manufacturer’s joint compound in place. It eventually yielded to the gentle persuasion of a propane torch, applying just enough heat to wreck the compound’s grip.
The good thing about a plywood clamp is that I don’t form a deep emotional attachment to it: make one when it’s needed to fit the pipe at hand, don’t worry about a precision fit, regard it as a consumable, and move on.
Having managed to mislay my dingy yellow kickstand plate, I made two more and this time hit ’em with fluorescent red paint. Ought to be unforgettable for another few years…
In theory, you’re supposed to apply a white undercoat. I hosed ’em down with many drippy, runny coats of red and it’s all good. This ain’t art and they get thrown on the ground, so what’s the point of being fancy?
The hawk who’s been keeping the chipmunks and squirrels under control paused for a moment atop the utility pole out by the garden. He left instantly after I appeared around the edge of the roof, leaving me no time to fight the camera automation into a better exposure, but it’s good to know he’s on patrol.
A few months ago he had a squirrel in a Mexican standoff inside a pine tree, circling the trunk amid all the branches. Eventually the squirrel made a break for it, got about five feet out from the trunk, and wham that was the end of the story: once those claws go in, they don’t come back out.
Notice the noonday sun refracted through his cornea onto his upper cheek (or whatever it is that birds have there). This was with the 1.7X tele-extender on the Sony DSC-H5 zoomed in pretty nearly all the way; if it weren’t for all fringing and blown highlights, it’d be a neat picture.
The Smell of Molten Projects in the Morning 