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.
The Smell of Molten Projects in the Morning 