In the Old Days, nobody had digital nothin', and life didn't entirely suck. It's become so cheap and easy to get such a variety of lousy, boring sounds out of a guitar that we sometimes forget an important fact: Lousy, boring sounds are a pointless waste of effort.
It's also easy to feel nostalgic for the days when you didn't need to be a double-E to figure out how most of your gear worked. Tape echo devices are like that, and so are Leslie cabinets. Anything with moving parts is cool. Take for example the venerable and glorious Echoplex. What was it? It was a tape echo machine.
Tape echo works on a very simple principle: You take some magnetic tape, much like in those old Bon Jovi cassettes you've got hidden where your friends won't see them. (Yeah, don't think I'm not onto you...) You run that tape past a record head, and you record sounds onto the tape. Beyond the record head, you put a playback head, and play the sound back from there while recording with the other head at the same time.
So the sound goes into the machine, and you put it on the tape. The tape moves forward a bit until it goes past the playback head, at which point it comes back out of the machine. A brief period of time has elapsed: Presto change-o, an echo! By varying the speed of the tape, you can change the length of time before the sound echoes. You can do the same by moving the playback head farther from the record head or nearer to it. You can add more playback heads too, if you like.
What we've got there just echoes once, but you can pull another nice trick: When you play the sound back, you send it out to the speakers. Okay, but with wires you can send the signal somewhere else, too. So you send some of it back to the record head and put it back on the tape again, along with whatever new sound is coming in. You throw in a volume knob for this feedback signal, so you can control how long it takes to die out: If you re-echo the echoed sound at 90% of its original volume, it'll keep going for a while; if you re-echo it at 25% of its original volume, it'll be gone quite soon. 1
Of course, your Bon Jovi tapes are only about twenty-five minutes or so long on each side, so if you erased those and used them for a tape echo, you'd have to stop playing every so often and flip the tape over. There's no good way to work around that, but there's a lousy way which was used back in the 1960s and 1970s when dinosaurs like the Echoplex ruled the Earth: Make a loop. Take eight or ten feet of tape, wind most of it around a little reel, and splice the ends together. Don't wind it too tightly, because you'll be pulling tape out of the middle of the reel and adding it back on the outside, so all the layers of tape wound around the reel will have to slide against each other. Put some dry lubricant on it to help the process along. You put the whole thing in a user-friendly cartridge2. When the dry lubricant all wears off, or when too much of the iron oxide coating on the tape wears off, throw the loop away and put in a new one. When the echo unit becomes "obsolete" and nobody's making loop cartridges any more, you're SOL.
You can do the same thing with digital electronics, but you'll be missing most of what makes the whole excercise worthwhile. What's nice about tape echo is the way the signal degrades as it gets recorded and re-recorded, and pushed through analog circuitry (preferably using vacuum tubes) in between. Everything gets nice and murky and organic. You could probably come close to simulating that in software that if you had a few years to spend on it, but you'd be modelling an extremely complicated system, and that's not so easy. I've never heard a decent simulation yet. Here's why: With electromechanical toys you get all kinds of weird phenomena for free just by using things that have physical properties, but in a digital simulation you have to crank all that stuff by hand, so to speak: You can run a piece of tape past a record head without an intimate knowledge of all the physics that's happening. It just goes ahead and happens on its own3. In a simulation, nothing "just happens". In my view, digital signal processing would do better to play to its strengths, and do things that are completely impossible in nature. That's where it gets interesting.
If you want to hear tape echo in action, there are a lot of places to go, but the best I can think of offhand is the Rolling Stones' song "Shine a Light" on their Exile on Main Street (1972) album. There's a tape echo on the lead guitar throughout the song, and just before the intro there's a little something-or-other that echoes a bit in the silence. That little something-or-other is just plain wonderful.
All may not be lost. There's a reissue of the Echoplex being made as of May, 2001: http://www.rockhardinc.com/theplex.html See also "Make an analog echo box" right here on E2.
1At 90%, it takes about fifteen or twenty echoes before they're too quiet to hear. At 25%, the first echo will be a bit quiet, and the second echo will probably be inaudible already, depending on what else is in the mix. Fifteen or twenty echoes is a hell of a lot. Signal processing is best used sparingly.
2This is how eight-track tapes worked, by the way.
3The behavior of stuff like this will also be affected by all sorts of strange arbitrary factors in its environment, not excluding atmospheric pressure, humidity, and the phase of the moon. I'm not kidding: My tube amps have always had good days and bad days. Tube amplifiers are like plants. You have to talk to them to keep them happy.