The Amazing Echoplex

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...stalking and playing a classic guitar effect

(Thanks to Cameron Schmitz for use of his pic.)

ECHOPLEX THEORY OF OPERATION


The Echoplex Transport

Mechanical
The endless tape loop is housed in the white cartridge in the photo above. The tape is a special self-lubricated formula and is wound in the cartridge with the oxide facing out. This approximately seventy-five foot tape travels clockwise around both the cartridge and tape deck. It exits the inside of the loop through the diagonal slit in the cartridge (under the orange label above) and heads into the white plastic reversing idler wheel on the right. Via the reversing idler, it changes direction by about 340 degrees and heads into the head stack. Holdback tension is maintained by the felt pad riding on the copper spring that presses against the tape at the top of the reversing idler. The holdback tension holds the tape against the heads. The three heads are in contact with the tape across the bottom of the picture, and we'll discuss their functions in a minute. At the left of the heads is the metal capstan. The tape winds around it approximately 100 degrees and heads back towards the cartridge. When the unit's power is turned on, the rubber pinch roller is drawn into contact with the tape against the capstan by a solenoid, pinching it against the capstan. Turned by a large AC induction motor that makes up a large part of the weight of the unit, the capstan pulls the tape out of the cartridge and through the head stack. As the tape exits the capstan, it is drawn into a slit in the left of the cartridge and is wound back onto the outside of the loop. The two bobbins keep the loop loosly formed as the tape moves from the outside to the inside of the pack and the exit, thus completing its cycle.

Electronic
There are two head systems that support the two modes in the Echoplex. The switch on the far left chooses between "Echo" and "Sound on Sound" modes. First, the echo system. Echo is accomplished by recording the guitar's signal on to the tape and playing it back. The tape playback signal is mixed with the with the original guitar signal and Voile'! echo. The "Echo Volume" control blends the output of the record head with the guitar signal. In the Echoplex, the delay time is set by moving the record head closer to and further away from the playback head. The further away the two are, the long the delay time. So far, our description gives you one echo. In order to get more than one repeat, the output of the playback head is mixed with the original signal and fed back into the record head. The "Echo Sustain" control varies how much of the playback signal is fed back to the record head.

So, looking at the picture, we see three heads in contact with the tape across the bottom. The center head, with what looks like a big wheel below it, is a combined record and erase head. The tape goes by the erase head section and any previous signal is erased from it. It then goes by the record section of the same head where a new copy of the guitar is recorded onto it. From that head the tape moves to the echo playback head on the left, where the recorded signal is played back, mixed with the original signal, and sent to the output. Remember that the delay time is set by the distance between the record and playback heads. The spike in the middle of the "wheel" below the record head is the handle that allows you to move the record head back and forth along the bridge and thus vary the delay length. The handle protrudes through the tape system cover and the attached pointer protrudes through the front and points at a scale that allows you to remember and reproduce delay time settings.

The "Echo Volume" control has the ability to blend original and echo signals and achieve a mix anywhere from pure original signal to wholly delayed signal. The "Echo Sustain" control has enough gain to allow you raise the volume of the delay signal fed back into the record head enough for the echoes to "run away" or actually increase in volume. It is my understanding that the extra gain was originally provided to allow you to keep up with the diminishing sensitivity of the heads as they wear. The runaway effect was just a pleasant side-effect. In the Echo mode, a footswitch inserted into the left-hand "Echo/Off" jack will allow you to switch the echo signal on or off. This is '70s technology, so there are no echo "tails" when the signal is switched off.

Now let's switch the system to "Sound on Sound" mode. In this mode, the erase portion of the erase/record head is turned off as well as the left-hand playback head, and the far-right playback head is switched on. As a result, the signal is recorded on the tape and the tape must then travel all the way through the cartridge loop before it returns to the right-hand playback head and is played back. The tape is about two minutes long so the loop returns in about... uh... two minutes. Because erase head is switched off, you can play along with the recorded loop and your second performance will be added to the first and show up again when the tape comes back around. In this mode, a footswitch inserted into the "Playback" jack allows you to alternate between recording and playing back and playback-only. The red "Recording" light will light up when you are recording.



Notes on typical tweaks needed on a vintage machine:
1. On first contact with a machine, check the calibration of the two main knob pointers and adjust if necessary. Every EP-3 I've encountered has had at least one knob out of whack – usually the right one ("Echo Volume"). Because there is no key-way on the right knob's smooth shaft, the knob loosens up and drifts. Make sure it swings from where “0” would be (below “1” on the left) to “9” on the right. If not, loosen the set screw and set it right. It makes it hard to wrap your head around the knob settings and work from any memories you have if the pointers are all weird.

2. This one is important: Always set the "Record Level" control per the instructions. That's the little screw control recessed in the hole between the "Echo Sustain" and "Echo Volume" control knobs. It's the only way to win the signal to noise battle. If a low signal is recorded on the tape, the "Echo Volume" (playback level) control must be jacked up to get a good mix. The variable available to us is the record level, which establishes the level of the audio signal recorded above the tape noise, which is a constant. Because tape noise is a constant, the higher the printed signal, the lower the "Echo Volume", and thus the noise floor, can be set. As delivered, my unit seemed to be a little noisy. I found the "Record Level" was pretty low, requiring me to jack up the Echo Volume and thus the tape hiss, until I set it properly. My little tweak was rewarded by a "crunch" sound and a jump in the audio level - the pot could have needed exercise after being stored so long. Here's how you adjust the unit: Begin by cleaning the heads. Set Echo Sustain at “0” to get one repeat. Now, using the "Record Level" control, you want the volumes to match when the "Echo Volume" control is at its far left (dry) and far right (echo) positions. Voile’! The unit is optimized. Enjoy!!!

For more info on delays and how they work, go

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