A transistorized spring reverb device
Here now the circuit diagram of an effect device with a spring spiral unit. The input stage works with positive feedback (bootstrap), whereby the guitar pickup is loaded only slightly in terms of impedance and consequently the sound of the electric guitar is hardly adulterated. The drive amplifier works as a push-pull amplifier with transformers. This, and the very small sized capacitor at the input of the driver stage (0.1μF) ensures that the exciting system of the reverb unit are only offered frequencies that can be processed by it clean and without overshoot. The signal from the outlet system of the spring spiral unit is mixed together with the un-reverbated original signal tapped at the collector of the input stage. The in this way produced total signal is ultimately routed via a further amplifier stage to the output of the device.
A special feature of this reverb unit is that the original signal and the signal from the reverb unit can be adjusted independently of each other. In this way extreme effects can be achieved, as they were sometimes used in psychedelic rock. In addition, you can completely suppress the original signal and use the device then without a special switching as a insert effect with a mixer.
The device is operated from a transformer plug. Since that provides a rippled DC voltage, located in the supply line is a smoothing circuit, consisting of two electrolytic capacitors and a filter choke. In order to keep the hum small, quite large smoothing capacitors likewise are used in the two preliminary stages (470μF each).
In the driver stage of the push-pull amplifier, this is not necessary because of the close transfer range of the reverb spiral. Despite the very simple circuit and the use of a comparatively cheap reverbation system (Monacor RE-4), the properties can certainly compete with these of then quite expensive professional devices.
The following photos give an impression of the construction of the sample device. On the front panel are the input sensitivity, bypass and reverb level knobs as well as the on/off switch.
Viewed from the back you can see in the open device the mixer circuit board, which is mounted directly behind the front panel. It serves to combine reverb and original signal. In the middle of the device you can see the spring-reverb system, which takes up the most space in the device. On the back wall are the 6.3mm jack sockets for input and output.
Looking from the front into the open device, the driver amplifier for the reverb system screwed to the back of the device is visible. On the board is also the smoothing chain for the power supply. On the far right side, the DC socket for the hollow plug of the power supply can be seen on the inside of the rear panel.