VOX AC30 Custom Classics

Spring Reverb 

In the analog domain there are basically two types of spring reverb drive and recovery circuitry used in guitar amps – valve and solid-state. Both have their pros and cons. A valve circuit has an inherent soft-clipping characteristic that’s rich in even-order harmonics and is pleasing in terms of both sound and feel. As valve circuits involve a transformer it is relatively expensive. Other drawbacks of a valve driven circuit are that the valve creates heat and eventually wears out with use. Also, particularly with high-gain circuits, there’s a lot of radiation from the wiring and transformer which can cause instability and noise. The solid-state option is cheaper but has usually less headroom and a harsh clipping characteristic in the drive circuit. Because of these reasons, solid-state reverb drive and recovery circuits were chosen and Steve decided to develop a brand-new circuit to get over the drawbacks normally associated with solid state designs.

Designer Steve Grindrod says “Although some of the solid state reverb circuits already out there are pretty good, it has always been my impression that no-one, and I include myself here, has taken them far enough in getting a really good sound or maximum benefit out of the reverb. I’ve managed to find a special MOSFET that works in the same mode that a valve does and, even though its gain characteristics are slightly different, you could unplug a valve from an amp’s preamp circuit, solder this MOSFET in its place and the amp would work without you having to change or add any other components. And, being a MOSFET, it has the even order harmonics similar to those you get from a valve. The MOSFET is being used in both the drive and recovery stages of the Custom Classic’s reverb to emulate the soft clipping that naturally occurs in a valve circuit."

"In addition, I’ve used five special op-amps in parallel and as they are 60 volt devices as opposed to the regular 30 volt ones they give a lot of headroom and also provide a great deal of current/drive capability without saturation. They are wired in a circuit configuration that emulates the behavior and characteristics of a transformer.”

Dwell switch

The dwell switch allows you to select Low or High reverb drive settings. In effect, it is a “post fader effects send level switch” (in this case the effect is reverb). When using spring reverb you need to be driving the input as hard as possible in order to maximize the signal-to-noise ratio while getting good reverb depth. If the circuit is not driving the springs hard enough then you’ve got to add more gain to the recovery stage which will add noise and possibly cause instability and feedback.

Most guitar amp reverb units (especially those guitar amps capable of high gain settings) have drawbacks because you have to compromise how much drive you put it in verses.