| The EF86 frequently failed or became microphonic forcing the tube to be replaced often. The need to change this tube frequently became a customer service problem for Vox. By 1960, Vox grew weary of the problems caused by the EF86, and the preamp circuit was redesigned without the troublesome tube.
For many years, it was difficult and expensive to locate and replace an EF86 tube, but Sovtek has recently reissued it.
The control panel had a three-position rotary switch to select the Vib-Trem effect. A second three-position rotary switch controlled Vib-Trem speed. A third single-position rotary switch controlled the "Brilliant" tone effect. Both channels had a volume control. A "Cut" control rolled off treble response. An egg shaped single button foot switch with the Vox logo would also engage the Vib-Trem effect.
The Vib-Trem circuit in the AC30/4 is extremely complex. In fact, nearly half of the electronic parts in the amp are needed to create the Vib-Trem effect.
Vox head engineer Dick Denney told the story about the development of the Vib-Trem circuit in an interview with Guitar Player magazine. Denney recounts that he was experimenting with the vibrato circuit on a Wurlitzer home organ when the phone rang. As he stood up to answer the phone, he accidentally knocked a screwdriver into the organ, and it rested across some live electronic components. After the call, he discovered that the organ was now making the exact vibrato and tremolo effect he was hoping to achieve, courtesy of an accidental dropping of a screwdriver into the circuit. Denney carefully noted the parts that the screwdriver had "shorted" and incorporated this modified Wurlitzer circuit into Vox AC-15 and AC-30 amps.
The power amp had four EL-84 tubes, each rated at approximately 8 watts, in two push-pull pairs. A "Class A' self biasing output circuit was employed, and no negative feedback was used. The combination of the cathode biased small bottle EL-84 tube without negative feedback even further enhanced the harmonically rich overtones of the EF86 based preamp section.
Two 8 ohm Celestion 12" G12 Alnico speakers were wired in series presenting a 16 ohm load to the AC-30 amplifier section.
The AC30/4 is most frequently found with tan or "fawn" vinyl and brown diamond Vox grill, as shown in the picture above. Leather handles, a black control panel with gold legend, chicken head knobs, and pressed brass vents are typically found on the earliest versions of this amp. No corner caps wre installed. Slightly later versions may be covered in a smooth black vinyl and have a red control panel.
|
