The Vox AC30/4 "Four Input" Combo Amplifier (1960 - 1961)



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Tom Jennings, president and founder of JMI (the parent company of Vox) was keenly aware of the impact that celebrity endorsees would have on the sales of Vox amplifiers. Jennings scored a major victory in 1959 when the UK instrumental band the "Shadows"
replaced their Selmer amplifiers with the first generation Vox AC-15. In addition to producing instrumental guitar hits such as "Apache" and "FBI," the Shadows were also the backup band for the UK singing sensation Cliff Richards. The Shadows introduced many young British musicians, including the Beatles, to Vox amplifiers.

Can You Make It Louder?
In a televison documentary about Vox produced by the BBC entitled, "Vox Pop: How Dartford Powered the British Beat Boom," Shadows member Bruce Welch recalled a 1960 conversation with Vox chief engineer Dick Denney. Welch asked Denney if there was any way to boost the output of the AC-15. The sound of a fifteen watt AC-15 was easily overcome by an auditorium of screaming girls. At times, the Shadows couldn't hear their own guitars.

Denney decided that the best solution would be to build sort of a "twin" AC-15. While maintaining the two channel AC-15 preamp design, the power could be increased from fifteen to thirty watts by adding a second pair of EL84 tubes. A second 12" speaker could be added by increasing the width of the AC-15 cabinet a scant seven inches. JMI soon developed the new amp Denney envisioned, naming it the AC-30/4, or four input AC-30. The Shadows took delivery of three AC-30/4 amps in mid 1960.

EF86 Preamp Tubes
While most guitar amplifier manufacturers were incorporating dual triode tubes such as the 12AX7, 12AT7 or 12AU7 into their preamp circuits, Dick Denney decided to include a pentode tube, the EF86. Denney so preferred the additional gain and rich harmonics offered by the EF86 that he designed it into the preamp circuits of the Vox AC-2, AC-4, AC-6 and the Normal channel of the AC-10 and AC-15 amplifiers. As the AC-30/4 and AC-15 shared the same preamp circuitry, the AC-30/4 also utilized an EF86 tube in the Normal channel.

Shortly after the AC30/4 amp was introduced, an unforeseen engineering problem arose. The EF86 tube used in the AC-30/4 preamp circuit proved to be susceptible to damage from excessive vibration. At the lower output levels of an AC-4, AC-10 or AC-15 amplifier, this was not a large issue. In the AC-30/4, the strong vibrations caused by a 30 watt power amp powering two 12" speakers was often more than the EF86 could tolerate. As the EF86 started to break down, it often would became microphonic. Howling and ringing tones from the failing EF86 tube would at times accompany the sound of the guitar playing through the amp.



EF86 Tube
Control Panel, Jacks and Controls
The AC30/4 featured a black and gold anodized control panel divided into five boxes. The box at the far left was labeled "Inputs." The amp had four inputs. two each for the Vibrato and Normal channels. The input jacks for the Vibrato channel had white nuts, the Normal channel input jacks were black.

Moving to the right, the next control panel box enclosed the Vib/Trem controls. A three position "Speed" control adjusted the speed of the Vib/Trem. A second panel mounted rotary switch labeled "Switch" toggled between the tremolo and vibrato effects. An egg shaped foot switch turned off the Vib/Trem circuit.

The volume controls were in the control panel box labeled "Volume."

Next came the "Tone" control section. The "Brilliant" rotary switch affected the treble response of the Nornal channel. The "Bass" control rolled off the treble response of both channels.

The power switch, power indicator lamp, fuse and a mains voltage selector were on the right side of the control panel.

Cabinet Design
The AC30/4 was the first AC-30 to incorporate the traditional "split front panel" design shown at left. However, the early AC30/4 amps delivered to the Shadows still had a "TV" front cabinet.

Speakers
While the AC30/4 shown at left had a pair of Celestion G12 Alnico "Blue" speakers, some early AC30/4 amplifiers were fitted with an earlier version of the same speaker, shown at right. These were painted tan and did not include the pressed steel magnet cover found on later Celestion Alnico 12" speakers.



AC-30/4 Bass Amplifier
JMI offered two models of the AC30/4, one for "normal frequency" and a second for "bass frequency." The 1961 JMI Vox price list offered the AC-30/4 Normal for £99.15 and the AC30/4 Bass for £103.19. The only differences between the AC30/4 Normal and the AC-30/4 Bass were the values of seven capacitors in the preamp of the Normal channel. Read more about this in the AC-30/4 "Under the Hood" web page.

AC-30/4 - AC30 "Top Boost Tone" Comparison
Many consider the AC-30/4 to have the best tone of all AC-30 models. My ears tell me that the Normal (EF86) channel of the AC-30/4 offers greater mid range response while the AC-30/6 Top Boost has superior treble and bass response. The AC-30/6 also tends to be a bit cleaner than the AC-30/4. The Beatles featured the brilliant tone of the AC-30 Top Boost on many of their early recordings.

The AC-30/6 Replaces the AC-30/4
JMI introduced the AC-30/4 in mid 1960 but it's reign at the top of the Vox product line was brief. The 1961 Vox catalog introduced the three channel, six input AC-30/6 but made no mention of the AC-30/4. The AC-30/4 continued to be offered in the 1961 price list, probably to reduce existing inventory. The AC-30/4 made no appearances in Vox catalogs and price lists after 1961.

A number of reasons have been offered for the decision to retire the AC-30/4. First, it was not unusual for an English band to pool their money to buy one good guitar amplifier and share it. Having three channels with individual volume controls, the AC-30/6 offered the ability for three instrumentalists to play through the amp simultaneously. Secondly, the AC-30/6 circuitry did not include the troublesome EF-86 tube. This eliminated the problems with microphonics common to the AC-30/4. Finally, the unique design of the AC-30/6 was not merely an adaptation from the AC-15. The AC-30/6 had a new and unique circuit design that was preferred by Vox lead engineer Dick Denney.

AC-30/4 Head
Vox introduced an AC30/6 six input head with a matching 2x12 speaker enclosure in the 1962 catalog. This "AC-30/6 Super Twin" was the first amp head produced by Vox. It was covered in fawn vinyl. In early 1963, the AC-30/6 Super Twin, along with all other amps in the Vox line, transitioned away from fawn to a pebbled black vinyl. Later that year, Vox again shifted from the black pebbled material to black basket weave vinyl. How does this history of the AC-30/6 Super Twin head and vinyl coverings relate to an AC-30/4 head?

It appears that a supply of unsold AC-30/4 chassis may have lingered at JMI many years after it disappeared from the Vox catalog in 1961. I was recently shown an AC-30/4 "Super Twin" head covered in black basket weave vinyl. This AC-30/4 head also had one pin corners and a black plastic Vox logo handle. This is significant because it ties the shipment of this AC-30/4 head from JMI to no earlier than late 1963.

It would seem that JMI was eliminating their AC-30/4 chassis inventory by mounting them in head cabinets covered in black basket weave vinyl and shipping them to America. Perhaps JMI discovered that mounting the AC-30/4 chassis in a head cabinet was adequate to isolate a microphonic EF86 tube from the speakers. The original owner purchased this AC-30/4 head new from Zeb Billings Music, a Thomas Organ/Vox dealer in Milwaukee WI in late 1964.




North Coast Music offers a large selection of replacement parts for AC-30 amplifiers.
Many of these items are manufactured exclusively by North Coast Music under license to Vox.




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Photos and editorial content courtesy Gary Hahlbeck, North Coast Music


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