JMI Vox AC-30/6 Normal, Treble and Bass Models
1960 - 1967
A Detailed Look "Under the Hood"

© 1996 - 2023 The Vox Showroom, all rights reserved. No use on online auctions, eBay or Reverb.

Vox introduced their first amplifier aimed at the professional musician, the AC-15, in 1958. An improved, second generation version of this 1x12" dual channel 15 watt combo amp was designed in 1959. The new AC-15 preamp section featured an EF86 tube and the Vox "Vibravox" circuit.

For some, the AC-15 was just not loud enough. Vox addressed this problem in 1960 by introducing the AC-30/4, a new model that doubled the output power of the AC-15. While retaining the preamp and control circuitry of the second generation AC-15, the
AC-30/4 added two more EL84 power tubes to create a 30 watt power amp. A pair of 12" Celestion alnico speakers handled the additional power output. The AC-30/4 cabinet was the same height and depth as the AC-15 but 7" wider.

AC-30/4 Control Panel

........EF-86 Tube

Shortly after the AC30/4 amp was introduced, an unforeseen engineering problem arose. The EF86 tube used in the AC-30/4 preamp circuit (photo at left) proved to be susceptible to damage from excessive vibration. At the lower output level of the 17 watt AC-15, 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 deteriorate, it often would became microphonic. Howling and ringing tones from the failing EF86 tube would at times accompany the tone of the guitar playing through the amp.

Vox introduced the AC-30/6 in the second half of 1960. Like the AC-30/4, the AC-30/6 was a 2x12" 30 watt combo amp. Unlike the two channel, four input AC-30/4, the AC-30/6 was a new and unique design by lead Vox engineer Dick Denney that had three channels and six inputs. The AC-30/6 replaced the troublesome EF86 based preamp circuit from the AC-15 and AC-30/4 with a new design based on ECC83 (12AX7) tubes. The AC-30/4 and the AC-30/6 were simultaneously available from Vox in late 1960 and early 1961.

Vox decided to retire the AC-30/4 by the middle of 1961 but the AC-30/6 was retained. A number of reasons have been offered for this decision. 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 prefered by Vox lead engineer Dick Denney.

V1 ECC83 ½ used for preamp of Normal channel
½ used for preamp of Brilliant channel
V2 ECC83 Phase Inverter
V3, V4
V5, V6
EL84 Power tubes
V7 ECC83 ½ used for preamp of Vibrato channel
½ used as a gain stage for Vibrato circuit
V8 ECC82 Modulator for the Vibrato Circuit
V9 ECC83 Oscillator for the Vibrato Circuit
-- GZ34 Tube rectifier for power supply
The original, three channel, six control AC-30/6 evolved into three unique models. First came the AC-30/6 Normal, introduced in 1960. Next was the Vox AC-30/6 Bass model in 1961. Vox introduced the AC-30/6 Treble model in 1964, after the "British Invasion" of America was in full swing.

The circuitry for the AC-30/6 Normal was detailed on Vox service schematic OS/065. The AC-30/6 Bass model was found on Vox schematic OS/057 and the AC-30/6 Treble on OS/056.

The circuitry for the AC-30/6 Normal, Bass and Treble models included four ECC83, one ECC82, four EL84 and one GZ34 tubes. The service schematics numbered the tubes from "V1" through "V9." The tube numbers in the chart at left correspond to the factory schematics and chassis photos near the top of this page. This illustrates the function and chassis location of each tube.

The AC-30/6 Chassis
The ingenious chassis design of the JMI Vox AC-30/6 combined a pressed steel horizontal base with a vertical assembly made of aluminum. This chassis metal work was adapted from the second generation AC-15 and the AC-30/4.

The steel base of the chassis supported the power supply and output amp sections of the AC-30. The use of steel in the chassis base not only provided strength but also electronically isolated the high voltage (and hum producing) section of the power supply and output amp from rest of the amplifier. The base of the chassis bolted to a slider board to allow the chassis to be removed from the cabinet like a drawer.

The power transformer and output transformers were located at opposite ends of the chassis base to provide proper balance. "L" brackets mounted to the top of the power and output transformers strengthened and supported the vertical aluminum portion of the chassis.

The vertical aluminum section of the chassis enclosed the preamp circuitry and secured the control panel. Aluminum is even less likely than steel to pick up hum and oscillations from the power supply and output amp section of the amp. This served to further electronically shield the preamp from the power supply.

Coupling and Bypass Capacitor Values Used for
Voicing AC-30 Normal, Treble and Bass Amplifiers
AC-30/6 Normal
AC30/6 Bass
AC30/6 Treble
.047 uf
.1 uf
.047 uf
500 pf
1000 pf
500 pf
.047 uf
.047 uf
.01 uf
.15 uf
.15 uf
.047 uf
.047 uf
.047 uf
.01 uf
.15 uf
.15 uf
.047 uf
.0047 uf
.01 uf
.0022 uf
220 pf
Tone Shaping: AC-30/6 Normal, Bass and Treble Models
Unlike stereo equipment that provides even response across the entire audio spectrum, guitar amplifiers are intentionally designed to perform in a somewhat narrower frequency band. Guitar amplifiers are voiced to accent certain frequencies while trimming others. This tonal shaping is most commonly accomplished by the careful selection of coupling and bypass capacitors.

Coupling capacitors serve two important functions. They are used to separate the audio signal coming off the plate (output) of a tube from the 200+ DC voltage required for tube operation. They also serve to determine the point where the low frequency audio response of the amplifier is trimmed, generally at the rate of 6db per octave. In this way, the values selected for the coupling capacitors become a major contributor to the audio response of the amplifier.

Introduced in 1960, the AC-30 Normal was the original AC-30/6. The "Normal" provided an even distribution of bass and treble frequencies. The AC-30/6 Bass was introduced in 1961. The circuitry of the AC-30/6 Bass enhanced the low frequency response of the amplifier. The AC-30/6 Treble appeared in 1964. The circuitry of the AC-30/6 Treble amp was designed to accentuate high frequency reponse. The only differences between these three amplifiers were the values of the coupling and bypass capacitors shown in the table above.

All one needs to know to understand the table above is that as the value of the capacitor increases, more bass frequencies are allowed to pass. Conversely, decreasing the value of the capacitor allows less bass frequencies to pass. The capacitor values and part numbers in the table come from the factory schematics. Following are two examples of how the value of the coupling capacitors affects the tonal response of the AC-30/6.

C2 was the coupling capacitor for the "Normal" channel on all versions of the AC-30/6. It was located between the plate (pin 6) of V1 and the volume control for the Normal channel. C2 had a value of .047 uf in the AC-30/6 Normal amp and .1 uf in the AC-30/6 Bass amplifier. The .047 uf coupling capacitor in the Normal version of the AC-30 started to roll off the bass frequencies at ~68 hz. The .1 uf coupling capacitor in the Bass version of the AC-30 started to roll off the bass frequencies at ~32 hz. In simple terms, the Normal channel of the AC-30/6 Bass amp had more low frequency response than the Normal channel of the AC-30/6 Normal amp.

A second example involves the AC-30/6 Treble amplifier. C5 was a coupling capacitor located in the "Brilliant" channel between the volume control and the screen of the phase inverter (V2). C5 had a value of .047 uf in the AC-30/6 Normal amp and .01 uf in the AC-30/6 Treble. The .047 uf coupling capacitor in the Normal version of the AC-30 started to roll off the frequencies below 68 hz. The .01 uf coupling capacitor in the Treble version of the AC-30 started to roll off frequencies below 300 hz. In simple terms, the Brilliant channel of the AC-30/6 Treble had much less bass than Brilliant channel of the AC-30/6 Normal. This gave the listener the impression that the AC-30/6 Treble had enhanced treble response. In reality, it just had less bass.

Treble Bypass Capacitors - AC-30/6 Treble
Treble bypass capacitors were utilized to passively accentuate the high frequency response of Vox AC-30/6 Treble amplifiers. These 220pf capacitors were installed between the hot and wiper terminals of all three volume controls (see photo at left). The capacitors allowed treble frequencies to bypass the level adjustment of the volume control. Stated another way, treble was not affected by adjustments to the volume control, the treble frequencies always remained at the maximum level. At lower volume levels, this offered the perception of an increase in treble output. However, as the amp was turned up, the perceived amount of treble would decrease in relation to the rest of the audio signal.

AC-30/6 Top Boost Circuit
JMI Vox also developed an optional tone control circuit for the AC-30/6 called "Top Boost." The Top Boost circuit added treble and bass controls to the Brilliant channel of the AC-30/6. This subject is discussed in depth in the Vox Showroom at the AC-30/6 Top Boost "Under the Hood" web page.

The Tone Control
As was common for amplifiers designed in this period, the Vox AC-30/6 was equipped with a single, global tone control circuit to roll off treble response. This tone circuit wasn't located, as one might expect, in the preamp area. It was located in the circuitry between the phase inverter and the power amp tubes. Here is how it worked.

The audio signal from the preamp went to the phase inverter tube (V2 on the AC-30/6 schematic). The phase inverter (or "phase splitter") circuit divided the audio from the preamp into two opposing signals that were 180 degrees out of phase with each
other. These out of phase signals were sent to each half of the "push-pull" power amplifier circuit. One side of the phase inverter supplied the signal to the "positive" side of the power amp, the other side of the phase inverter supplied the signal to the "negative" side of the power amp.

The tone control circuit straddled the connection between these out of phase audio signals with a 250k control "Tone" potentiometer (VR3 on the AC-30/6 schematic) and a capacitor (C10 in the table above). Rotating the 250k control reduced the resistance in VR3, allowing the opposing signals from the phase inverter to combine. Were it not for capacitor C10, combining these opposing signals would cause them to completely cancel each other out. Capacitor C10 only allowed the treble frequencies to pass, in turn cancelling only them.

The value selected for capacitor C10 was adjusted to create the desired tonal effect in AC-30 Normal, Bass and Treble amps. (see table above).

The .0047 uf capacitor used to C10 in the AC-30 Normal amplifier created a 1350 hz treble roll off point for the Tone control. This Tone control circuit would only roll off frequencies greater than 1350 hz. Frequencies less than 1350 hz would be unaffected by the Tone control.

The .01 uf value chosen for the AC-30 Bass amplifier created a 650 hz treble roll off point. The .0022 uf value for C10 in the AC-30 Treble amplifier raised the treble roll off point to about 2900 hz.

Vox Vibravox "Vib/Trem" Circuit
The Vox Vibravox or "Vib/Trem" circuit was originally designed for the AC-15 but it also trickled up to the AC-30/4 and AC-30/6.
Many amplifiers include tremolo. Tremolo applies amplitude modulation, or pulsed volume, to the signal. This is not to be confused with vibrato which applies frequency modulation, or wavering pitch, to the signal. The Vox Vibravox circuit included both vibrato and tremolo.

A vibrato circuit is far more complex than the tremolo circuit frequently installed in other amplifiers. The shaded area of the schematic for the AC-30 Normal (shown at left) illustrates this. The Vibravox circuit required over 70 components and three tubes (V7, V8 and V9). Nearly half of the parts in the AC-30/6 were dedicated to Vibravox.

In an interview with Guitar Player magazine, Vox lead engineer Dick Denney admitted he "reverse engineered" the Vibravox circuit from a Wurlitzer console organ.

Power (Mains) Supply
The AC-30/6 power supply included an electrostatically shielded power transformer, a GZ-34 rectifier tube, a choke and a dual 16 uf 450 volt filter cap (C39 and C40).

The primary (input) side of the power transformer had taps (connections) for the five local AC mains voltages an AC-30 might encounter. These taps connected to the voltage selector on the control panel. The secondary (output) side of the transformer had a 280 VAC, 160 ma tap for the B+ voltages, a 6.3 volt 6A tap for the preamp tube heaters and a 5 VAC 2 amp tap for the GZ-34 heater.

The GZ-34 was a full wave rectifier tube that converted the 280 VAC from the secondary of the power transformer to about 320 volts of pulsed DC. The GZ-34 rectifier tube had a natural tendency to exhibit a slight "sag" in output voltage when the amp was pushed toward the limit. These momentary drops in voltage caused a bit of audio compression to occur in the output of the AC-30. Many feel that the audio compression created by an overdriven AC-30 is an essential component of Vox tone.

For filtering, the AC-30/6 power supply included a 10 henry choke straddled by two 16 uf, 450 volt smoothing capacitors (C39, C40). Positioning the choke between two filter capacitors created a "capacitive pi" filter circuit, a superior, noise-free design.

30 Watt Power Amplifier
JMI adapted the 30 watt AC-30/6 power amplifier circuit from the 15 watt power amplifier circuit originally developed for the AC-15. To double the power output of the AC-15 power amplifier, Vox added another pair of EL84 output tubes to the AC-30/6. Each pair of output tubes were wired to each other in parallel. Vox also increased the size of the power supply to accommodate the additional current requirements of the extra pair of output tubes.

Three key design concepts were combined in the AC-15 and AC-30 power amplifier section to create the characteristic "chimey" Vox tone. The ingredients were: EL84 output tubes, no negative feedback in the power amp and a Class A "self biased" output stage.

Denney's design used small bottle EL-84 power tubes as the first component of the signature Vox tone. The EL-84 is a highly efficient tube. Each is capable of producing 7.5 watts with a relatively low plate voltage of 300 to 340 volts. Compare this to the other output tube commonly used by Vox, the EL34. It required plate voltages in the 425 to 490 volts range.

The efficiency of the EL-84 also had a downside. EL-84 tubes were a bit more prone to distort due to their reduced "headroom." Simply stated, when pushed hard, the distortion level could creep up into the 7 percent area. This distortion was normally controlled by the incorporation of a circuit design called "negative feedback." Negative feedback sends a bit of the signal coming out of the amplifier back to the input of the power amp. Negative feedback not only cleans up undesirable distortion, it also removes some of the pleasing even order harmonics from the amplifier output.

After listening tests, Vox lead engineer Dick Denney decided he preferred the harmonically rich tone and the natural sounding overdrive created by the EL84 when negative feedback was not employed. Although unconventional in design for the period, eliminating the traditional negative feedback circuit in the power amp became a major contribution to Vox tone.

The final ingredient involves the method of biasing the output tubes. Bias is a controlling voltage sent to the control grid to keep the current passing through the tube within safe prescribed limits. Most tube power amps have a manual bias adjustment for the output tubes, typically adjusted from time to time by a trained technician.

Denney discovered that his EL84 power amplifier design sounded better when the traditional manual bias adjustment was abandoned in favor of a self biasing or "Class A" output circuit.

North Coast Music offers a large selection of replacement parts for the AC-30/6 amplifier.
Many of these items are manufactured exclusively by North Coast Music under license to Vox.


The VOX Showroom!

Photos and editorial content courtesy Gary Hahlbeck, North Coast Music

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