JMI Vox AC-30/4 Normal and Bass Models
1960 - 1961
A Detailed Look "Under the Hood"

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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 in the "Normal" channel 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 based on the AC-15. While retaining the preamp and control circuitry of the AC-15, the AC-30/4 added an upgraded power supply and an additional pair of EL84 tubes to increase the output power from 15 to 30 watts. The AC-30/4 cabinet was the same height and depth as the AC-15 but 7" wider, making room for a second 12" speaker.

AC-15 Chassis

Vox head engineer Dick Denney so liked the higher gain and rich harmonics of the EF86 pentode tube that he designed it into the preamp circuits of Vox AC-4, AC-10 and AC-15 amplifiers. As the AC-30/4 and AC-15 shared the same preamp circuitry, the AC-30/4 also included an EF86 tube in the preamp stages of the Normal channel.

When JMI introduced the four input AC-30/4 in mid 1960 it was their "top of the line" amplifier but it was joined by the six input AC-30/6 before the end of the year. The AC-30/6 replaced the single element EF86 preamp tube used in the AC-30/4 with a dual element 12AX7. This facilitated the addition of a third channel in the AC-30/6. Thoroughout the first part of 1961, AC-30/4 and AC-30/6 amps were sold concurrently by JMI. By the time the 1961 Vox catalog was released in mid 1961, the AC-30/6 was included but the AC-30/4 was not.

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.

V1 EF86 Preamp of Normal channel
V2 ECC83 Phase Inverter
V3 - V6
EL84 Power Output tubes
V7 ECC83 Preamp of Vibrato channel
V8 ECC82 Modulator for the Vibrato Circuit
V9 ECC83 Oscillator for the Vibrato Circuit
-- GZ34 Tube rectifier for power supply
AC-30/4 Tube Chart
A number of opinions have been offered why JMI would decide to retire the AC-30/4 in favor of the AC-30/6. 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.

The AC-30/4 Schematic
The circuit diagram for the AC-30/4 is found on JMI schematic OA/032, dated April 29, 1960. The schematic indicated that the drawing was for "works use only," or as an factory manufacturing assembly reference. Schematics that were supplied to outside service organizations started with the letters "OS."

The title box for schematic OA/032 offers the name "AC-34" for the AC-30/4. This was probably a misprint was caused by confusion between the written and spoken version of the amp's name. While it was "AC-30/4" in print, it was "AC thirty four" when spoken. The model number was listed correctly in all other JMI printed materials.

Haddon Power Transformer - AC-30/4

Haddon Output Transformer - AC-30/4
The AC-30/4 Chassis
The ingenious chassis design of the JMI Vox AC-30/4 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.

The steel base of the chassis supported the power supply and output amp sections of the AC-30/4. 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 Haddon 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.

It is interesting to note that the control panel area of the AC-30/4 chassis was punched to accommodate six input jacks (see photo above). This allowed JMI to use the same chassis to build both the AC-30/4 and AC-30/6. The black and gold AC-30/4 control panel covered the two additional input jack holes.

Tone Shaping: AC-30/4 Normal and Bass 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.
Coupling and Bypass Capacitor Values Used to
"Voice" AC-30/4 Normal and Bass Amplifiers
AC-30/4 Normal
AC30/4 Bass
.01 uf
.05 uf
.01 uf
0.1 uf
.01 uf
.0.1 uf
.01 uf
0.1 uf
.01 uf
0.1 uf
.01 uf
0.1 uf
.005 uf
.01 uf

The only differences between the AC-30/4 Normal and AC-30/4 Bass 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 schematic OA/032.

Tone Controls
The AC-30/4 featured two tone controls, "Brilliant" and "Bass".

The Brilliant control affected only the Normal channel. The circuit for the Brilliant control had only two components, a two position rotary switch and a 250 pf capacitor. The simple circuit rolled off treble response above 5000 hz.

The Vox AC-30/4 was also equipped with a tone control named "Bass" to roll off treble response for both channels simultaneously. 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-30/4 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-30/4 schematic) and a bypass capacitor (C11 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 C11, combining these opposing signals would cause them to completely cancel each other out. Capacitor C11 only allowed the treble frequencies to pass, in turn canceling only them.

The value selected for capacitor C11 was adjusted to create the desired tonal effect in AC-30/4 Normal and Bass amps. (see table above).The .005 uf capacitor used for C11 in the AC-30/4 Normal amplifier created a 1350 hz treble roll off point for the Tone control. The .01 uf value chosen for C11 in the AC-30/4 Bass amplifier created a 650 hz treble roll off point.

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-30/4 (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-30/4 were dedicated to Vibravox. An external egg shaped foot switch enabled and disabled the effect.

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-30/4 power supply included a Haddon power transformer, a GZ-34 rectifier tube, a choke and a dual 16 uf 450 volt filter cap (C40 and C41).

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 285 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 285 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-30/4 power supply included a 10 henry choke straddled by two 16 uf, 450 volt smoothing capacitors (C40, C41). 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.


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Photos and editorial content courtesy Gary Hahlbeck, North Coast Music

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