The Vox 730 Head - An "Under the Hood" Look at the Upper Chassis
Power Supply and Power Amp Circuits (1966)

Vox 730 Serial #3022

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The story surrounding the power amp circuitry of the Vox 730 is a most curious tale.

The schematic for the 730 power amp sections (OS/114) was released on February 28, 1966. The 730 preamp schematic (OS/117) was dated April 25, 1966 with a revision on November 15, 1966. Most if not all 730 amps were produced in 1966.

While investigating the serial numbers posted in various web sites and speaking to various 730 owners, I have found that the existing serials seem to be concentrated between #3001 and #3100. While perahps 100 to 150 UL730 amps may have been produced in 1966, the number of surviving amps has diminished over the years. Some may have been abandoned over service issues such as transformer failures. Many were recalled from retailers by Jennings. Additionally, Vox scrapped a number of 730 amps that were returned to the factory by dissatisfied customers. These amps were often exchanged for the all solid state amps introduced by Vox in 1967.

If you own a 730, please email your serial number to the Vox Showroom at so that I may continue to refine the production count.

The 730 chassis was comprised of three parts. The chassis base was a rectangular steel box with an open bottom. It enclosed the preamp circuitry. The steel upper chassis was riveted to the base. It was formed to create an electronically shielded "valley" to mount the transformers and other power supply components. The upper chassis then extended over the top of the EL84 and ECC83 tubes, serving as a heat shield for the cabinet. An anodized aluminum front panel completed the chassis. It included the control panel and front escutcheon and was fastened to the base and upper chassis.

The power and output transformers for the UL730 were manufactured for Vox by Drake. No end bells were installed on these transformers. The output transformer was rotated 90° to the power transformer to minimize 60 hz hum.

Figure 1 - CZ4 Brimistor

730 Power Supply Components
The power supply for 730 serial #3022 included the power transformer, a "Brimistor" (see Figure 1), the components on the power supply tag strip and a number of filter capacitors.

The "primary" side of the power transformer connected to the wall current. The primary had five input "windings" that allowed the 715 to operate on 115VAC, 165VAC, 205VAC, 225 VAC or 245VAC mains voltages. The correct mains voltage was manually selected from a five position rear panel rotary switch.

Three "secondary" power transformer windings connected to the internal circuitry of the 730. One winding provided ~25 volts to power the solid state preamp. A second winding supplied 6.3 VAC for the tube heaters and lamps. The third, or "high tension" (HT) winding, powered the B+ supply. Let's have a closer look at the B+ supply.

730 B+ Power Supply
As depicted on JMI schematic OS/114, the 730 B+ power supply provided DC operating voltages for the plates and screens of the EL84 output tubes and the plates of the ECC83 inverter tube.

A bridge of four BY100 diodes and C56, a 32 uf 450 volt capacitor provided about 330 VDC of plate voltage for the output tubes (Figure 2, Point A). This is followed by R94, a 470 ohm, 1 watt resistor and C54, another 32uf 450 VDC capacitor, to supply screen voltage for the EL84 tubes (Figure 2, Point B). R83, a 22k 1/2 watt resistor and C50, yet another 32uf 450 VDC capacitor, power the ECC83 inverter tube (Figure 2, Point C).

Vox amps are often loaded with surprises, no exception here. There are major differences in the B+ supply circuit depicted in schematic OS/114 and the B+ supply in the incredibly original 730, serial #3022, shown on this page.

Like the OS/114 730 schematic, the B+ supply for serial #3022 started with a bridge of four BY100 diodes (see Figure 3). Following the diode bridge was a thermally sensitive resistor called a "Brimistor" (see Figure 1 above). The CZ4 Brimistor in the 730 presented a high resistance to the B+ circuit at room temperature, limiting the "turn on" voltage spike from the power supply. After thirty seconds of operation, a heating element inside the Brimistor caused this resistance to drop to a negligable amount, allowing the full B+ operating voltage to enter the circuit.

It is preferable for vacuum tubes to be warmed by their heater filaments prior to applying the full B+ voltage to their plates. The Brimistor automatically limited the amount of voltage passed to the tubes until they had been sufficiently warmed.

Figure 2 - 730 B+ Power Supply as Depicted on OS/114 Schematic

Figure 3 - 730 B+ Power Supply with Brimistor - Serial #1022

The current exiting the Brimistor entered R99, a 110 ohm, 5 watt wire wound resistor and C57/C58, a twin element 32 uf 450 volt filter capacitor. The positive terminals of capacitors C57/C58 were connected to the center tap of the output transformer, in turn providing the plate voltage for the EL84 tubes (Figure 3, Point A). The circuit continued with R83, a 22k step down resistor and C50/C56, a twin element 16uf 450 volt capacitor. This provided the plate voltage for the ECC83 inverter tube (Figure 3, Point B).

Unlike the circuit depicted in OS/114, the JMI 730 factory schematic, the B+ circuit for 730 serial #3022 did not include a tap for the EL84 screens. The screen supply for 730 #3022 was supplied by two taps on the output transformer. There will be more about that subject later on the page.

730 Power Amp Circuit as Depicted on OS/114 - the JMI Factory Schematic
Like the thirty watt Vox AC-30 it was intended to replace, the Vox output stage of the 730 had four EL84 tubes in a traditional cathode biased, parallel push-pull circuit with no negative feedback.

V1, the ECC83 phase inverter tube, converted the audio signal coming from the solid state preamp into two equal but opposing waveforms. These waveforms entered the power amp at C51 (Input 1) and C52 (Input 2) as illustrated in Figure 4 below. One pair of EL84 tubes (V2 and V3) were wired together in parallel to amplify the signal from C51. The second pair of EL84 tubes (V4 and V5) were wired together in parallel to amplify the inverted signal from C52. These "push-pull" signals combined in the output transformer (T2) to create a 30 watt RMS output stage.

Figure 4 - 730 Power Amp Circuit as Depicted on OS/114 Schematic

The 730 amplifier stage illustrated in JMI factory schematic OS/114 has a common cathode resistor (R89) and common cathode bypass capacitor (C55). This portion of the circuit is highlighted in blue in Figure 4 above.

JMI schematic OS/114 also shows that all four EL84 screen grids receive their bias voltage from a common point in the B+ power supply. This portion of the circuit is highlighted in yellow in Figure 4 above.

"Ultra Linear" Power Amp Circuit From 730 Serial #3022
There were major differences between the traditional Vox power amp circuit illustrated in the OS/114, the 730 factory schematic and the cutting edge power amp stage of 730 serial #3022 (and perhaps all other 730 amps). The output stage in 730 serial #3022 incorporated the Ultra Linear power amp design that had previously only been used in esoteric hi-fi amplifiers. The Ultra Linear circuit design supplied bias voltage to the screen grids of the output tubes in a revolutionary way.

David Hafler and partner Herbert Keroes found that harmonic and intermodulation distortion in tube amplifiers could be significantly reduced by powering the screen grids of output tubes from taps in the output transformer. By using screen grid taps in the output transformer to provide negative feedback to the power amp in a new and innovative way, Hafler and Keroes' Ultra Linear circuit topology provided greater usable power with lower distortion and improved bass response. It also eliminated the need for a fixed screen grid power supply. The Ultra Linear circuit design was awarded a fourteen year US patent in 1955 and Hafler and Keroes licensed their technology to various manufacturers of audiophile hi-fi tube power amps including Harmon Kardon, Marantz, Dynaco and Scott.

Figure 5 - "Ultra Linear" Power Amp Circuit from 730 Serial #3022

The yellow highlighted circuitry in Figure 5 illustrates the pair of output transformer taps that supply the bias voltage to the screen grids of the EL84 tubes in 730 serial #3022. Compare this to JMI schematic OS/114 in Figure 4 where the screen grids are connected to the B+ power supply.

The position of these screen grid taps in the primary winding of the output transformer is critical for optimum Ultra Linear performance. For amps running EL84 tubes in Ultra Linear mode, the UK tube manufacturer Mullard recommended the screen grid taps should be located at 43% of the winding measured from the center tap to the ends of each coil (see Figure 6 at right).

Figure 6 - Screen Taps for Ultra Linear Operation

Hafler and Kehoes also suggested that the output tubes in Ultra Linear power amps should be equipped with separate cathode resistors and bypass capacitors. 730 serial #3022 had a cathode resistor and bypass capacitor for EL84 V2-V3 and a second cathode resistor and bypass capacitor for EL84 V4-V5, as highlighted in blue in Figure 5. The OS/114 730 factory circuit diagram showed a single cathode resistor and bypass capacitor for all four output tubes (Figure 4).

The Vox OS/114 730 Schematic Conundrum
Based on component date codes, it seems reasonable to assume that the power transformer, output transformer, Brimistor, filter capacitors and power supply resistors in 730 amp serial #3022 are original factory parts. It is also obvious from the photos and circuit diagrams shown above that neither the power supply nor power amp circuitry of UL730 serial #3022 matches the circuitry illustrated on OS/114, the factory "power end" diagram. How can this be possible?

As it seems likely that less than one hundred fifty UL730 amps were produced in total, it begs another question. Were some 730 amplifiers produced with the traditional power amp design from OS/114 while others were constructed with the Ultra Linear style power amp design never documented by Vox? While the answer is left to speculation, there are a number of reasons to suggest that all UL730 amps probably included the Ultra Linear style power amplifier circuit.
  • As Vox ramped up to produce the UL730 and UL430 in the spring of 1966, power and output transformers needed to be ordered and produced. These transformers would not be an "off the shelf" item for Drake, the company that produced the transformers for the 730 and 430. The transformers would be custom made for Vox and Drake would probably require a significant minimum order to produce them.

    We know that Drake produced at least one screen tapped "Ultra Linear" output transformer for the UL730 Ultra Linear circuit as it was installed in serial #3022. If they made one, it would seem reasonable that Drake would have produced a significant number of the UL730 and UL430 Ultra Linear output transformers for Vox.

  • It is unlikely that Drake would have produced both a traditional output transformer (less screen grid taps) and an Ultra Linear style output transformer (with screen grid taps) for amps produced in such limited quantities as the UL430 and UL730.

  • Most would agree that in 1966 Vox was still trying to produce "clean" sounding amps. The Ultra Linear circuit would enhance the "clean" response of the 730 while also improving bass response and the usable power of the 430.
One might wonder why Vox chose not to document the Ultra Linear amp circuit on OS/114. Was it perhaps that they were trying to hide something?

The US patent issued to David Hafler and Herb Kehoes in 1955 for the Ultra Linear circuit design would be in effect until 1969. During that time frame, Hafler and Kehoes collected a licensing fee from manufacturers incorporating the Ultra Linear circuit into their power amplifiers.

JMI had a history of "borrowing" circuit designs from other manufacturers. Dick Denney admitted that the Vibravox (Vib/Trem) circuit in the AC-15 and AC-30 was copied from a Wurlitzer home organ. The Top Boost circuit installed in AC-30 amplifiers appears to be from the Gibson GA70 amp. To avoid paying a license fee, JMI designed a reverb pan that narrowly bypassed the patents on the Hammond Type 4 reverb system.

Based on this history, it would seem unlikely that JMI would have wanted to pay Hafler and Kehoes to license the Ultra Linear circuit design. Perhaps JMI included the traditional EL84 power amp circuit in the OS/114 schematic to divert attention from the Ultra Linear circuit Vox was actually installing in their UL730 amps.

Based on the information provided above, it seems likely that the Ultra Linear circuit was installed in all UL730 and UL430 amplifiers. It is also worth noting that all of the other "hybrid" amps in the UL series (710T, 715, 760 and 7120) had fixed screen grid power supplies, and as a result, would have traditional, not Ultra Linear, power amp circuits.

UPDATE - MAY 2017: It has been reported to the Vox Showroom that UL730 serial #3052 and UL430 serial #1052 both have the "Ultra Linear" output transformer windings described on this page. Further, master Vox restoration tech Mitch Colby recently restored a UL730 (serial #3021) and a UL430 (serial plate missing). Mitch noted that both amps had the Ultra Linear output stage. All of this lends support that all UL730 and UL430 amplifiers likely utilized the Ultra Linear output circuit.


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Photos and editorial content courtesy Gary Hahlbeck, North Coast Music

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