The Vox UL430 Head - An "Under the Hood" Look at the Upper Chassis
Power Supply and Power Amp Circuits (1966)

Vox UL430 Upper Chassis Views

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Introduced in 1966, the UL430 was a 30 watt "hybrid" bass head with a solid state preamp and a tube power amp. Vox also produced the UL730, a 30 watt hybrid guitar head with tremolo, reverb and fuzz.

The schematic for the UL430 power amp section (OS/114) was released on February 28, 1966. The UL430 preamp schematic (OS/119) was dated April 20, 1966. Most if not all UL430 amps were produced in 1966.

The UL430 and UL730 chassis was comprised of three parts (Figure 1). 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 UL430 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 - Side view of a UL730 chassis, similar to the UL430 chassis

Figure 2 - CZ4 Brimistor

430 Power Supply Components
The power supply for UL430 included the power transformer, a "Brimistor" (see Figure 2), 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 UL430 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 UL430. 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.

430 B+ Power Supply
As depicted on JMI schematic OS/114, the UL430 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.

JMI Vox schematic OS/114 showed that 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 3, 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 3, Point B). R83, a 22k 1/2 watt resistor and C50, yet another 32uf 450 VDC capacitor, power the ECC83 inverter tube (Figure 3, 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 real UL430 and UL730 amps.

Like the OS/114 430 schematic, the B+ supply for the UL430 started with a bridge of four BY100 diodes (see Figure 4). Following the diode bridge was a thermally sensitive resistor called a "Brimistor" (see Figure 4 above). The CZ4 Brimistor in the UL430 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 power supply with the Brimistor (Figure 4) automatically limited the amount of voltage passed to the tubes until they had been sufficiently warmed.

Figure 3 - 430 B+ Power Supply as Depicted on OS/114 Schematic

Figure 4 - 430 B+ Power Supply with Brimistor

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 4, 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 4, Point B).

Unlike the circuit depicted in OS/114, the JMI UL430 factory schematic, the B+ circuit did not include a tap for the EL84 screens. The screens were supplied by two taps on the output transformer. There will be more about that subject later on the page.

UL430 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 UL430 had four EL84 tubes in a traditional cathode biased, parallel push-pull circuit. Unlike the AC-30 and UL730, the UL430 power amp stage included a negative feedback loop. Amps employing negative feedback limit power amp distortion, a plus for a bass amplifier.

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 5 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 5 - UL430 and UL730 Power Amp Circuit as Depicted on OS/114 Schematic

The UL430 amplifier stage illustrated in JMI factory schematic OS/114 showed a common cathode resistor (R89) and common cathode bypass capacitor (C55). This portion of the circuit is highlighted in blue in Figure 5 above.

JMI schematic OS/114 also shows that all four EL84 screen grids (pin 9 on the EL84) receive their bias voltage from a common point in the B+ power supply. This portion of the circuit is highlighted in yellow in Figure 5 above.

"Ultra Linear" Power Amp Circuit
There were major differences between the Vox UL430 and UL730 power amp circuit illustrated in the OS/114, the schematic provided by Vox to their service techs and the cutting edge power amp stage of real UL430 and UL730 amps. The actual output stages in these amps 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 6 - Actual UL430 and UL730 "Ultra Linear" Power Amp Circuit

The yellow highlighted circuitry in Figure 6 illustrates the pair of output transformer taps that supply the bias voltage to the screen grids (pin 9) of the EL84 tubes in UL430 and UL730 amps. Compare this to JMI schematic OS/114 in Figure 5 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 7 at right).

Figure 7 - 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. All UL430 and UL730 amps had one 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 6. The OS/114 factory circuit diagram for the UL430 and UL730 power amp stage indicated a single cathode resistor and bypass capacitor for all four output tubes (Figure 5).

The Vox OS/114 UL430 and UL730 Schematic Conundrum
It should now be obvious that Vox did not depict the Ultra Linear power amp circuitry on OS/114, the factory service schematic for the UL430 and the UL730. Was it perhaps that Vox was 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. It is also worth noting that all of the other "hybrid" amps in the UL series (UL710T, UL715, UL460, UL760, UL4120 and UL7120) had traditional screen grid power supplies, and as a result, would not have the Ultra Linear power amp circuit.


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