The Vox UL430 Head - An "Under the Hood" Look at the Lower Chassis
Solid State Preamp and Tube Phase Inverter Circuits (1966)



Vox UL430 Chassis Serial 1052

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Introduced in May of 1966, the Vox UL430 was a "hybrid" bass amp that combined a modular solid state preamp circuit with a 30 watt tube power amp stage. This page takes an "under the hood" look at the lower chassis of the UL430.

The Push Towards Solid State Amps
A newly signed distribution contract finalized in the summer of 1965 between JMI and Thomas Organ allowed Thomas to design and produce their own Vox products for the North American market. As a first priority, Thomas Organ wanted to eliminate tubes from their amps. The new Vox amps built by Thomas Organ would be transistorized.

JMI Vox lead engineer Dick Denney traveled from the UK to California in October 1965 to get a first hand look at the progress made toward the conversion from tube to solid state amp circuitry at Thomas. Denney witnessed how transistorization allowed Thomas to add a plethora of features to their solid state preamps. Transistor circuits cost less to build and could be more dependable than their tube counterparts.

Thomas Organ made a compelling case to their UK partners for the advantages of modular solid state amp design. Introduced by JMI in 1966, the 30 watt UL430 was a "hybrid" bass amp that combined a modular solid state preamp with an all tube power amp.

Two Channel Solid State Preamp
The 1966 Owner's Manual for Vox UL Series bass amps noted that the UL430, UL460 and UL4120 had two transistorized channels: Bass (channel one) and Normal (channel two). Both channels had the same layout: two high impedance inputs, volume, bass, middle and treble controls plus a "Top Boost" slide switch. The manual also indicated that unlike the larger UL guitar amps (UL715, UL730, UL760 and UL7120), the UL bass amps did not include reverb, tremolo or distortion.

The preamp circuitry for the Vox UL Series bass amps was assembled on two identical hand wired tag strips. It was unusual for Vox to construct a solid state circuit such as the UL430 preamp on tag strips. Most solid state circuits are assembled on printed circuit boards. The preamp tag strips were mounted vertically to a three sided folded steel channel inside the chassis pan. A photo near the top of this web page shows the location of these hand wired preamp tag strips.

The schematic for the UL430, UL460 and UL4120 preamp (Jennings Musical Industries drawing number OS/119, available for purchase from North Coast Music) establishes that both channels share the same voicing and solid state circuitry.

Tube Phase Inverter Circuit
The Vox UL430 utilized an ECC83 (12AX7) as the phase inverter for a 30 watt power amp stage powered by a quad of EL84 tubes. The supporting capacitors and resistors were mounted to a pair of phase inverter tag strips located near the tube sockets (see photo above).

Photo Credits
The Vox Showroom would like to thank Steve Walsh for his generous contribution of the UL430 chassis photos used on this page.





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Photos and editorial content courtesy Gary Hahlbeck, North Coast Music


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