JMI Vox LW30 "Light Weight 30" Guitar Amplifier

The Vox LW30 "Light Weight" guitar amplifier is a very close cousin to the Vox T.60 bass amplifier. It shares the same control panel and germanium transistor based output power amp circuitry as the T.60. To the best of my knowledge, this amp only appeared in the 1964-65 JMI Vox catalog and I suspect it was not produced in very great numbers. There is no evidence from Thomas Organ/US Vox literature that this amp was ever imported by Thomas and offered in the United States.

Vox had obtained a bit of experience in solid state design during the development of the Continental Organ. One of the members of the Vox Continental engineering team, Les Hills, created a radical new solid state power amp concept that was incorporated first into the T.60 and soon after into the LW30 guitar amp. Unlike tube amplifiers that required a heavy output transformer, Les Hills' design coupled the output transistors directly to the speakers. Eliminating the cost, size and weight of the output transformer would help to create a trimmer amplifier package. Additionally, as solid state amplifiers did not have the high voltage and current demands of a tube amplifier, the power transformer could be modest in size, reducing weight and bulk from the final product.

The completed design for the Vox T.60 and LW30 amps used a pair of germanium output transistors that were capable of about 30 to 40 watts RMS and 60 watts peak output power. The amp had two channels, each with dual inputs. The first channel was on the upper, larger control panel and an exact clone of the T.60. The second channel had a volume and two tremolo (labeled "Vibrato" on the amp) controls and was located on a smaller control panel on the rear back panel.

A black vinyl Vox logo handle and two brass vents graced the top of the cabinet. A rotary voltage selector allowed the amp to be manually adjusted for the mains voltage anywhere in the world.

The compact, open backed enclosure was covered in traditional Vox basket weave vinyl and featured three 10 watt 10" stamped frame Goodmans speakers with Alnico magnets. An impedance value of 15Ω was imprinted on the rim of each speaker (see photo at left). As the trio of 10" 15Ω speakers was wired in parallel, a total load of 5Ω (15Ω ÷ 3 = 5Ω) was presented to the output stage of the amplifier.

After the amp was sold to the general public, the electronics for the T.60 and LW30 failed repeatedly and at first Vox couldn't understand why.

Eventually, Vox discovered the problem causing the large number of T.60 and LW30 amp head failures. The power amp design was prone to go into high frequency oscillation. In simple terms, the amplifier continuously generated an extremely high pitched squeal at full power that was above the range of human hearing. This drove the power amp section past the safe design limits for the output transistors, causing meltdown. A successful solution to this output transistor failure problem was never implemented.

By 1966, the LW30 was discontinued with the completion of the second generation of Vox solid state amps. The new solid state Conqueror, Defiant and Supreme guitar amps featured dependable silicon output transistors with large heat sinks. This corrected the high frequency oscillation problem that plagued the LW30. The new models also had a a number of features not found on the LW30, such as reverb, distortion, Tone X and MRB.


The VOX Showroom!

Photos and editorial content courtesy Gary Hahlbeck, North Coast Music

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