Vox V-5 Student Amplifier
"A Look Under the Hood"

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The Vox V-5 "Student" amplifier was the entry level tube amplifier from the Vox division of the Thomas Organ Company. It only appeared in the 1965 "Vox-King of the Beat" catalog.

The parts list for the Vox Student amp was very brief. It had a power and output transformer, two input jacks, a pilot lamp, an on/off switch with a volume control, three tubes, six resistors and six capacitors. The Vox Student amp did not even include a fuse.

The tube complement was a 35W4 rectifier tube, a 12AU6 preamp tube and a 50C5 audio output tube.

1960's Era Westinghouse "All-American Five" Radio

The "All-American Five" Radio Circuit
If none of these tubes are familiar to you, it would be for good reason. These tubes were rarely used for guitar amplification. These tubes were commonly a part of a five tube circuit design used in an inexpensive class of AM table-top radios known as the "All-American Five." An example of one such "AA5" radio is shown at left.

An AA5 radio did not include either a power transformer or a traditional power supply. The tubes in the traditional AA5 radio circuit were connected directly to the 120 line voltage without the benefit or safety offered from a line isolation (power) transformer.

The tube heaters (or filaments) of all five tubes in an AA5 radio were wired in series, like an old-fashioned string of Christmas tree lights, and connected directly to the 120 VAC line. This feat was accomplished through some creative tube engineering. The first numbers in a tube designation reveal the tube's required filament voltage. The 35W4 required thirty-five volts at the filaments. The 12AU6 required twelve volts and the 50C5 required fifty. The converter and IF amplifier tubes each ran off twelve volts. The combined desired filament voltages of these five tubes when wired in series (35 + 12 + 50 + 12 + 12) was 121 volts, just a little more than the 120 VAC line voltage. As a result, the series chain of filament heaters in AA5 radios could be connected directly to the 120 VAC line voltage without a power transformer.

A tube circuit also requires a B+ voltage supply. The plate of the half-wave 35W4 rectifier tube in an AA5 radio was connected directly to the 120 VAC line voltage. Used in this fashion, the output of the 35W4 rectifier tube produced a B+ voltage of about 145 volts DC.

The lack of a power transformer in the AA5 design presented an electrical hazard when servicing the radio. Depending which way the plug was oriented in the electrical socket, it would be possible that the hot, rather than the neutral side of the 120 VAC line voltage would be present on the chassis. This presented an electrocution hazard when servicing an AA5 radio. Most AA5 radios were equipped with an AC power cord safety interlock that prevented the radio from being powered up without the back installed.

The V-5 Student Amp Circuit
Unlike the AA5 radio design, the Vox V-5 Student included a power transformer to isolate the chassis from the AC line. The V-5 Student amplifier transformer had a 120 volt primary and a dual tapped secondary.

One of the secondary taps was connected to the plate of the 35W4 rectifier tube and provided the B+ voltage for the circuit.

A second tap, rated at about 55 volts, powered the tube filament heaters. The heaters of the 35W4 rectifier and 12AU6 preamp tube were wired in series to this tap. The heater of the 50C5 output tube was wired to this filament tap in parallel.

Like the AA5 radio, the Vox V-5 Student was capable of 2 to 3 watts RMS.

The V-5 Student Amplifier Repair Schematic
Thomas Organ never produced a repair schematic for the Vox V-5 Student Amplifier. In response to numerous requests, North Coast Music has assembled an original, detailed schematic for the V-5 Student. The schematic includes the wiring scheme, parts list, and voltage information on the power transformer. You can order a copy of this schematic by clicking here.

What Year Was My V-5 Vox Student Built?
After looking at hundreds of Thomas amplifier serial numbers, I have come to the conclusion that the first digits appear to provide a clue to the year the amplifier was produced. Serials starting with "9" were from the first full year of production, 1966. A paper serial tag fastened to the V-5 chassis with almost certainly show a serial tag starting with a "9," indicating a 1966 production date.



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Photos and editorial content courtesy Gary Hahlbeck, North Coast Music

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