The Vox NT15H Night Train 15 Amplifier
2009 - 2013
A Look "Under the Hood"

Vox introduced the NT15 Night Train 50 amp head in 2009. It was replaced by the the NT15H-G2 Night Train 15 amplifier introduced by Vox in 2013.

The NT15H Night Train 15 featured a compact, all metal enclosure finished in black and chrome. The chrome plated and ventilated upper portion of the enclosure protected the tubes and fastened to the lower portion of the enclosure with ten phillips machine screws. The lower portion of the enclosure, finished in black paint, served as the chassis pan to contain the NT15H electronic circuitry. Silk screened control panel nomenclature embellished the front and rear portions of the chassis pan.

Circuit Board Construction
The NT15H Night Train amplifier was constructed on two printed circuit (PC) boards. The diode rectified power supply, preamp and power amp circuitry were found on the main circuit board. The speaker output jacks were mounted to a second circuit board.

Circuit Design
The NT15H Night Train circuit featured an all tube audio path that included two 12AX7 preamp tubes and two EL84 output tubes. The circuit design for the NT15H was a collaboration between famed US boutique amp designer Tony Bruno and Vox Amplification in the UK.

The signal from input jack was fed through a 10k and a 27k resistor before entering a V3a, one half of a 12AX7 preamp tube. The output of V3a was directed to the tone stack (tone controls). The output from the tone stack was then sent back to V3b for gain recovery. The "Bright/Thick" switch allowed the musician to bypass the tone stack and use V3b for extra gain.

V4 (12AX7) was the phase inverter, or perhaps more properly called the "phase splitter." It divided the audio signal coming from V3b into positive and negative waveforms. These signals were then directed to the EL84 power tubes V1 and V2.

A three position standby also allowed the selection between operating the EL84 power tubes as triodes (producing 7.5 watts) or as pentodes (producing 15 watts).

Power Supply
The NT15H Night Train was designed for operation on 120, 220 or 240 VAC. Voltage selection involves an adjustment to the internal circuitry that requires the assistance of an authorized Vox service center.

Separate diode circuits rectified both the B+ (350 VDC) circuit and the tube filament heater (6.3 VDC) circuit.

Hand wired amplifiers normally control 60 hz hum by braiding the filament heater wires between tube sockets. Braiding these wires created a "humbucking" effect that canceled the 60 hz hum from the AC filament heater circuit. As it isn't possible to "braid" the traces on a printed circuit board, a DC filament heater circuit, as found in the NT15H Night Train amplifier, is a very effective solution to reduce filament enduced 60 hz hum.

The main power fuse was located in a "drawer" just below the socket for the power cable. When operating the amp at 120 VAC, a T1.25AL/250V fuse was required. For 220 or 240 VAC mains, a T630mAL/250V was correct.

Two additional fuses were located on the printed circuit board. FS1, a T160mAL fuse, was on the B+ circuit of the amplifier. FS2 protected the tube filament circuit with a T6.3AL fuse.

An additional 1k ohm fusible link (R1) was also located in the B+ circuit of the power supply. Failures of fusible link R1 require replacement by an authorized Vox service center.

The rear panel of the amplifier indicated that the amp was built in Vietnam.


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Photos and editorial content courtesy Gary Hahlbeck, North Coast Music

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