Vox V-2 Tube Pacemaker Amplifier
"A Look Under the Hood"


2_Footer
© 1998 - 2017 The Vox Showroom, all rights reserved. No use on online auctions, eBay or Reverb.

The V-2 Vox Pacemaker was similar to the V-3 Vox Cambridge Reverb amplifier. It shared most of the circuitry, the cabinet and speaker with the Cambridge. The primary difference was that the
Cambridge had reverb but the Pacemaker did not. Both were manufactured in 1966 by the Thomas Organ Company of Sepulveda, CA under license to Jennings Musical Industries of Dartford, Kent UK.

Point-to-Point Hand Wired Chassis

A view of the tidy hand wiring found in a JMI AC-30 chassis...
The Vox V-2 Pacemaker chassis featured point-to-point hand wired construction. Point-to-point construction implies that the electrical components (resistors, capacitors, etc) are installed and soldered by hand to terminal strips. Additional hand soldered wires would connect the components installed on the terminal strips to the transformers, tube sockets and filter capacitors. No printed circuit boards would be used on these amplifiers.

The quality of the point-to-point hand wired construction found in JMI Vox amps built in the UK was tidy and masterful (see photo at left). Compare this to the view of the open Pacemaker chassis shown above. It is my opinion that the design and workmanship of the hand wired Vox amps built by Thomas Organ does not approach the quality one would find in a JMI Vox amp.

A two piece steel chassis housed all of the preamp, power amp and power supply circuitry for the V-2 Pacemaker anplifier.


The stamped and formed steel chassis pan was the backbone of the chassis. The T1 power transformer, T2 output transformer, C26 filter cap, and tube sockets were all mounted to the outside of the chassis pan. Locating the high gain preamp circuitry inside the grounded chassis pan shielded it from stray electronic fields emanating from the transformers.

This formed steel chassis pan was also shared with the Vox V-1 Pathfinder, V-3 Cambridge Reverb and V-8 Berkeley Super Reverb amplifiers. It was also incorporated into the solid state versions of these amps produced in later years.

The second part of the chassis was the control panel. This included the rotary controls, jacks and switches. It was fastened to the top of the chassis pan with two machine screws.

Preamp Circuitry
Please refer to the chassis diagram at the top of this page when reading this section.

A 12AX7 tube labeled "V2" powered the preamp stage of the V-2 Pacemaker amplifier. The first section on the tube, V2a, increased the signal coming from the input jacks. The second half, V2b, was used for gain recovery after the tone control stack. This preamp circuit was shared with the V-1 Pathfinder, V-3 Cambridge Reverb and V-8 Berkeley Super Reverb.

One half of a second 12AX7 tube, labeled "V3b" was used in the tremolo circuit. This tremolo circuit was as well found in the V-1 Pathfinder, V-3 Cambridge Reverb and V-8 Berkeley Super Reverb.

Phase Inverter
Aside from lacking reverb circuitry, the phase inverter is one of few areas where the V-2 Pacemaker deviates from the circuit design of the V-3 Cambridge Reverb amplifier.

In simple terms, the phase inverter (or "phase splitter") is a circuit that divides the audio signal coming from the preamp into positive (peak) and negative (trough) components. Using the classic "push-pull" output amplifier design, the "positive" side of the audio from the phase inverter tube is sent to one output tube, the "negative" side of the audio from the phase inverter is sent to a second.

V-3 Cambridge Reverb
Phase Inverter Circuit
V-2 Pacemaker Phase Inverter Circuit

Phase inversion can be accomplished using just one half of a dual triode tube using a "cathodyne" or "split load" circuit (see image at near left). Phase inversion can also be accomplished by a "Long-Tail" or "Schmitt" circuit that uses both halves of a dual triode tube (see image at far left).

Unlike the V-3 Cambridge Reverb, which used a "Long-Tail" or "Schmitt" phase inverter circuit, the V-2 Pacemaker incorporated a "cathodyne" or "split load" phase inverter.

From a design standpoint, it made sense that Thomas designed the V-2 Pacemaker with a "cathodyne" inverter. The V-2 Pacemaker used only one triode of the V3 tube to power the tremolo circuit. The second triode of the V3 tube was not needed for any other function so using it for the phase inverter would seem a wise design choice.

The V-3 Cambridge Reverb required all six triodes from three preamp tubes to power the preamp, tremolo and reverb circuits. As it was already necessary to add an additional 12AX7 tube to the Cambridge Reverb for phase inversion, incorporating a "long-tail" phase splitter circuit seemed a practical engineering decision.

Schematic Error
The factory schematic appears to have an error in the value listed for resistor R17 (see the schematic fragment of the V-2 Pacemaker Phase Inverter Circuit schemaic shown above). The schematic lists R17 to be 5.6k when it appears to be actually 56k.

Power Supply
An EZ81 (V1) tube recifier served to rectify the AC output of the HT secondary of the power transformer (T1). The pulsed DC output from the EZ-81 was filtered and smoothed by a vertical twist lock style filter capacitor (C26). This 350 volt electrolytic filter capacitor had three sections, 40uf, 30uf and 10 uf. The V-2 Pacemaker schematic publishes the power supply output voltages at 322 VDC, 310 VDC and 220 VDC.

Power Amplifier
Two EL84 output tubes (V4 and V5) powered the classic Vox "cathode biased, no NFB" (no negative feed back) output stage.

Many thanks to Josef Appell who allowed the Vox Showroom to take photos of his Vox V-2 Pacemaker amplifier.



2_Footer

.................



The VOX Showroom!


Photos and editorial content courtesy Gary Hahlbeck, North Coast Music


Any and all material presented herein is protected by Copyright.
© 1998 - 2017 The Vox Showroom and North Coast Music, all rights reserved

The images and editorial content in this web site may not be copied or reproduced
in online auction sites such as eBay and Craig's List. Sellers may provide a link
to the Vox Showroom web site if they wish to refer to this copyrighted material.


URL: http://www.voxshowroom.com/us/amp/v-2_hood.html