Triplett 631 VTVM

Triplett 631 Vacuum Tube Voltmeter battery replacement

Stuart Schmitt, 14 August 2016
 

Introduction

The Triplett 631 is a vintage analog multimeter with a vacuum tube voltmeter. That is, in some operation modes, the analog meter is driven by a vacuum tube circuit, allowing for a high input impedance (11 MΩ) compared to the impedance of the panel meter’s electrical movement. In the 631’s original design, the plate voltage for the 1R5 vacuum tube is 45 V, supplied by two 22.5 V “XX15” carbon-zinc batteries made by the Burgess Battery Company. These batteries (or the equivalent Eveready 425-P) haven’t been available for decades, so I designed a replacement using AAAA cells mounted on custom printed circuit boards (PCBs).

The sections of this page are:

About the PCBs

The three PCBs each hold ten 1.5 V AAAA cells; thus the three boards in series supply the necessary 45 V. The battery clips are of surface-mount design in order for the PCBs to stack directly on top of each other without shorting any through-hole contacts. It is advisable to place small pieces of 1/8 in (3.5 mm) heatshrink tubing on battery clip contacts that could short with a neighbor.

PCB front Battery adapters
PCB back

OSH Park is an excellent place to buy custom printed circuit boards. A single order is for three copies of a design, which is really convenient for this project. I have shared my design publicly for easy ordering. My Gerber files are also available for download.

Wiring

The two XX15 batteries were wired in series with a center tap. In my design, the two batteries are replaced by three PCBs containing 10 batteries in series. The three PCBs are themselves wired in series. A center tap is available on each PCB, but is only used in one of them. The electrical connections are Molex KK 254 headers, which are inexpensive and readily available. The wiring is shown below.

+22.5 V to vacuum tube socket 0 V to solder stud at R9 –22.5 V to splice at R4 & R5 New wiring

The original wiring is too short, so I replaced the battery wires all the way back to their solder joints. In my specimen, Triplett’s original wiring was orange for ±22.5 V and blue for 0 V. In my rewiring, I chose a more sensible scheme of blue for −22.5 V, black for 0 V, and yellow for +22.5 V.

Assembly

One final caution is that the metal battery retention clip must be covered in electrical tape or heatshrink tubing to prevent shorts. A piece of foam can be used between the new battery holders and the case to keep everything snug.

Batteries installed

The vacuum tube’s filament battery is a D cell, and it is required for VTVM operation (and also the ×1 Ω, ×10 Ω, and ×1 kΩ ranges). I did not have a D cell at the time of the photograph, so a C cell and some extra wires are present instead.

Bill of materials

Manufacturer Part Quantity Description Mouser Digi-Key
Keystone 50 60 AAAA surface-mount battery clip 534-50 36-50K-ND
Molex 22-05-3031 3 KK 254 header, 3 position, right angle, tin plated 538-22-05-3031 WM4301-ND
Molex 22-01-3037 3 KK 254 receptacle, 3 position 538-22-01-3037 WM2001-ND
Molex 08-50-0114 9 KK 254 female terminal, AWG 30-22, tin plated 538-08-50-0114 WM1114-ND
any Wire, AWG 24 or 22, preferably three different colors
any 30 AAAA battery affordable 60-pack on Amazon
a PCB fab 3 My PCB design purchase from OSH Park

Additional references

Final note: Comparison of XX15 batteries and AAAA adapters

XX15 comparison 1 XX15 comparison 2