Audio Research VT-100:

Adding a tube timer

A simple and inexpensive way to measure running hours on output tubes.


This mod is fairly simple, and easily adaptable to other equipment.  Several manufacturers, including ARC, now include timers to measure running hours on tubes, and I decided to add one to my VT-100.  A simple solution (which ARC uses on some of their amps) is to mount the timer inside the chassis on a PCB.  I wanted an easier to read location, and chose the rear panel, just to the right of center near the top. 


The timer I used is a Curtis #701FR001O1248D2060A, from DigiKey, DK part # 267-1003-ND ($20 each as of 5/07).  This timer works on a supply voltage of 12 to 48VDC (21.5V in this case), and uses <1ma current.  It holds the timer value when turned off, and can be reset when tubes are changed.

To install it in the VT-100, I drilled, cut and filed a small hole in the rear panel to mount the timer, and drilled 2 holes for the mounting screws, all using the template provided with the timer - see photo above for my finished installation.


Below: For electrical connections, I tied into +21.5VDC (pre-regulator) supply for tube heaters (V1 – V4)  - the white wire with red trace below, and to ground - the white wire with blue trace below.   This is the left (from the front) side PCB, near the front, lower side.



Below: inside view of the timer.  Pins are numbered 1-4 going clockwise from the upper left.  Pins 1 and 3 are connected together on the timer, shown below.  In addition, a wire from pin 1 is left hanging (with a heatshrink insulator), to be used to reset the timer.


To reset the timer to zero hours when changing output tubes:

With the amp powered on, momentarily connect the green wire (+21VDC power supply) to pin 4(normally no connection).


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