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).