Figure 1: A simple meter lamp dimmer circuit
In the past two issues, I wrote about vacuum tubes. This month, the filament-related theme continues. Light bulbs have been getting the nudge from LEDs for years, yet there are still plenty of old-fashioned VU meters to keep lit. In one month, I’ve encountered several meter lamp — challenged consoles — only one meter out of dozens remains lit.
I am often asked whether it’s better to leave equipment powered 24/7 or turn it off. I believe professional gear should tolerate being “power-cycled,” especially if it is not in use every day. Heat dries out capacitors and wastes energy. Either way, bulb life is an issue, especially when replacement requires more than a little wrist action.
Whether for outboard gear, consoles or recorders, finding the right bulbs can be a chore. Sometimes the replacement procedure adds salt to the wound, especially when the bulbs must be soldered — or worse, they’re soldered inside the meter.
Many popular bulbs in vintage ’70s-era American-made gear use variations on the T1-3/4 “midget” series (bi-pin, screw, flange, groove and wedge). LEDtronics has LED alternatives that draw about half the current of a conventional incandescent bulb and last at least 10,000 hours. (One 24/7 year is 8,760 hours!) The catch is that some distributors require a sizable minimum order. However, according to an LEDtronics representative, its LED replacement for the common 7387 bulb is $6 each in quantities of one to nine. Not bad for a 2-track bus, but pricey if you’re doing 24 (or 48!) VU lamps.
Figure 2: An SSL console meter with clear protective cover removed to access the lamps. At right, the incorrect Type 7387 bulb is installed (for comparison purposes); at left is an LED replacement.
Many years ago, working in a poor studio somewhere in Jersey, I installed meter light dimmers on a 3M tape machine. By going easy on the juice, we got a lot more mileage out of those 48 bulbs; their only remaining punishment was to endure fast-wind shock therapy every three minutes or so! During a more recent Trident rackmount compressor upgrade, the bulbs and mounting hardware were completely missing. Using what was handy, I installed a very simple dimmer circuit, seen in Fig. 1.
Type 7387 is similar to what MCI used in its meters and costs just $0.65 each in 100-piece quantities. This version is rated at 28 volts, 40 mA, so it’s conservatively “burning” when fed by a standard 24V supply. The lamps were adjusted to burn only as bright as necessary. Creative variations for this type of mod include adding a photo-resistor so the circuit self-adjusts for ambient light using a timer. If the meter lamps had a dedicated power supply, then it could be trimmed (or modified) down a few volts.
Bulb access for the Trident 65 meters requires removing the clear front cover. You’ll need a soldering iron to get them out. Talk about fear factor! Just try to find the perfect bulb: one that’s the same brightness and physical size as the others. Somewhat easier are the Soundcraft TS-12 lamps that are soldered to a PCB located behind the meter. These “grain of wheat” — style replacements (12V @ 50 mA) are available from www.mcminone.com as part #25-1370 — unbeatable at $0.26 each in small quantities.
By contrast, just about everything about SSL’s 4000 Series was designed for service. It’s possible to shut down individual eight-module “buckets” while leaving the rest of the console functional. The bi-pin — style bulbs are socketed inside the meter. Figure 2 shows a standard incandescent and an LED replacement; the latter gets surprisingly hot without a filament!
SCORCHED TASTE BUDS
Last on our theme of “stuff that gets hot” is advice for assistant engineers: Use a candy thermometer when steaming milk for my lattes and cappuccinos! Boiling water is 212° F (100° C), but steam under pressure can be considerably higher. Either way, 140° F is all it takes.
Eddie welcomes all to latte-practice. Be sure to bring your fave LPs and 45s. Visit www.tangible-technology.com if ya can’t travel.