Hi All,
I received an email from djunk regarding a problem he's having with his amplifier, and he indicated that he's using his speakers as his load. Rather than tell him individually about a better mousetrap and then posting it here, I figured I'd go here first.
The obvious problem with using speakers as test loads is that if the amp goes into oscillation--either sonic or ultrasonic--the drivers are in serious jeopardy of being destroyed. The equally obvious cure is to use an 8Ω dummy load on the output. Radio Shack sells an 8Ω 25W resistor (can't remember stock #) that's good for most short term testing. But unless the potential oscillation is in the audible frequency range, where it can be heard as "singing" in the OT itself, there's no way of knowing anything's awry. Sure, you could monitor the output with a 'scope or AC voltmeter, but how many DIYers have the luxury of a meter (or 'scope) for the output plus another meter to measure B+ or bias or whatever?
The answer is the lowly incandescent light bulb--specifically, a #93 bulb, which can be found at most hardware and auto parts stores. It's the bulb used in lots of older high intensity lamps, rated at 12.8V @1.5A.
Get a couple of #93s and solder leads directly to the bulb, or use a socket (also available at auto parts stores). Terminate the leads with whatever your amp's output connectors will accept, and connect the bulbs to the amp(s).
The bulbs will present a reasonable load to the amp, regardless of what tap on the OT you connect to. More important, the bulb will glow if oscillation of any frequency is present.
If you want to be ultraconservative in this approach, use a #47/1847 bulb as a starter. It's rated at 6~8V @ 0.15A and will let you know RIGHT NOW if something's wrong.
Neat, huh?