A not-so-dummy load

[erichayes]

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?

[TomMcNally]

I did the same think Derek did - fire up a howling amp on a pair of crummy little Radio Shack Minimus 7 speakers on the test bench. Even though it was only on for seconds, it burned the wire up that hooks to the woofer in one of them. The speaker worked fine after soldering the wire back on to the push on.

Another thought - my big 95 watt per channel (4 EL-34's per side) Velleman kit came with two 8 ohm 5 watt resistors for dummy loads - fine for setting the bias etc. A quick touch with a finger will tell you if there is a problem - as they'll get hot fast. Obviously you need something more substantial to do any serious testing.

When I was a kid playing with CB radios, a # 47 was the dummy load of choice on the RF output. Whistle in the mike and watch it get brighter !

[mesherm]

I have a set of double bananna plugs with 8 ohm 5w resistors in my box for new amp startups. I used to use my multimeter in AC mode but now I have acquired a Velleman HPS 10 hand-held scope which works fine for me and was only $100.

[EWBrown]

Back in my early ham radio days (1980), as a "novice", I was testing out a Yaesu FT-101 with a 100 watt light bulb "dummy load" in a ceramic socket, and a few feet of RG-58U co-ax cable, all laid out on the bench. Managed to make a CW contact another ham in Kansas on 40 meters (7 MHz) with this set-up.

A few years later, a bunch of us "locals" had converted SSB CB sets to operate in the 28 MHz ham band, and we used 50 ohm resistor dummy load "antennas" for strictly local communications, and these would be good for a few miles, when we didn't want to deal with long-distance "skip".

/ed B in NH KB1MZ FN42cs

[erichayes]

Here it is.

[EWBrown]

The #47 light as a CB dummy load reminds me of a funny prank that a ham radio buddy of mine, Steve, pulled on some CB'ers way back when he was a teenager. His CB "buddies" kept telling him to give up on that "kid stuff" of morse code on the Novice bands, and come join the "Big Dawgs" on the "He-man's Band", where the "action" was Yellow_Light_Colorz_PDT_11 .

One of his CB buddies really wanted a "linear" amp to boost his RF power, so our young hamlet built up this very impressive looking chassis, which had eight 1625s (similar to 807s) wired up the plate caps, made a couple of big impressive copper tubing coils, and all that powered this thing was a hefty 12 volt trannie to light up the filaments. No high B+ voltage, at all.

The CB set connected to it's "input" which lit up the #47 bulb, which in turn shined (and flickered) onto a cadmuim sulphide photocell, which, in turn, with a few simple components, controlled a big whopping SCR (silicon controlled rectifier). This, in turn controlled a "dummy load" bank of ten 100 watt lightbulbs (ergo, one kilowatt).

He invited his CB buddies over, told them to bring their favorite five watt rig, and he would demo his "linear" amplifier to them. They fell for it, hook, line and sinker...

They all immediately wanted to buy one, but he always had some creative excuse why he couldn't get the right parts to build them, and strung them along for a good long time before the "big dawg" CBers finally realized that they had been "had" but good..

So, I guess that one was a big dummy load for big dummies Yellow_Light_Colorz_PDT_06

/ed B in NH