Cheapest "listening amp" (signal tracer) imaginable....

[eleventeen]

Garage sale/thrift store > used set of powered computer speakers --- usually have headphone jack on them, so that if you don't get the fidelity you wish from the small speaker, you can signal trace in silence, thru 'phones-- all plastic enclosed so they're insulated ---with power supply and pilot light and on/off switch, all done.

Or....probably trivial to open up the case & run the speaker leads out to a bigger speaker if that's your preference. <<Wait, no need, just plug your bigger spkr into the headphone jack.

$1, maybe $2.

Make sure you perform the isolating of the low-voltage world of transistors from the HV world of tube amps, typically by inserting a series capacitor (of almost any size but at least 600 volt rating) before the input to the speaker(s)

The only work is deciding exactly how you are going to change the stereo 1/8" male plug on the speaker input to something useful, ideally, resembling a probe. You could chop off the plug, after all, the thing cost you $2, tops...and fabricate something inside a clear plastic tube....or, leave the plug on, get a female-to-female 1/8" adapter, plus a male-to-male cable into the other jack---and chop up the male end of the aux cable. Spiritually, I like that a little better because that leaves you with the computer speakers perfectly intact, should you need them. Otherwise, for $1-$2 total you can go to town, hack away, and not look back. I have even gotten these for free from recycler dudes because they contain essentially nothing of any scrap value.

And guess what, if you blow it up? Big deal. Next garage sale.

The idea that this is an amp, with power supply, with pilot lamp, with on/off switch, with volume control, all done, all insulated...is a crazy bargain.

[shortfuse]

Just what I think I need for tonight's project.  I have several sets of old computers speakers setting in the garage doing nothing.  You said cap value does not matter just 600V rated, have that also.  I also have extra probes with a female banana adapter in and a male banana to connect the probe from the cap.
My question is would the tip of the 1/8 jack go to the probe tip and the sleeve go to an alligator clip attached to the input jack sleeve?

[eleventeen]

If I'm understanding you right....

Tip (or...could be ring) of 1/8" leading to the spkr is your "hot" which you protect with a series-connected 600 v cap of pretty much any size (.01 would be just fine) and the sleeve/shield is your ground which gets alligator-clipped to the chassis (or signal-ground) of whatever you're fooling with. So yes, sleeve-to-sleeve is (external world) ground to (inside the speaker) ground. With the speakers powered up, you should be able to get hum when you touch the tip of the 1/8" plug.

You are going to abandon the "ring" connection and use only one speaker....but you do not know (and thus you should test) if the ring drives  the spkr with the amp in it or to the slave speaker. I am assuming you really only want one speaker on your bench, eg; you're going to delete the slave speaker entirely. So you power up the (powered) speaker and leave the slave spkr off. When you touch the 1/8" input plug, which gives you the hum? The tip or the ring? You prolly need a pin or a small piece of wire to make sure you're touching only the ring if you have human-sized fingers.

The issue is whether you want to preserve the 1/8" stereo plug at the end of the cable leading to the spkrs. Hmmm. I say, chop it. If you do that, you can just connect your probe-with-series-cap to BOTH on the internal wires inside that teensy coax and not care about tip or ring.

[jim]

Great idea!! Love it.  Goes with cheapest signal generator (a finger)! Jim

[sluckey]

Another cheap option is to use another amp.

[jjasilli]

There was another recent thread on this. 

sluckey:  Yes, but a computer speaker or 2 can be left hanging on the wall; always handy, takes-up no space.  I'm thinking: run the speaker plug into a box with a matching jack.  The box will contain the protective cap.  On the other end of the box: a ground wire with a clip to attach to the amp chassis; and a hot signal lead with a probe on the end.  The probe could optionally be plugged into an alligator clip.

[BrianS]

Just wanted to say that I "built" one of these a couple weeks ago and used it to diagnose two solid state amps and it worked like a charm.

Thanks!!

[eleventeen]

Very happy you got some use out of the idea! I wish I was better at troubleshooting SS amps. I have a $25 Peavey Bandit that I would love to fix but I am getting the feeling that I will destroy it before fixing it. It actually "works" but when viewed on a scope, there's obviously some very weird hum getting into half the signal. Indeed, half the signal as viewed on a scope has a giant sawtooth imposed on it. Big enough to overhear the output transistors but fortunately not blow them up outright.