Two complementary tools, both dead simple. A signal injector feeds a test tone into the amp at various points, starting near the speaker and working back toward the input — you listen at the speaker, and when the tone disappears, the dead stage is between the last good point and the current one. A signal tracer does the opposite: feed a guitar or any signal into the amp's input as normal, then listen with the tracer probe at each stage, working front-to-back, until the signal disappears or dies. Either one alone will find 90% of dead-stage faults, and building both costs about the price of a coffee if you're scavenging parts.
Parts List — Things You Likely Already Own
| Qty | Designation | Item | Where to find it | Notes |
|---|---|---|---|---|
| 1 | — | Small powered computer speaker or old phone/aux amp | Junk drawer | Needs its own amplifier — don't use passive speakers |
| 1 | — | Smartphone or tablet | Your pocket | Tone generator app, or just a "1kHz sine tone" video |
| 1 | — | 3.5mm aux cable (cut one end off) | Old headphone/aux cable | One for each probe you build |
| 1 | C1 | Film capacitor, 0.1μF, rated 400V+ | Old electronics, or a spare from your parts bin | The critical safety part — do not substitute a cheap low-voltage ceramic if you're probing plates |
| 1 | R1 | Resistor, 100k–1MΩ, 1/4W | Any junk board | Limits loading/current, protects both the amp stage and your speaker input |
| 2–3 | — | Alligator clip leads | Everyone has some of these | For probe tip and ground |
| 1 | — | Small project box or electrical tape | Optional | Keeps it tidy, not required |
That's it — no 555 timer, no signal generator IC, no scope. If you'd rather build the more permanent IC-based version instead, see the electronics-parts edition.
Building the Tracer Probe
- Cut the male end off an aux cable, strip the tip (signal) and sleeve (ground) conductors.
- Solder the 0.1μF cap in series with the tip lead, then the 100kΩ resistor in series after it.
- Attach an alligator clip to the free end of the resistor — this is your probe tip.
- Attach a second alligator clip to the ground/sleeve lead — this clips to the amp chassis.
- Plug the other end of the aux cable into your powered speaker's aux/line input.
Use: with the amp on and a guitar (or a tone from your phone) feeding the input jack, clip the ground lead to chassis, then touch the probe tip along the signal path from the input jack toward the speaker. You should hear the signal at every stage, getting progressively louder — each gain stage amplifies it. Where it disappears or drops dramatically, that stage is the suspect.
Building the Injector Probe
Identical build, just swap what's on each end:
- Same cap + resistor in series on the tip lead, alligator clip on the end.
- Ground clip on the sleeve lead, goes to chassis.
- Plug the cable into your phone's headphone jack (or a USB-C/Lightning adapter dongle if no jack).
- Play a steady tone — a 1kHz test tone video/app works fine, or even music with the volume moderate.
Use: with the amp on and volume up, clip ground to chassis, then touch the probe tip starting at the power tube grid and work backward toward the input, listening at the speaker each time. You should hear the injected tone, amplified more the further back/upstream you inject. Where the tone stops coming through, the stage between that point and the last working point is dead.
Typical Test Points — Standard 2–3 Gain-Stage Amp, Front to Back
| Order | Test point | What you should hear |
|---|---|---|
| 1 | Input jack / grid of V1 | Raw, quiet signal |
| 2 | Plate of V1 (first gain stage) | Amplified, inverted signal |
| 3 | Grid / plate of V2 (second gain/tone stage) | Louder still, tone-shaped |
| 4 | Phase inverter output (both plates, if long-tail pair) | Two signals, roughly equal level, opposite phase |
| 5 | Power tube grids | Strong signal, near full amplitude |
| 6 | Output transformer primary (plates) | Loud, some HF rolloff |
| 7 | Speaker jack / OT secondary | Full signal at speaker level |
Reading the Results
- Signal present and growing at each stage → that stage is working, move on.
- Signal suddenly vanishes → the stage right before that point is the fault (bad tube, open coupling cap, dead resistor, cold solder joint).
- Signal present but distorted/weak at a specific point → bias or cathode bypass cap issue at that stage, not necessarily total failure.
- No signal anywhere, even right at the input → check power supply / B+ first before chasing signal stages.
Notes & Tips
- Cap voltage rating matters. Use 400V+ film caps for probing plate circuits. A cheap 50V ceramic can arc over and pass DC through — right into your speaker or phone.
- If your test speaker seems too quiet or too loud at different points, that's expected — each gain stage multiplies the signal, so trace loudness changes are normal, not necessarily faults, unless the signal disappears entirely or turns to noise/distortion.
- Keep the ground clip solidly attached to chassis at all times while probing — a floating ground gives false readings and hum.
- This build works for finding dead or noisy stages. It won't measure exact voltages or waveform shape — for that you still want a multimeter and, ideally, a scope. But for "why is my amp dead/silent," this gets you to the faulty stage in a few minutes.
Once you've localized the fault to a stage, the DIY Bias Probes and Ohm’s Law / Power Calculator help confirm what's actually wrong with it.