Safety: amplifiers
PedalScope currently measures pedals only. Amp measurement is a planned mode with its own guided setup and software interlocks. This page exists now because the most dangerous mistakes are the ones made early, with pedal-mode habits and an amp within reach.
If you take nothing else from this page, take the two rules every amp measurement will require you to acknowledge:
Never run a tube amp without a load connected. An unloaded tube amp under signal can arc across and destroy its own output transformer — often silently, sometimes not immediately, always expensively.
Never connect a speaker output directly to your interface. Speaker outputs deliver tens of volts into inputs built for about four. One patch cable from speaker-out to line-in can end an interface.
Why speaker outputs are different
Everything in the pedal loop lives around a volt. An amplifier’s speaker output is a power circuit:
A 15 W amp pushed into overdrive puts roughly 11 V RMS across an 8 Ω speaker; a 50 W amp roughly 20 V RMS, with peaks higher still. Line inputs are typically built for 2–4 V.
The danger runs both directions: the interface’s line input is destroyed by the amp’s voltage, and a tube amp that loses its load (a speaker cable used as the “measurement” path, a dummy jack, a broken cable) can destroy itself.
There is no “just for a second, at low volume.” Master volumes drift, channel switches boost, and a sweep is by design a signal that explores the amp’s range.
What a proper amp rig looks like
When amp mode ships, its wizard will require a declared load device between the amp and the interface — one of:
Reactive load box with a line out (best: presents a speaker-like impedance, so the amp behaves like itself).
Resistive dummy load with a line out (safe, though the amp’s feel and measured behavior shift slightly — a resistive-load fingerprint is a fingerprint of a slightly different amp).
Attenuator or load box in front of a real speaker, using its line/DI tap.
The load must match the amp’s impedance tap and carry its full rated power. The line-out pad value matters too — amp mode will verify your declared attenuation at low power before allowing any loud measurement, and PedalScope will never emit into an amp path without a declared load.
The gentler hazards
Not destructive, but worth knowing before amp mode arrives:
Effects-loop sends (“preamp only” measurements) can run several volts hotter than line level on some amps. Start input gains low.
Ground loops: an amp and an interface on different outlets invite hum into every measurement. Same outlet or power strip; use a transformer- isolated DI on the line path if hum persists; never lift the amp’s safety ground.
Heat and duty cycle: high-drive measurements dissipate the amp’s full power into the load as heat. Load boxes and output tubes both appreciate breaks between long runs.
Until amp mode ships
Measure pedals and other line-friendly boxes (preamp pedals, DI-out devices at line level). If a device’s output is a speaker jack, it waits for amp mode — there is no safe improvisation around that rule.