Tube Amp Archive

Working on a Traynor K4 keyboard amp with a crackling 12" subwoofer. It will crackle every few seconds when the volume is 50% or better. I measured the ohms on this speaker which is at 2.4 ohms and should be four. Is this too low?

2.4 ohms DC is all inline with 4 ohms impedance, which BTW is mostly inductive....

Agree and add: being a subwoofer meaning lots of movement you might have some bad contact such as cracked/bad soldering at terminals, twisted but not soldered cable connections getting "rusty"(whatever copper becomes) , loose crimped connectors, or if it's a combo cabinet, remember that subwoofer shakes everything.
A tried and true troubleshooting tip is hitting cabinet at different points with a rubber mallet or at least a closed fist, and listen for "cracks" or "pops".
Also try the speaker alone (disconnect it from anything else) driven by another amp (a Bass head would be fine) and receiving some Bass torture, such as some Synth lines , one of those Car test CDs or some bass heavy music.
Sometimes it's just some loose speaker mounting nuts driving you nuts .

Check it for voice coil rub.

Best if the speaker is removed but the idea is to push the cone evenly around the voice coil, push the speaker cone gently and see if you feel or hear the coil rubbing against the magnet.

Speaker was bad. No coil rub. A 200 watt amp in the Traynor K-4 was driving this speaker. It would crackle every few seconds and only for a split second. Otherwise it sounded fine. Removed the speaker and hooked it up to my 20 watt Marantz stereo and it was farting like crazy. New speaker ordered!