low frequency hum solved

[Colas LeGrippa]

Holas amigos !


Just a few words to share this with you. I was very surprised to hear a low frequency noise as turning on one of my builds last week. I do pay much attention when I build an amp to keep any kind of ( strange )noise away. Lead dress, elevated heaters, good grounding and so on. When I play my guitar through one of my builds, I don't wanna hear a mosquito singing on my music. My amps are dead quiet when turned off and dead quiet when turned on ( before I hit a string....).  This noise was caused by a defective 12ax7. I know by experience that tubes are the first cause of problems in a tube amp and right away I started to tap on the preamp tubes until the faulty tube said ''hello, it's me ''. Big low hum caused by a def. pre. tube. Heaters of that tube gone ?


Colas

[HotBluePlates]

... I was very surprised to hear a low frequency noise as turning on one of my builds last week. ...  This noise was caused by a defective 12ax7. ... Big low hum caused by a def. pre. tube. Heaters of that tube gone ? ...

Probably heater-to-cathode leakage.

In a leaky tube, if you don't have a very large cathode bypass cap (couple-hundred µF), the heater hum will mix with the audio. So if you used a smaller value to shape bass (or maybe even a 25µF), you'll have hum. But if you have a 250-330µF bypass cap handy, try tacking it in when your leaky tube is in the socket. The hum will probably go away.