11-02-2023, 09:53 PM
It helps if you can characterise the hum, as different kinds of faults produce different kinds of hum.
* Earth loop hums (other than those involving the rectifier and HT secy CT) and those caused by problems with the heater circuit tend to have a fundamental at 50Hz - approxmately the lowest G on a piano - and tend to be 'smooth' sounding with few harmonics.
* HT smoothing issues (with full-wave rectifiers) tend to manifest as predominantly 100Hz because that is the ripple frequency leaving the rectifier. This is one octave higher, so they sound 'high' in pitch relative to classic 'mains hum'.
* Earth loops that are picking up the charging pulses from the rectifier will also be 100Hz and up and might be quite buzzy
* Hum being picked up by a signal pathway is likely to be centred on 50Hz but also include higher harmonics and noise, and sound more 'open' like a guitar amp with its level turned up with nothing being played.
Listen to how it changes as the amp warms up and cools down. Does it fade in first and then out somewhat as the valves warm up and the NFB takes effect? Does it go off suddenly with the mains switch or only fade out as the reservoir runs out of HT?
* Earth loop hums (other than those involving the rectifier and HT secy CT) and those caused by problems with the heater circuit tend to have a fundamental at 50Hz - approxmately the lowest G on a piano - and tend to be 'smooth' sounding with few harmonics.
* HT smoothing issues (with full-wave rectifiers) tend to manifest as predominantly 100Hz because that is the ripple frequency leaving the rectifier. This is one octave higher, so they sound 'high' in pitch relative to classic 'mains hum'.
* Earth loops that are picking up the charging pulses from the rectifier will also be 100Hz and up and might be quite buzzy
* Hum being picked up by a signal pathway is likely to be centred on 50Hz but also include higher harmonics and noise, and sound more 'open' like a guitar amp with its level turned up with nothing being played.
Listen to how it changes as the amp warms up and cools down. Does it fade in first and then out somewhat as the valves warm up and the NFB takes effect? Does it go off suddenly with the mains switch or only fade out as the reservoir runs out of HT?
