hum overtone on certain bass notes

[ulysses]

hey guys.

i'm jamming on an old jazz bass into my home made SB12 bass amp clone.

i'm getting a weird overtone hum on certain notes. ie, if i hold play the low g note it has a weird overtone -- its almost like a hum version of the overtones of the note -- it vibrates at the same frequency as the note. the interesting thing is that if i continue to hold the note on the bass the note eventually rings out, but the weird overtone hum keeps going until i lift my finger, then it stops immediately. if i hold the note then drop the tuning on the string (while still holding the note) then the frequency of the overtone hum changes too, speeds up or slows down depending on whether the string it tightened or loosened.

i tried it with my strat, and it does a similar thing, but much softer, it think its having less of an effect due to the fact that the guitar is an octave higher.

thinking something has going wrong with either the power tubes or the filter caps in the amp. anyone experienced a similar thing?

cheers

edit: this guy has a similar problem https://www.thegearpage.net/board/index.php?threads/sub-harmonic-distorted-note-blown-speaker-or-tube.860811/

[digi2t]

I've heard that happening on guitar amps with power filtering problems. You get ghost notes, but usually more around certain notes/frequencies. Sometimes it's the output transformer, but that's generally more rare. Try a different speaker too, just to be sure it's not cone cry you're hearing.

[samhay]

Low G on the bass is 49Hz.
Do you live in a part of the world where the mains/utility frequency = 50Hz?

[ulysses]

thx for the replies guys

yeah i live in aus, 230v 50hz.

the strange thing is that it does it from low C down, but it loudest at G.

so i suspect it is power filtering. does anyone know of a quick hack to confirm if it is power filtering?

the amp has 6 el36 power tubes so i pulled four, leaving two at a time to check if it's the tubes. it's not, unless all the power tubes are nackered.

cheers