"N_Cook" wrote in :

What is the likely cause of this problem.
Just as well I keep repair records as I met this amp 5 years ago with
the same untraceable fault. Owner and me had forgotten it re-appeared
when he used the bass with a different bass amp.
Previously reported as a fuzzy sound when he used the bass E string.
This time just playing the F on the bass E string , a rattly buzz,
like a spiral wound guitar string vibrating against a fret and loss of
the actual F note. I've not seen the bass or transducer , assuming its
a piezo element - make Ashworth transducers, what could cause this
symptom, disappearing for 4 years? Both bass and amp have been around
the world a few times on cruise ships so perhaps excessive heat or
humidity



Input dependend oscillations?
for retain freguencies and /or source impedances oscillations at certain
parts of the waveform.
Sounds just like you described.
And a change of source will make it appear/disappear.
Remedy: carefully changing the HF bandwidth a bit, and improve
dc power decoupling.
A small cap across inputs also is worth trying.