has been going on for about six months now , but not every time I power it up and never
when I’m actually trying to figure where the sound is coming from . And I never see any
light flash (other than the normal mullard flash) even with the room lights off .
About once a month when I power it up after about 8-10 seconds it makes a kind of
loud snap-arcing noise . Only on a cold initial start up , after its warm if you shut it off
and power it up again after a few minutes it dosen’t make the sound . And not every time
its turned on either . I’ve taken the bottom off and visibly looked for loose connections
and burnt looking components , nothing odd looking in there , and I also re-tensioned and
cleaned the tube socket terminals .
But it still sounded like it should and no bias problems !
So this weekend I was moving components around and when I put the ST120 back in
place I forgot to plug the input rca's into the back of the preamp (oops , too many wires back there!)
and when I turned it on , after 8-10 seconds I got the snap sound again but about 2 seconds later I got
a real scary LOUD snap-arcing sound followed by a bright flash show , Of course I wasn’t
looking directly at the amp so I only caught the flash from the corner of my eye so I don’t
know what exactly flashed , Power tube , rectifier or what ??? It also hummed loudly
until I plugged the cables back in , then the humming went away , most likely from the
inputs not being plugged in .
But now again it still sounds perfect , I listened to it all day yesterday , still quiet , no hum noise ,
nothing to indicate a problem other than the intermittent snap noise on start up .
The bias is still good , three tubes were at .550v the other was at .560v , not enough variation to suspect a
problem . There is obviously a problem somewhere . Any suggestions where I should start
looking ? A bad resistor , Capacitor , tube , solder joint ? This is the first problem I’ve had with
this amp in over three years . Would a voltage check find a intermittent problem like this
It still sounds good and the snapping noise only happens infrequently .