ST120 PROBLEM

My ST120 is making a snapping - arcing like noise sometimes when I power it up . This
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 .

your amp can/will hum if the inputs are left floating/open, so yes, as you've said, you either need to short the inputs, say if you're testing/checking it or make sure your input cables are plugged in.
Is your ST-120 fitted with the start up time delay pcb?
Are you powering the amp up first followed by the preamp?, if you do that, the amp tends to make cracking noises when the preamp kicks in, so it is best to start the preamp first followed by your power amp.
Another possibility is that your rectifier tube could be on its way out, which model are you using? I had a troublesome rectifier tube, flashing, cracking sound, since then, I have switched over to the Weber Copper top SS rectifiers.

The not plugging the cables in was just a stupid mistake .

I've been running a Mullard GZ33 rectifier for a while , It might be my imagination but I think it sounds smoother than the Weber .
I ordered a new set of KT120 tubes , I was tempted to buy the KT150's to see how they sounded , but they are still too much money right now !
The tubes did not have a real lot of hours on them since the amp was not in my main system .This weekend I moved it back down to bi-amp with my newer Infinity's so it will get a lot more use there now .

May be I'll try the Weber again when I get the new tubes , but I have another NOS Mullard just in case . I've heard tubes crackle before this noise is different .
It did not make any noises tonight ! It could go a couple weeks before making it again .

MontanaWay wrote:your amp can/will hum if the inputs are left floating/open, so yes, as you've said, you either need to short the inputs, say if you're testing/checking it or make sure your input cables are plugged in.
Is your ST-120 fitted with the start up time delay pcb?
Are you powering the amp up first followed by the preamp?, if you do that, the amp tends to make cracking noises when the preamp kicks in, so it is best to start the preamp first followed by your power amp.
Another possibility is that your rectifier tube could be on its way out, which model are you using? I had a troublesome rectifier tube, flashing, cracking sound, since then, I have switched over to the Weber Copper top SS rectifiers.

What MontanWay said, if it's helpful. My preamp makes some terrible racket on warming up, the noise peaking at about 8-12 seconds, so I just let it burn for a minute before lighting the mains. Did you get the snapping (not meaning the hum) when the pre-amp was unplugged?
I have red-plated output tubes because of dirty sockets but never got snapping, just a bunch of fuzz and a lot of fireworks.
In the slight possibility it's a pre-amp issue, you might check the connections on that little disc capacitor that bridges the contacts on your pre-amp's power switch. It's there for a reason.
Just a layman's thoughts.

Its not a preamp problem The preamp its connected to doesn't make a difference . I've had it happen when it was connected to either of the SP14's or the S.S. ML preamp .

My first Sp14 build when first built would make a heck of a racket when it was powered up or down till it "warmed a few seconds then go quiet until I changed out the tubes. The new SP14 makes a little start up tube noise too , but not enough to annoy. This is a totally different sound and definitely in the amp . It doesn't make a difference , It will make the noise if I power the amp up first or the preamp first .

When I powered it up with the cables accidentally unhooked it made the snapping noise then a couple seconds later It made a MUCH louder snap and a light flash , it never flashed ever before . That is what made me concerned enough to ask about the problem . I did not see any red plate at the time . One of the tubes must have flashed from the disconnected inputs . The hum was because of the not connected cables I'm sure .
I just want to catch any problem now before it becomes a bigger problem .

I've thought that the sound comes from the left side of the amp ?
Again It has not made the sound in the last couple days . Go figure .

it could be ANYTHING, but my guess is a rectifier tube (if you have one) or a capacitor somewhere, although you wouldn't see a flash with a capacitor . . .
If you are SEEING something, it's probably a tube.

The beauty of intermittent problems is either they get worse, in which case you can identify them, or they go away. Can't lose either way. Just keep a fire extinguisher handy. if it's the latter.
GZ-33s are pretty tough, but I managed to nuke one with a bad 5-pin on one of my output tubes. (Bob L. helped me work this one out.) Nothing intermittent about it, though, just one big flash and the smell of tubular death. Would have enjoyed the light show if it had been somebody else's system.

Well I received my new tubes . And after installing them and a new GZ33 (I tried it with a Weber also ) I noticed that only V2 would not hold a bias after it warmed up . The other three were good . So I pulled the cover to measure the bias resistors ,and they all measured good . 9.9-10.1 ohms ? I had ordered four bias resistors when I built my SP14 recently so I figured I may as well install them while it was apart . Well after installing them V2 bias is steady as a rock . I guess the resistor was braking down under load . But Then I notice V7 bias starting to creep up slowly as it warmed up , so I re-checked inside nothing looked amiss , I re-flowed the solder joints on that socket and the bias then held steady but that bias pot had to be 1/4 turn more counterclockwise than the other pots . After a bit more testing and moving tubes around it looks like I might have a bad or weaker tube . Hopefully he will do something about it . I haven heard back yet .

Its weird , I never had a bias problem with this amp before , even with the old worn tubes and snapping sound the bias held steady . But I did notice the new tubes biased up at 1/4 turn where the original tubes biased at 1/2 turn . I guess the difference in the new tubes showed up the weak resistor that was probably damaged by the bad tube that I guess was in V2 spot .
Would it do any damage if the amp was run like this with one pot turned more than the others till I find out whether he will exchange the tube ?

And I know all about intermittent problems they are a PITA !

Where the bias controls are in relationship to each other really means nada as long as you can get all four into the correct range and they're steady. The differences you're seeing is more than likely just that the individual components that drive each tube add up to slightly different overall resistance. That after all is why you put a VR in a circuit in the first place, allowing you to fine tune it.

Could be a matching problem on the tube itself - did you replace the entire quad? Pony up for whatever passes for matching from your supplier?

I did have a VR here go bad, and that resulted in a red plate on the associated tube. Went ahead and replace all the pots AND the bias resistors while I was in there.

Interesting experiment once things settle down would be to swap tubes and see if the sockets still bias up with the same "discrepancy" ... I'd still just let it alone for now until you're sure the amp's performing well and consistently.

They were "supposed" to be a matched quad .
And I did move the tubes around to different sockets and that is what makes me think its a bad tube as the bias issue moved with the tube .
I figured if they all biased correctly it shouldn't matter if the pots are not in the same position but I just wanted to be sure .

I received a new tube for the bad one no questions asked . The fellow was real nice ! Took care of it right away .

It seems also that one of the new bias resistors I just installed was bad also bad causing the bias on V7 to wander up , luckily I found that out before getting the new tube by using it with one of the old tubes and keeping an eye on the bias. Now with all new resistors and all four new good tubes the bias stays steady and all four bias pots are in closely the same position .

I noticed right after the tube blew originally that the amp now had a slight hum from both channels that had never been there before. You can only hear it within a foot of the speakers , so its not Real bad . I was hoping that it was just the tubes or bad bias resistor and after replacing them the hum would go away . It didn't . I bought new driver tubes thinking one could have been damaged and put them in hoping that would be the problem but that wasn't it either . So I guess I will have to take a close look at the board components when I get a chance .

At least except for the light hum it is back to sounding good no more snapping noise so far. It was a little "flatish" sounding that is opening up nicely as the tubes start to break in .

I bought two Amperex 7062's and put one in the center spot and two Siemens ECC801s in the outer spots .
I replaced the high mid range drivers in the speakers with newer used drivers about a month ago and they sounded a little harsh-bright sounding afterwords . I was almost thinking of putting the original ones back in , but this tube combo so far toned that problem down and it sounds better than the Mullard 4024's I had in it . Hopefully it does not change much when the tubes fully break in .