SVT annoying buzz

[Seans]

My Blueline SVT has developed an annoying buzz, this happens on certain notes of my bass and more so in channel 2, it sounds like a loose connection or a valve vibrating, so I take the back off to see if it's a loose preamp valve, these all are fine and not crackling in the bases, now the amp is on all through this and I decide to plug back in and try, well it's stopped, after a bit of head scratching, I realize that the fan is disconnected, so back in it goes and so the buzz comes back. I clean the pre connection block and try various positions of the fan wires but no change, so is the fan shot?, it is the original but it's silent and running perfectly?. Any ideas?.

 

[beans-on-toast]

If the buzz is only happening on certain notes, it sounds like you are getting a resonance.

One thing that you can do is to isolate the fan to determine if it is the source of the noise. First try carefully tightening the mounting screws. If that doesn't help, remove the four screws and separate the fan from the metal screen and the fan guard. Then test the fan just by itself. Look for anything that is loose. Also check the fan guard and see if there are any breaks in it that could be vibrating.

If the source is the fan, a cleaning might help. Sometimes those old Rotron fans can get funky and just need to be replaced.

 

[Seans]

Ok, just cleaned and tightened everything, tested again with amp isolated from speaker vibration, it's now worse and I feel the fan is just amplifying the problem as it is now buzzing without the fan, but I hadn't noticed this before now. I get the feeling I have a dry solder joint somewhere, got a gig on Friday too.

 

[Selta]

No offense, but it doesn't sound like you're equipped to check this out. Take it to a tech. A good one should be able to have it back to you by Friday.

 

[beans-on-toast]

These type of noises can be difficult to track down because the noise can be non directional and there are a lot of things to check. You need to methodically break the problem down and look at each thing one at a time. The noise can be mechanical caused by a vibration, or electronic coming out the speaker. You have to determine which this is. Separate the head from the cab and see if you can determine where the sound is coming from, the speaker cab or the head.

While you play and generate the noise, it would help of someone else could carefully poke around and apply pressure on various things to see if they can stop a mechanical noise as well. A tech should be doing this as it is very dangerous to poke around in an SVT head!

If the source of the noise is electronic, it could be many things from a solder joint to a bad component. There could be something loose in one of the tubes that is vibrating. Sometimes the noise is just mechanical, other times it can also be amplified within the tube and turn into electronic noise. Then it would be tech time.

If it is just mechanical and from the SVT head:

- Make sure that there isn't something else in the room near the head that is vibrating.

- Pop off the front grille and the rear screen and rear metal plate on the power amps chassis. These can be the source of a lot of vibrations.

- Rule out the fan which you have done.

- Make sure that the cabinet feet are tight and that the cabinet is not vibrating on a hard surface that it is sitting on. Check the screws on the bottom of the cabinet that hold the power amp. A loose washer can vibrate.

- On each side there are two nuts to check for tightness that attach the handles.

- Check that the metal plates are secure that cover the power amp and pre-amp circuit boards.

- Pressing upward on the pre-amp might allow you to isolate the noise. Remove the pre-amp and check all the screws that hold the metal plates that form the preamp chassis. Check the screws that hold the circuit boards in place. Check that the sliding tube shields on the pre-amp are fully extended and tight. Check all the nuts on the front panel. It could also be that the entire pre-amp assembly is vibrating. It slides in and is held in place with only four screws. Sometimes the pre-amp itself needs to be shimmed to hold it secure.

- Same goes for the power amp. Pull it and inspect that everything is secure. All the jacks, screws that hold the caps down, the circuit board mounts, the nuts on all the transformers.

 

[Seans]

Forgot about a reply on this thread, Ok, had a look round on the resistor front and being apparently unequipped to check this out I found it could have been one of the 10 ohm 5w wirewounds as one was giving spurious readings, also one of the R 39 22ohm 2w was reading 60ohms, so i replaced the lot with some nice Allen Bradleys etc and gigged on the friday, perfect. Cheers, guys.

 

[Seans]

These type of noises can be difficult to track down because the noise can be non directional and there are a lot of things to check. You need to methodically break the problem down and look at each thing one at a time. The noise can be mechanical caused by a vibration, or electronic coming out the speaker. You have to determine which this is. Separate the head from the cab and see if you can determine where the sound is coming from, the speaker cab or the head.

While you play and generate the noise, it would help of someone else could carefully poke around and apply pressure on various things to see if they can stop a mechanical noise as well. A tech should be doing this as it is very dangerous to poke around in an SVT head!

If the source of the noise is electronic, it could be many things from a solder joint to a bad component. There could be something loose in one of the tubes that is vibrating. Sometimes the noise is just mechanical, other times it can also be amplified within the tube and turn into electronic noise. Then it would be tech time.

If it is just mechanical and from the SVT head:

- Make sure that there isn't something else in the room near the head that is vibrating.

- Pop off the front grille and the rear screen and rear metal plate on the power amps chassis. These can be the source of a lot of vibrations.

- Rule out the fan which you have done.

- Make sure that the cabinet feet are tight and that the cabinet is not vibrating on a hard surface that it is sitting on. Check the screws on the bottom of the cabinet that hold the power amp. A loose washer can vibrate.

- On each side there are two nuts to check for tightness that attach the handles.

- Check that the metal plates are secure that cover the power amp and pre-amp circuit boards.

- Pressing upward on the pre-amp might allow you to isolate the noise. Remove the pre-amp and check all the screws that hold the metal plates that form the preamp chassis. Check the screws that hold the circuit boards in place. Check that the sliding tube shields on the pre-amp are fully extended and tight. Check all the nuts on the front panel. It could also be that the entire pre-amp assembly is vibrating. It slides in and is held in place with only four screws. Sometimes the pre-amp itself needs to be shimmed to hold it secure.

- Same goes for the power amp. Pull it and inspect that everything is secure. All the jacks, screws that hold the caps down, the circuit board mounts, the nuts on all the transformers.

I think its was one of the 10 ohm 5w Wirewounds even possibly dancing in its ceramic hold.

 

[beans-on-toast]

Glad that you resolved the problem.

With a bit of maintenance, these old amps can be kept in top form and they sound fantastic.

 

[Seans]

At the gig it just sounded so smooth and together, bit like when you change the oil on your car, it just gives that little hint when running that everything's just fine.