SVT CL issue

[Jb4]

I've been lurking in this forum for I don't know how long, but need help with my head and would like some advice before I take it to a tech as the closest one isn't exactly close.

It's an Ampeg SVT CL, I believe the final year they were made is USA by SLM(if not, pretty close to the end). I purchased used and it made an overly distorted, staticky sound as soon as I plugged in to play. I changed all the tubes opting it would go away, and when playing with gain and volume both low, it's not bad; still there at times, but I was able to deal. However anytime take the gain and volume up, it starts to crackle and distort again. I play through a 410 hlf and I've tested the cones with 9v and see no issues. My bias is all green and I usually let the amp warm up before I play.

I keep all my mid and treble at midpoint for the most part, with the bass level pretty much around zero. I play passive Jazz bass and I use a practice Ampeg ba110 along with an Ashdown head and cab at rehearsal studio with no issues. This happens whether I run through a loaded pedal board or direct into the head.

I have a video of the head making the sound but not sure how to load it in the thread if possible at all. I can't think of what else to add so ask away and I'll do my best to answer. Thanks in advance.

 

[Jb4]

I figured easiest way to do video was YouTube upload. Give a listen and let me know what you think. Thanks.

 

[Jb4]

I'd like to add, I didn't pick any differently in that video to make the distorted sound. It just intermittently happens. Thanks.

 

[seamonkey]

Good on you, posting the problem and a video. Hope some experts jump in to help.

To me, It sounds like a mechanical rattle, like something hitting a cone, like a loose wire.
Open the cab and look.

To check if it's a microphonic tube take the head off the cabinet and move it away from the speaker.
Try sending the effects send to another power amp (effects return) to see if it's just in the pre-amp

 

[Wasnex]

I'd like to add, I didn't pick any differently in that video to make the distorted sound. It just intermittently happens. Thanks.

If you changed out the tubes and the amp has the same problem, about the only other things you can do is fiddle with the controls and install a short 1/4" cable between the Preamp out and Power Amp in.

Sometimes controls have a bad spot that will produces this sort of sound. So if you adjust the control away from the bad spot the sound will clear up. The Power Amp In jack has a switching contact that breaks the connection with the preamp. If this connection is corroded or dirty it can cause this sort of distortion. Plugging in a 1/4" cable between the Preamp Out and Power Amp In bypasses the switching contact.

The other possibility that comes to mind is the amp has a broken solder connection. A tech will need to open up the amp and identify where the problem is, and then repair it.

There are other potential causes for the problem as well.

You can plug a different preamp into the Power Amp In. You can also take the Preamp Out and run it into another amp. However, this Preamp Out is a line level signal, so you need to plug it in to a location that expect line level rather than instrument level. This will allow you to determine if the problem is in the preamp or power amp. Of course, it will still be necessary for a technician to open the amp, track down the problem, and perform the repair.


Make sure you cables are all good and if you bass is active, make sure you have a fresh battery in it or try another bass.

 

[808State]

If something crakles or distorts and is intermittent

It is usually either a pre tube with a cathode to heater short that only appears after warmup/ cool down.
Cathode to heater shorts will form from vibration or moving the amp.
It is not a full constant short but temperature change from the heater warming up will make it more
obvious and intermittent. Sometimes it warms up and wont be there, sometimes it warm up and will be there.

Otherwise it could be tube sockets need solder touchup. Same thing hot/ cold cycles make a bad connection obvious or not obvious and will be random.

In the video it sounds like plate rattle or possibly bad solder on a tube socket.

When you say you replaced all the tubes was that just the preamp tubes?
There is 2 small signal tubes on the power amp board. One 12ax7 and One 12au7
One could be microphonic, or has a random cathode heater short.

Basically over time the plates rattle loose the mica plates dont hold them well.
So certain notes can make all kinds of weird noises.
Otherwise second guess the new tubes in the pre. Usually ok out of the box.
But the can get rattled to death in shipping.

 

[Brich]

I've been experiencing the same issue with a svt 3 pro and using a 410hlf

 

[808State]

I've been experiencing the same issue with a svt 3 pro and using a 410hlf

If it is a very strange metal ping or crackle with certain notes. It is likely a Pre tube gone bad.
Tube pre only. 4x 12ax7 and 1x 12au7
You can tap on the tubes and if you get a metal ping or other random noises, that is the one.
That metalic crunchy sound is the tube getting vibrated by certain notes and the tube has gone microphonic. The mica spacers arent holding the plates well anymore. If it is random after hot / cold cycles the heaters will randomly smack against the cathode. Same thing vibration/ moving the amp eventually rattles things loose in the tube.

Millions of tube recommendations, But I have had good luck with the more expensive Sovtec 12ax7LPS

 

[Wasnex]

There is 2 small signal tubes on the power amp board. One 12ax7 and One 12au7

Three small tubes, one 12AX7, and two 12AU7s. The 12AU7s are drivers.

No worries swapping out the 12AX7.


The second triode of the 12AU7s control the bias of the 6550s, so IMHO it's a good idea to bias the amp for minimum current before you roll a fresh 12AU7 into the amp. Then when you turn the amp back on, follow the bias procedure in the manual.

Why? A fresh 12AU7 may pull significantly more current than a worn old tube. Higher current in the drivers, means lower bias and higher current in the output tubes.

IMHO, Ideally you would swamp both 12AU7s as each driver tube biases three of the 6550s. But for troubleshooting it's okay to replace one.

 

[Jb4]

I appreciate everyone's responses. Definitely a nice amount of info to digest and look into. I did change all pre and power amp tubes so no old tubes after I purchased. I usually let the tubes warm up for like 20 minutes before I play. The sound distortion/clipping comes and goes regardless of level(I had it super low practicing today will finger picking very lightly and still heard it). I really don't think it's speaker cab as I plugged in a GK amp I had lying around and it sounded pretty clear. Although I have been looki for a reason to get 810 Heritage cab....

 

[beans-on-toast]

I appreciate everyone's responses. Definitely a nice amount of info to digest and look into. I did change all pre and power amp tubes so no old tubes after I purchased. I usually let the tubes warm up for like 20 minutes before I play. The sound distortion/clipping comes and goes regardless of level(I had it super low practicing today will finger picking very lightly and still heard it). I really don't think it's speaker cab as I plugged in a GK amp I had lying around and it sounded pretty clear. Although I have been looki for a reason to get 810 Heritage cab....

Using an instrument cable, patch the preamp out into the power amp in of another amp. If you can't do that, plug into the input of another amp, just watch the SVT's level. If it works fine, the preamp isn't the problem. You will have to have the SVT's speaker connected during this test. So both amps will be putting sound out.

You mentioned changing tubes. See the attached CSB bullitens. The power tube amp board should be inspected.