Svt-CL making an ugly distorted buzzing noise at lower frequencies

[Mrtoodamnsabre]

So I’ve been playing this thing the same as I always have an it appears I’ve woken up to a situation where the e string distorts in an ugly way. Played multiple basses through it and same thing with all of them. Since it’s a cl I run the master at full blast the gain at 8:00 and the volume on the bass is usually very low, I try to shoot for equal to the gain usually, so if the gain is at 8 the bass is only 1/3 or 1/4 of the way turned up.

not sure how this would cause the sound to get wacky like this? Could it be something simple like biasing?


It’s in an 810 cabinet if that helps any

haven’t serviced it ever, and only had it for a year

 

[basscooker]

Isn't the svt300 a solid state?

 

[Mrtoodamnsabre]

It’s a tube it’s the svt cl, the little red bias light is on all the time. And it seems like the distortion gets worse the longer it runs? I don’t really push it far enough to blow speakers I think, there’s been like a few times where a ton of noise went through when I’ve accidentally unplugged a instrument cable it before the standby kicks in or thought I could plug it into a new bass and not get a huge thundering noise, but I’m usually pretty gentle with it in general, mostly play very low volumes. A blown speaker would be symptomatic on the a string as well I’d imagine? I’m using a four string active Sabre bass, I’ll admit the bass has a deeper range than normal but it wasn’t doing this a week ago

 

[seamonkey]

Page 9
https://ampeg.com/pdf/SVTCL_OM.pdf

 

[basscooker]

Yeah my bad, did you change the thread title? Lol.

 

[Mrtoodamnsabre]

Yeah my bad, did you change the thread title? Lol.

yes! Lol

 

[Mrtoodamnsabre]

Page 9
https://ampeg.com/pdf/SVTCL_OM.pdf

it’s saying that an over biased tube can cause distortion? Everything else is very clean sounding, and the sound is very big and Loud and stuff! This is all low end distortion that just appeared the today out of nowhere weirdly enough

 

[rickdog]

If "the little red bias light is on all the time", the amp is not biased correctly, and won't operate correctly. Yes, this can cause distortion that may not always be evident.

Follow the bias procedure given in the Owner's Manual (linked above) exactly. If the amp still won't bias correctly, the manual says that "a number of tube problems can be diagnosed by the user." Use the information in the table to determine what is wrong and what action to take.

My guess, based on your description of the problem, is that a tube failed and is no longer operating. Some tube failures can damage other components in the power amp. In this case, replacing the failed tube won't fix the problem. I experienced this in my SVT-CL: a tube that arced internally caused a screen resistor to fail (blew it like a fuse). Unless you have appropriate tech skills (I'm fortunate that I do), that would be time to take it to a good tech.

 

[Mrtoodamnsabre]

user." My guess, based on your description I experienced this in my SVT-CL: a tube that arced internally caused a screen resistor to fail (blew it like a fuse). Unless you have appropriate tech skills (I'm fortunate that I do), that would be time to take it to a good tech.

probably gonna take it to a tech soon as I think biasing stuff and electricity and stuff is spooky to me and my room is too small to throw me around in safely!

One question though and it might be wishful thinking cause I don’t wanna retube so soon, (I suspected I might have to though when I bought it so I saw it coming eventually) but I was wondering if Amps “fall out” of bias, as is over time they have to be rebiased in order to prevent these kinda sounds etc.

 

[rickdog]

I was wondering if Amps “fall out” of bias, as is over time they have to be rebiased in order to prevent these kinda sounds etc.

Yes, as the tubes age and wear their characteristics change, and the bias may need to be adjusted.

But a problem that "just appeared ... out of nowhere" seems to me more likely to be due to a tube failure, not just bias drift.

 

[Mrtoodamnsabre]

Yes, as the tubes age and wear their characteristics change, and the bias may need to be adjusted.

But a problem that "just appeared ... out of nowhere" seems to me more likely to be due to a tube failure, not just bias drift.

Ah that could be it, it’s around 350 to retube I’d imagine right? I mean it’s a very slight distortion, once it hits a certain level it sorta clips and the sound distorts in this weird way. The reason I suspect trouble is because it didn’t used to do this at all, and it’s doing this at fairly low volumes, the amp is always loud enough so I can hear the fullness of the sound but low enough to where the neighbors aren’t gonna be mad at me.

it’s like the sound pancakes, gets squished or something, it sounds like a digital clipping almost. Like a whole frequency range below A is missing and just registering as a kinda grimy noise with no definition. Which I’d imagine is what happens when one tube is gone and the others remain maybe! Thanks for the help!

 

[Mrtoodamnsabre]

Also can I plug the bass into the power amp in to see if it’s a pre amp or a power tube gone bad?

 

[Wasnex]

The bias procedure is simple and you do not need to open up the amp, so you will not be exposed to high voltage. If the tubes can be biased, it may return the amp's headroom to normal.

Here is the chart that shows the codes for the LEDs.

The amp has six output tubes. They are organized into two circuits of three tubes. Each circuit has a bias control and a set of Green and Red indicator LEDs.

From the manual:

Here is how the amp normally operates:
If the bias controls are turned all the way CCW, all of the bias indicator LEDs should go out. As you turn one of the bias controls CW, the associated Green LED should come on first. This happens when each of the three tubes in the circuit pulls approximately 15mA. This is an "AND" function: tube 1 AND tube 2 AND tube 3 must satisfy the minimum current target for the Green LED to come on.

As you continue to turn the bias control CW, eventually the associated Red LED should come on. This happens when one of the tubes in the circuit pulls approximately 30mA. This is an "OR" function: when tube 1 OR tube 2 OR tube 3 exceeds the max current range, the Red LED comes on.

At this point, both Red and Green should be lit. Back the bias off just a touch until the Red LED goes out and the bias is set for the first circuit.

Repeat for the second circuit.

If it's not possible to adjust for Green/Green with no Red, consult the chart above.

To answer your question, yes the operating characteristics of tubes change as they age. Over the life of a tube, the ability for the cathode to emit electrons eventually begins to decrease. This may result in the current decreasing. But in my experience, frequently tubes tend to start running hotter as they age. This can occur as the control grid becomes less effective at holding the electrons back, or if the tube has leak that contaminates the vacuum with impurities.

When one tube gets colder and another tube gets hotter it really degrade how well matched they are. So you might wind up with Red before Green...or worse Red and no Green (see the chart above).

You don't typically need to do a complete tube job. The normal lifetime of the small signal tubes is considerably longer than the output tubes. A full matched set of 6550 runs about $270.

Also, if only one of the output tubes goes bad, it may be possible for a tech to install another tube with characteristics similar to the remaining good tubes. It may even be possible to sort the output tubes into a different order that provides a better current match. It's helpful if the tech has a tube tester.

If the amp continues to distort when the bias is adjusted to Green/Green, there are a few things you can check. The problem could be in the preamp or the power amp. The amp has Preamp Out and Power Amp In jacks on the back. The jacks are basically at the junction between the preamp and power amp. The first thing to try is to plug a short 1/4" to 1/4" jumper between these jacks. The Power Amp in jack is switching and sometime the contacts can get dirty or corroded. Inserting a jumper bypasses the switching.

If you have another amp with an Effects loop, the second thing you can do is try running the Preamp from the SVT into the other amp's effects return, and running the other amp's preamp out or effects send into the SVT's Power Amp In.

Also can I plug the bass into the power amp in to see if it’s a pre amp or a power tube gone bad?

You can do this, but the signal level of your bass is unlikely to be hot enough to drive the power amp to full power. I believe 0.25V RMS is required for full power.

If you can figure out where the problem is, you can try rolling a new tube into the amp. The preamp uses two 12AX7s. The power amp uses one 12AX7 and two 12AU7s. You can replace any of the 12AX7s with a new 12AX7. I suggest rolling a good tube through each tube slot. In other words, you only need one spare 12AX7.

I suggest a bit more caution if you want to replace the 12AU7s, because they control the bias for the output tubes. Before pulling the 12AU7s, turn the bias controls CCW so the bias indicator LEDs go out. Then, after you install new 12AU7s, follow the bias procedure in the manual.

Why? A new 12AU7 may be stronger and pull more current than the old tubes. If this occurs, it will pull down the bias voltage, which increases the current passing through the output tubes. To prevent this, you set the bias really cold before messing with the 12AU7s.

 

[agedhorse]

Rather than just replacing all the tubes, I would recommend having a QUALIFIED service tech first troubleshoot the problem and identify the cause. Then repair what’s actually bad.

I see a lot of perfectly good tubes replaced in an attempt to fix problems that are unrelated to an entire tube set.

 

[Mrtoodamnsabre]

The bias procedure is simple and you do not need to open up the amp, so you will not be exposed to high voltage. If the tubes can be biased, it may return the amp's headroom to normal.

Here is the chart that shows the codes for the LEDs.

View attachment 4503451

The amp has six output tubes. They are organized into two circuits of three tubes. Each circuit has a bias control and a set of Green and Red indicator LEDs.

From the manual:
View attachment 4503464

Here is how the amp normally operates:
If the bias controls are turned all the way CCW, all of the bias indicator LEDs should go out. As you turn one of the bias controls CW, the associated Green LED should come on first. This happens when each of the three tubes in the circuit pulls approximately 15mA. This is an "AND" function: tube 1 AND tube 2 AND tube 3 must satisfy the minimum current target for the Green LED to come on.

As you continue to turn the bias control CW, eventually the associated Red LED should come on. This happens when one of the tubes in the circuit pulls approximately 30mA. This is an "OR" function: when tube 1 OR tube 2 OR tube 3 exceeds the max current range, the Red LED comes on.

At this point, both Red and Green should be lit. Back the bias off just a touch until the Red LED goes out and the bias is set for the first circuit.

Repeat for the second circuit.

If it's not possible to adjust for Green/Green with no Red, consult the chart above.

To answer your question, yes the operating characteristics of tubes change as they age. Over the life of a tube, the ability for the cathode to emit electrons eventually begins to decrease. This may result in the current decreasing. But in my experience, frequently tubes tend to start running hotter as they age. This can occur as the control grid becomes less effective at holding the electrons back, or if the tube has leak that contaminates the vacuum with impurities.

When one tube gets colder and another tube gets hotter it really degrade how well matched they are. So you might wind up with Red before Green...or worse Red and no Green (see the chart above).

You don't typically need to do a complete tube job. The normal lifetime of the small signal tubes is considerably longer than the output tubes. A full matched set of 6550 runs about $270.

Also, if only one of the output tubes goes bad, it may be possible for a tech to install another tube with characteristics similar to the remaining good tubes. It may even be possible to sort the output tubes into a different order that provides a better current match. It's helpful if the tech has a tube tester.

If the amp continues to distort when the bias is adjusted to Green/Green, there are a few things you can check. The problem could be in the preamp or the power amp. The amp has Preamp Out and Power Amp In jacks on the back. The jacks are basically at the junction between the preamp and power amp. The first thing to try is to plug a short 1/4" to 1/4" jumper between these jacks. The Power Amp in jack is switching and sometime the contacts can get dirty or corroded. Inserting a jumper bypasses the switching.

If you have another amp with an Effects loop, the second thing you can do is try running the Preamp from the SVT into the other amp's effects return, and running the other amp's preamp out or effects send into the SVT's Power Amp In.



You can do this, but the signal level of your bass is unlikely to be hot enough to drive the power amp to full power. I believe 0.25V RMS is required for full power.

If you can figure out where the problem is, you can try rolling a new tube into the amp. The preamp uses two 12AX7s. The power amp uses one 12AX7 and two 12AU7s. You can replace any of the 12AX7s with a new 12AX7. I suggest rolling a good tube through each tube slot. In other words, you only need one spare 12AX7.

I suggest a bit more caution if you want to replace the 12AU7s, because they control the bias for the output tubes. Before pulling the 12AU7s, turn the bias controls CCW so the bias indicator LEDs go out. Then, after you install new 12AU7s, follow the bias procedure in the manual.

Why? A new 12AU7 may be stronger and pull more current than the old tubes. If this occurs, it will pull down the bias voltage, which increases the current passing through the output tubes. To prevent this, you set the bias really cold before messing with the 12AU7s.

thanks for the help and the level of detail in there, I think I’ll try to turn the little bias guys ccw as if it’s an issue of them running too hot I’d feel comfortable turning them down rather than If they’re running too cold. I might turn them up if that removes the problem. How much of a turn should I give them? Like a quarter? Or a full turn?

Rather than just replacing all the tubes, I would recommend having a QUALIFIED service tech first troubleshoot the problem and identify the cause. Then repair what’s actually bad.

I see a lot of perfectly good tubes replaced in an attempt to fix problems that are unrelated to an entire tube set.

absolutely! It’s a 2013 I believe and I don’t know how hard or gentle the old user was with it so I’m sure something at this point could go awry

 

[Dave W]

I’m using a four string active Sabre bass, I’ll admit the bass has a deeper range than normal but it wasn’t doing this a week ago

K.I.S.S.

Did you try replacing the battery in the bass? Sounds stupid, but try it before you go nuts spending money on much more expensive things first.

 

[brianrost]

You might want to open up the cab and check all the screws holding the drivers to the baffle board and even the screws on the back. I had a B25 cab given to me because "it was blown" according to the donor. Turned out only 3 screws (out of 12!!!) were still holding the back on and all the t-nuts on the drivers were loose. Tightened it up, problem solved.

Of course the tip from @agedhorse to get a tech to look at it is a good idea.

 

[Wasnex]

thanks for the help and the level of detail in there, I think I’ll try to turn the little bias guys ccw as if it’s an issue of them running too hot I’d feel comfortable turning them down rather than If they’re running too cold. I might turn them up if that removes the problem. How much of a turn should I give them? Like a quarter? Or a full turn?

absolutely! It’s a 2013 I believe and I don’t know how hard or gentle the old user was with it so I’m sure something at this point could go awry

IMHO gently turn until the RED led goes out or you hit the mechanical end stop.

If Green goes out before Red, the tubes are not matched.

 

[Mrtoodamnsabre]

You might want to open up the cab and check all the screws holding the drivers to the baffle board and even the screws on the back. I had a B25 cab given to me because "it was blown" according to the donor. Turned out only 3 screws (out of 12!!!) were still holding the back on and all the t-nuts on the drivers were loose. Tightened it up, problem solved.

Of course the tip from @agedhorse to get a tech to look at it is a good idea.

So I put a old trace Elliot hybrid on the cabinet and the same low end distortion is present. I think it’s the cab I can’t be 100% sure but I’m pretty sure it’s a cab issue and not an amp issue. How does one open up one of those anyways?

 

[Wasnex]

So I put a old trace Elliot hybrid on the cabinet and the same low end distortion is present. I think it’s the cab I can’t be 100% sure but I’m pretty sure it’s a cab issue and not an amp issue. How does one open up one of those anyways?

The front panel comes out. It can be a bit of a hassle and in my experience is hard to do without causing some minor cosmetic damage. Haven't actually done it on a modern cab, but on my 80s era cab the metal corners obstructed removal of the grill. The actual grill retainers are similar to this Dual Lock product:

The drivers are held in with screws. Careful you don't put a screw driver through one of the cones. A little slip is all it takes.


IMHO make sure and address the high current situation with the output tubes. The tubes wear out quicker when biased into the red.