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Ampeg SVT 3 Pro Problems...

S

sethplaysbass

Oct 27, 2009
10
0
Nashville, TN
www.myspace.com
#1
Hi guys, I just want to see first of all if anyone has the full schematics for the Ampeg SVT3-Pro head. It is a 2006 model, so not too old. I have been having a weird low rumble-ish distortion coming from it no matter what I do. I have plugged up a different head to my cab and it does not sound like that, so I know my cabinet is fine. My DI output of the SVT sounds perfect, so I think it is something with either the driver tubes (which are not in the signal path of the DI out, correct?) or the actual speaker output. Which I doubt. Because it really does sound like a bad tube. I just bought a 12AU7 tube, and put it in, but totally did not realize there are 2 driver tubes. Furthermore, only one of them is a 12AU7, the other is a 12AX7, and now I do not know which one is supposed to go where. I am planning on getting a new 12AX7 tomorrow so both the driver tubes will be new. But I need to figure out which spot the 12AU7 goes in and which the 12AX7 goes in. Also, if replacing these tubes does not fix my problem, then I don't know what to do! Thanks for your help!
 
#2
I suspect your Bias is off on the output stage.

I had the same problem.

I wouldn't try adjusting it yourself unless you have a function generator, an oscilloscope and an Isolation Transformer with a Current meter.

for some reason I guess Ampeg decided they didn't need to fix that issue. They use a really cheap low precision pot for adjusting the Bias. Its really touchy you could adjust it by ear but there is a really fine threshold once you cross it the amp starts running hot and will start drawing more than 3Amps at idle. You need to be able to monitor the current draw while adjusting the bias. Or you could fry your output stage.

Id take it to a good tech.
 
#3
The 12AU7 is the one closer to the front panel.

Bias might be an issue, but probably not, unless it is pretty low.

Noisy tube might well be it, so go for it. If that is not the problem, then it may be a problem best thrown to a good tech.......

If you want schematics, I have them, PM me.

BTW thanks for reminding me. I owed an answer to another guy on the same question, and your post prompted me that I needed to dig up his lost email and answer him....
 
#4
I had the same problem with a pair of 3-Pros. DI out and preamp out signals are clean, but the power amp section distorts. On head #1, it was a bias issue which is fixed. On head #2, the tech said I had some bad transistors. I'm not a tech so I can't tell you more than that. The guy working on mine said the parts to fix that head are still backordered - and they've had the head since May. Here's the link to my post on my problems, hopefully everyones responses help you as well :

http://www.talkbass.com/forum/showthread.php?t=535580&highlight=stiles72
 
#5
spode master said:
Its really touchy you could adjust it by ear but there is a really fine threshold once you cross it the amp starts running hot and will start drawing more than 3Amps at idle.
Id take it to a good tech.
Click to expand...
Yep! That happened with my head#1 - got it back and it ran for about 1-2 minutes then got hot and then the relay tripped. Took it back, and they re-adjusted it again. They say its fine now, and I can go pick it up this weekend. Hopefully they got it right!
 
#6
Yeah. Wow. Thanks Jerrold for that e-mail. Good resources to have! I switched the tubes, so the 12AU7 is in the right spot, and honestly, it was hard to hear that distortion last time I played it haha. It is always pretty difficult to hear, so I'm not giving up yet, and Im going to get a new 12AX7 to replace the other driver tube to see if that fixes it. If it does, that would be awesome. If not then I guess I will know that it is my bias like you guys were talking about. IF that is the case, unless I can find a really good tech who knows ampegs in Nashville, I would love to take it to him, but if not then I might just have to send it off to ampeg myself. Which sucks, because I really need this amp day to day. Thanks for all your help guys! And if you know of any good techs in Nashville please let me know! Thanks.
 
#7
sethplaysbass said:
Yeah. Wow. Thanks Jerrold for that e-mail. Good resources to have! I switched the tubes, so the 12AU7 is in the right spot, and honestly, it was hard to hear that distortion last time I played it haha. It is always pretty difficult to hear, so I'm not giving up yet, and Im going to get a new 12AX7 to replace the other driver tube to see if that fixes it. If it does, that would be awesome. If not then I guess I will know that it is my bias like you guys were talking about. IF that is the case, unless I can find a really good tech who knows ampegs in Nashville, I would love to take it to him, but if not then I might just have to send it off to ampeg myself. Which sucks, because I really need this amp day to day. Thanks for all your help guys! And if you know of any good techs in Nashville please let me know! Thanks.
Click to expand...

If its becomes noticeable as your note fades out then its probably the Bias, the crossing distortion is much less notice able when you are playing loud.

Personally I think all of us that have had the SVT pro 3 Biasing issue should email Ampeg and tell them to stop being such cheap asses and not only update the model with an upgraded precision multi-turn trim pot, but they should send current owners the replacement and pay for the install fee's.

The current pot is so touchy you could knock the Bias out if you fart to close to the thing.

I'm sure those of you who have dealt with the thing know what I'm talking about.

Its almost criminal that they opted for a component that probably cost them a nickle, where they could have used something that cost them 50 cents for something responsible for the stability of the output stage of the amp.

And we the customers have to pay a tech at tech rates to go re-adjust the thing periodically, when the problem could be avoided if they shelled out for a part that would cost them less than a dollar.

I like my SVT but I find the situation absurd.
 
#9
shaboda said:
can anyone send me the schematics?? with the test points and bias voltages marked on the drawings?? I have 2 svt3pro heads in need of some TLC!!

thanks!!
Click to expand...

You need a function generator an oscilloscope and a Variac with a current meter.

And you just measure across the output.

Don't try to adjust it yourself unless you have the right tools and know what your doing.

A competent tech should have the right tools.

If it was as simple as adjusting the Bias I would say you could do it with your ears, but its to easy to get the output transistors running too hot.
 
#11
Subscribed. Exact same problem here... bought a new 3-Pro and used it about 10-15 times with my band, but when playing alone I realized there was that kind of fuzz, especially in the mid frequencies. Many people said adjusting the mosfets bias solved the problem, yet there's no good tech in my area and I will probably have to send it back to where I bought it from for about 3-4 weeks (and more) :bawl:
 
#12
GizmoTheOne said:
Subscribed. Exact same problem here... bought a new 3-Pro and used it about 10-15 times with my band, but when playing alone I realized there was that kind of fuzz, especially in the mid frequencies. Many people said adjusting the mosfets bias solved the problem, yet there's no good tech in my area and I will probably have to send it back to where I bought it from for about 3-4 weeks (and more) :bawl:
Click to expand...

Doesn't sound specifically bias related.

You try on other cabs??

The sound of crossing distortion is less noticeable when playing loud its as the note decays or fades out that you hear a strange gritty distortion.
 
#13
Jerrold Tiers said:
The 12AU7 is the one closer to the front panel.

Bias might be an issue, but probably not, unless it is pretty low.

Noisy tube might well be it, so go for it. If that is not the problem, then it may be a problem best thrown to a good tech.......

If you want schematics, I have them, PM me.

BTW thanks for reminding me. I owed an answer to another guy on the same question, and your post prompted me that I needed to dig up his lost email and answer him....
Click to expand...

WHAT??????

can ANYONE tell me which place is the right place for the 12AU7 tube in the SVT 3 pro? elsewhere on here it says that it goes in the back, not the front, and this post suggests. where the heck does it go?

also can someone give me a link on exactly how to bias the MOSFET pot on this thing?

thanks in advance!
 

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