Ampeg SVT 3 Pro Distortion Problem ???

[stiles72]

Anyone else encounter this?

I have two Ampeg SVT 3 Pros that I use with my hard rock cover band. Recently they started distorting badly- both at the same time. I changed the tubes, and the problem persisted. Eventually I isolated the problem to somewhere in the power amp section. The preamp out is crystal clean (running it to a seperate amp). When you plug a bass or signal into the power amp in, the distortion remains.

The distorted sound does not diminish the level of the signal, and does not affect the tone stack. It sounds as if it is two seperate signals blended together at the same time - one clean, one distorted.

I took my amps to the closest authorized Ampoeg tech, and we can't figure it out and says he's going to send them off somewhere else.


On a side note - because I wasn't able to use my 3 Pro's this weekend, I ended up running my V4BH on my Simmons cabinet and I ran my Portaflex head on a 4x12. Beleive it or not that little Portaflex thumped! For only a 100 watt V4Bh and a 35 Watt Portaflex, they held their own compared to the 3 Pros and had tons of tone!

 

[DLEE]

There's a trim pot inside to adjust the mosfets,mine did the same thing years ago.You can adjust it yourself but if you do it wrong,POOF! there goes your power section. There was a thread about it a couple of years back. You need a small precision philips head screwdriver. Have someone play a note on your bass and SLOWLY turn the screw a little.When I say little I mean the thickness of a hair. I turned mine counterclockwise and the distortion immediatly went away and it never came back. If Jerrold Tiers is still around I'm sure he'll put you on the right course. Unless you're willing to see it go up in smoke I would have a tech do it with the correct testing tools.

 

[Jerrold Tiers]

If it's a buzz "behind the tone" that becomes really noticeable as the note dies out, probably bias.... best handled by a technician.

odd that both would do it, was it pretty cold that day?

 

[Jim C]

I recenly read this thread although can't find it again
The consensus was this adjustment should be made by a trained tech with test equipment as the trim pot is very sensitive and a small mis-adjustment can cause major damage.

 

[stiles72]

Thanks for the tips - we'll see if that does the trick

 

[stiles72]

If it's a buzz "behind the tone" that becomes really noticeable as the note dies out, probably bias.... best handled by a technician.

odd that both would do it, was it pretty cold that day?

Yes it was very cold the last night I used them. They seemed to work fine at the show but the next time I took them out of the trailer- distortion~~~

 

[hrgiger]

http://www.talkbass.com/forum/showthread.php?t=437406&highlight=mosfet+bias

 

[dmojo]

I have the exact same problem. You described it perfectly. I have been frustrated with this issue for a while now. The audio tech in my town can't hear the distortion, so they can't help me! I'm tempted to try it myself. Will research more before I do.

Good luck!

 

[bassman251]

There's a trim pot inside to adjust the mosfets,mine did the same thing years ago.You can adjust it yourself but if you do it wrong,POOF! there goes your power section. There was a thread about it a couple of years back. You need a small precision philips head screwdriver. Have someone play a note on your bass and SLOWLY turn the screw a little.When I say little I mean the thickness of a hair. I turned mine counterclockwise and the distortion immediatly went away and it never came back. If Jerrold Tiers is still around I'm sure he'll put you on the right course. Unless you're willing to see it go up in smoke I would have a tech do it with the correct testing tools.

I've heard the same thing mentioned before.

I though up to a point that was the ampeg sound for those types of amps. With just a hint of distortion in the background. Especially if its cranked up. I could see if the distortion is excessive being a problem.

 

[JimmyM]

On a side note - because I wasn't able to use my 3 Pro's this weekend, I ended up running my V4BH on my Simmons cabinet and I ran my Portaflex head on a 4x12. Beleive it or not that little Portaflex thumped! For only a 100 watt V4Bh and a 35 Watt Portaflex, they held their own compared to the 3 Pros and had tons of tone!

Slight correction...the B-15N is a 25w head, 30 if made in 65 or after. And people don't believe me when I say how loud it is, but it can easily carry a small/medium room at a reasonable stage volume. Don't play in an AC/DC cover band with it, but it can actually put out some juice, especially in a 412. Used mine with a 410 in the past...plenty of volume.