SVT III Pro Distorting

baddarryl · #1 09-25-2010, 02:30 PM

Hi all. I went music store, pawn shop hopping today and found an SVTIII Pro that everything seemed to work on it except it had a lightly distorted sound to it. Almost sounded like a blown speaker or a light distortion pedal on it. I tweaked all the knobs and it was ever present. I didn't test the volume of the amp as I didn't want to hurt someones cabinets with it. Thoroughly tested with different cabs, basses, and cords etc. Is this a common issue or an easy fix?

**JayAmel** · #2 09-25-2010, 02:35 PM

What are the settings ?

baddarryl · #3 09-25-2010, 03:24 PM

EQ flat, graphic off, gain anywhere from 3-10 volume 2, voltage plate from off to full. Did it in every setting. Not the usual over driven svt tone at all.

nightcityburn · #4 09-25-2010, 03:28 PM

Possibly some of the pre-amp tubes?

Luis Fabara · #5 09-25-2010, 03:31 PM

Most probably a bad tube, that or it is requiring biasing.

baddarryl · #6 09-25-2010, 04:33 PM

Am I correct that it has 3 12ax7's? I assume it is a pretty easy task to replace them? Can I bias it or do I need to take it to a tech?

How much would you bet that is the problem and if so what would it be worth?

vin*tone · #7 09-25-2010, 04:40 PM

It's probably biasing. There are a few threads on it here with some good advice from Jerrold T and others. You can do it yourself but it's not advised - you can stuff it up with a flick of the wrist.

baddarryl · #8 09-25-2010, 04:51 PM

I am honestly wondering if it is something I should take the chance on and buy, or is just a possible can of worms of problems? I would have to have a tech fix it.

TONYTHOMAS1956 · #9 09-25-2010, 05:42 PM

If it has any age on it,chances are It'll need the mosfits replaced. Just had to have mine done. It was distorted at low volume and not as noticeable qt higher volumes.
Made a big difference in tone as well.

baddarryl · #10 09-25-2010, 05:52 PM

Quote:

Originally Posted by TONYTHOMAS1956

If it has any age on it,chances are It'll need the mosfits replaced. Just had to have mine done. It was distorted at low volume and not as noticeable qt higher volumes.
Made a big difference in tone as well.

What did that cost you if you don't mind? Yes it look to have had some hours on it.

rickdog · #11 09-25-2010, 05:55 PM

Quote:

Originally Posted by baddarryl

I am honestly wondering if it is something I should take the chance on and buy, or is just a possible can of worms of problems? I would have to have a tech fix it.

Every used amp comes with a free can of worms

So do some new ones....

Did you try different speakers? It may not be the amp at all. Can you bring in a known good speaker to test with, something you can crank up?

Were you able to turn the volume up far enough to tell whether the distortion got louder, or just stayed at the same annoying low level?

If it's constant low-level, the bias adjustment is a good bet. If it's set too low, both the positive and negative output transistors turn off at zero crossings, causing what's called crossover distortion. As vin*tone said, the bias adjustment on these amps is very touchy, it really is a tech job.

Will they give you a reasonable time to bring it back for a refund, so you can take it to a tech and have it checked out?

birminghambass · #12 09-25-2010, 05:57 PM

Quote:

Originally Posted by vin*tone

It's probably biasing. There are a few threads on it here with some good advice from Jerrold T and others. You can do it yourself but it's not advised - you can stuff it up with a flick of the wrist.

I've owned 3 of these and they all needed bias adjustment to solve this problem at some point. Easy DIY but do a little research BEFORE you do it.

baddarryl · #13 09-25-2010, 06:00 PM

Quote:

Originally Posted by birminghambass

I've owned 3 of these and they all needed bias adjustment to solve this problem at some point. Easy DIY but do a little research BEFORE you do it.

That in mind worth a gamble if I could get it cheap enough?

Jerrold Tiers · #14 09-25-2010, 10:17 PM

The OP's problem sounds like a bias issue....

Quote:

Originally Posted by TONYTHOMAS1956

If it has any age on it,chances are It'll need the mosfits replaced. Just had to have mine done. It was distorted at low volume and not as noticeable qt higher volumes.
Made a big difference in tone as well.

Unless there is considerably more to the story, this sounds like someone sold you a load of what we might politely call "organic fertilizer"......... Sounds like you got 'took'....

In general there is NO REASON that mosfets need to be replaced due to age...... They work until they flat-out fail. If they DO fail, there is not much doubt about it..... generally the amp quits working at all.

With very old units from Ampeg, before the "matching" of mosfets, some amps might have had devices so mis-matched that they might fail early.... Most of those probably failed long ago, and were fixed with new matched ones. The ones that made it this far would have been decently matched "by accident", and should be good, proven by 15 years of use.

There has to be more to the story, because as-stated, it sounds shady....