ampeg svt3 wierdness

0xbow · #1 02-29-2012, 11:43 AM

can anyone help with some advice?
i bought an svt3 2008 recently that i run through a pf210
i was running a bfm monitor as an extension cabinet at home just as a test and it sounds like the speaker is blown-low frequency distortion-.
being brand new with eminence deltalite 2010 i was a bit taken aback so i pulled the driver to see if it was just something vibrating or a damaged cone but no.
then i hooked the monitor to my swr wm15 combo and what do you know the speakers fixed and is crystal clear.
the evidence is pointing to the svt3 as the problem which i couldnt previously hear because my other cabs are not as clear as the monitor.
am i doing something wrong?
what is the next step?

otherclef · #2 02-29-2012, 11:47 AM

I have a SVT3 pro... it may be just ampeg.
Could it be described as an extremely muddy bottom end??

RickenBoogie · #3 02-29-2012, 11:52 AM

2 things you can do, replace the preamp tubes, and clean the fx loop jacks. if neither fixes the problem, you'll need to find a qualified amp tech and take it in. It's not "just Ampeg".

otherclef · #4 02-29-2012, 01:43 PM

Be careful if open the chassis to try other preamp tubes.
touch the tube only and use one hand.

0xbow · #5 02-29-2012, 02:48 PM

the swr is more hifi than the ampeg but this is definitely distortion im hearing.
against my better judgement i had a look inside .the tubes are all of russian origin appear normal and the mosfets are as follows with no load ;1. 3.6mv 2. 3.2mv
3. 3.4 4. 2.2
5. 3.5 6. 3.2
7. 3.1 8. 4.8

stiles72 · #6 02-29-2012, 04:30 PM

Something to check... Run your 3-Pro preamp out to your SWR input. If the sound is distorted, then you could have a bad preamp tube. If the sound is clean, then you have a problem in the 3-Pro power section which means that either it's time to replace a driver tube or the MOSFET bias needs to be adjusted. You can change out the tubes on your own, but if that doesn't fix it - you need to take it to a tech to have the MOSFET bias adjusted. It should only be a bench fee, and it's a simple adjustment. After a few years, it's not uncommon for the bias to drift and is perfectly normal to need to have it readjusted.

**svtb15** · #7 02-29-2012, 04:41 PM

Wait.. This could be the SVT3 pro Mosfet BIAS issue that i described in a thread about 6 years ago or so..
It is a fuzzy sort of distortion that occurs even at low volumes....???
I will see if i can find the thread for you. if not try to search SVT 3 Bias

**svtb15** · #8 02-29-2012, 04:42 PM

This is just a few of many threads on the subject.. I cant find teh thread that i did about it.. but here a few
Ampeg SVT 3 Pro Problems...

little background distort in my SVT-3

Ampeg SVT-3 Pro have a natural fuzz ?

0xbow · #9 02-29-2012, 05:21 PM

ill have to get back to you on that .
the SWR is now in the car on the way to the gig.i take it you mean run a cable from the preamp out phone jack to the SWR input?.
i also have a sadowsky external preamp.if i connect that to the power amp in phone jack will that also isolate the preamp tubes?
ill try your suggestion when i get back

Jim C · #10 02-29-2012, 06:18 PM

svtb15 is on target; I had a 3 Pro that had been in for solder joing touch ups, only to have the same problem occur months later; the bias is also an issue for these amps

0xbow · #11 03-05-2012, 06:44 PM

well i read all the posts and threads i could find and i decided to have a go at adjusting the bias myself.
i just used a multimeter and proceeded cautiously.
the average before was 3.4mv and i got it up to about 23.8mv which was as good as i could get.the values fluctuate with temperature changes so i may recheck.the adjustment pot is a fool of an idea but i think someone who has a few clues and is careful can successfully do this.
the distortion disappeared immediately when i tested the amp so thanks very much to everyone who contributes to this wonderful resource.