My amp just died on a gig...

bass12 · #1 10-22-2010, 12:56 AM

...luckily I was able to go through the PA. I just came back from a gig where my Ampeg SVT III Pro basically died. I say "basically" because at first it wasn't giving me anything, then I would suddenly get some low-volume distorted sound, and then it would go clean before distorting again or cuting out completely. The amp has actually done this a bit before (fairly recently), but never to the point of not finally "settling in" and performing as it normally would. Does anyone have any idea what the problem could be (from my vague description). Could this be an issue with the tubes? The amp is around six years old and I've never had the tubes replaced. Thanks for any input you might have.

JimmyM · #2 10-22-2010, 01:00 AM

could be a bad tube, could be the mosfets needing rebiasing. if you have a spare tube or two, try switching out tubes one at a time. if it doesn't change, then it probably needs rebiased.

and yes, a tech needs to do it or you'll burn your amp up.

bass12 · #3 10-22-2010, 01:03 AM

Hey Jimmy - I was hoping you'd chime in. I'll take the amp to a tech as soon as I find one! I'm guessing that one of the Ampeg dealers in my area should have a capable enough tech on hand. I'll check...

onionpuzzle · #4 10-22-2010, 01:05 AM

did you have a DI pedal at the ready ?

bass12 · #5 10-22-2010, 01:32 AM

Quote:

Originally Posted by onionpuzzle

did you have a DI pedal at the ready ?

No, but I was able to go straight into the board with a 1/4" cable.

anderbass · #6 10-22-2010, 07:04 AM

Did you already try patching a cord across your fx-loop? If not give it a shot, might just be dirty switching contacts in your Return jack.

Low Sound Love · #7 10-22-2010, 07:18 AM

My svt3 pro did something similar not too long ago. well actually it didn't give any sound to the speakers at all, but the line output was still working properly.
The tech that repaired it explained what it was, but I forgot most of it

It had something to do with the tube gain section, something got fried and as a result something else did as well. One tube was replaced.
Most importantly the amp was easily fixed and is sounding even better then before.

Jefenator · #8 10-22-2010, 09:50 AM

Nice to have that PA option, huh?

I learned last month: our powered mixer w/ a couple of 12" mains isn't much of a bass rig, but it's 1000000 times better than nothing!

(I also learned, I should stop being a lazy schmuck and bring the LMII for backup.)

bass12 · #9 10-22-2010, 01:09 PM

I took my amp into the shop today. With any luck the tech won't have any difficulty identifying the source of the problem and I'll have the amp back within a week. We'll see...

dmusic148 · #10 10-22-2010, 01:11 PM

Quote:

Originally Posted by anderbass

Did you already try patching a cord across your fx-loop? If not give it a shot, might just be dirty switching contacts in your Return jack.

+1.

stiles72 · #11 10-22-2010, 03:48 PM

Quote:

Originally Posted by JimmyM

could be a bad tube, could be the mosfets needing rebiasing. if you have a spare tube or two, try switching out tubes one at a time. if it doesn't change, then it probably needs rebiased.

and yes, a tech needs to do it or you'll burn your amp up.

+1

Check your line out signals and if they are good and clean, it's probably in your last 2 driver tubes or the MOSFET bias needs adjusted. Had this happen to both of my 3pros. Anything beyond swapping tubes around - take it to a tech.

StyleOverShow · #12 10-22-2010, 03:52 PM

Six years and no problems, then perhaps a bad tube. Not too bad reliability, really. I have the same head, or anchor more accurately, and it has never needed to 'settle in', just warm up.

Any estimate on the damages?

stiles72 · #13 10-22-2010, 03:54 PM

MOSFET bias adjustment should just be a bench fee - mine was like $30

Jerrold Tiers · #14 10-22-2010, 07:48 PM

Mosfet bias doesn't do that, at least not if anything close to reasonable...... it can make some distortion.

Something in the tube driver section, or possibly a problem with the circuit that keeps the DC offset very low come to mind..... Those or maybe a problem with the circuit that provides the bias current to the mosfets (not the same thing as the actual bias circuit).