Ampeg SVT 3 pro cutting out? any help appreciated!

Phobic · #1 12-27-2011, 09:39 AM

not 100% sure if this is the right spot for this type of thing, but Ive got an Ampeg SVT 3 Pro head and it just keeps cutting out, or dropping the output level to near zero so I can just barely hear anything coming from my cab.

Ive tried new cables, a power conditioner, new power cord and still it's happening. It's intermitent too, so I can't always get it to happen, but last week I couldnt go 10 minutes without it cutting out.

Ive also tried using a different bass, using a wireless, not using a wireless, and just can't figure out exactly what the issue could be.

Any suggestions from you guys would be great!

Verminaard · #2 12-27-2011, 10:59 AM

https://ssl.eurotubes.com
get a full retube kit through these guys.
Call Phone #: (503)659-7401 and simply ask for a retube for the svt 3 pro.
It will sound better than it did before it started cutting out, and I think around $35.

RickenBoogie · #3 12-27-2011, 01:06 PM

Try putting a short cable in the effects loop, (send/return). Sometimes thoase jacks get dirty and cause this very problem.

dmrogers · #4 12-27-2011, 03:14 PM

Quote:

Originally Posted by RickenBoogie

Try putting a short cable in the effects loop, (send/return). Sometimes thoase jacks get dirty and cause this very problem.

+1. Try this first. Then you can check/change the tubes.

Actually it would be nice if you had some tuner cleaner to use with a 1/4 inch jack and clean all of the connections.

Phobic · #5 12-27-2011, 07:54 PM

Thanks guys, will try the cable and at the very least some compressed air to clear out some dust in the connections.

**duff84** · #6 12-27-2011, 07:55 PM

it happened to me before the fuse went dead, could need to replace the fuse in the amp, cheap fix cost a couple bucks and all. It could also be preamp tubes going dead too.

bstringrandy · #7 12-27-2011, 10:02 PM

I had this happen to my SVT3Pro a few years back. It was intermittent and happened with different cables and basses.

I took out the circuit board and found a cracked solder joint on the input jack connections. I reflowed the input jack solder joints and haven't had a problem since then.

Wasi · #8 01-21-2012, 08:37 AM

I have an Ampeg SVT 3 Pro that has a loud pop when relay engages... I opened it and adjusted the bias so I got 0 VDC at output and "pop" went away. Unfortunately the audio became distorted. Has anyone run into this problem and more importantly, does anyone have a fix. Thanks for helping

Jerrold Tiers · #9 01-21-2012, 08:51 AM

The OPs problem DOES sound like dirty jacks or a bad solder issue.

Quote:

Originally Posted by Wasi

I have an Ampeg SVT 3 Pro that has a loud pop when relay engages... I opened it and adjusted the bias so I got 0 VDC at output and "pop" went away. Unfortunately the audio became distorted. Has anyone run into this problem and more importantly, does anyone have a fix. Thanks for helping

Bias "should not" have fixed that, most likely, unless it was REALLY BAD.... and then you should have found the unit to be almost un-playable....

DC offset may be in the non-adjustable part of the bias circuit. There are some transistors that form a "constant current source" and if there is a problem in them or with their power supply, you would get a problem somewhat similar to what you describe.

Maybe a leaky capacitor on the power amp tube drive could also cause it.

Wasi · #10 01-21-2012, 09:02 AM

Yes!!!, I was wondering if it could be a bad cap. Looking at the schematics I got off line didn't realy point to anyone specific. Do you have any recommendations

Jerrold Tiers · #11 01-21-2012, 10:31 AM

Quote:

Originally Posted by Wasi

Yes!!!, I was wondering if it could be a bad cap. Looking at the schematics I got off line didn't realy point to anyone specific. Do you have any recommendations

Why certainly!

Find the real problem and fix it!

The output stage is simple, just a set of Mosfets as a 'follower"...... driven through capacitor coupling from the 12AU7...so most likely something is injecting DC into the drive point (leaky capacitor, bad part in current source, even a leaky Mosfet)..... OR something is not "pulling it out" (unbalanced constant current source, bad DC balance correcting op-amp circuit).

Your job is to find out what/which it is, and fix that. Or you can delegate the fix to a tech.

because of the DC balance op-amp, quite a bit of unbalance will get compensated, but if there is too much it will get "almost" compensated, leading to DC offset. In the process, the bias may get messed up, but it isn't the true problem.

Wasi · #12 01-21-2012, 03:37 PM

Okay, thanks for the leads... I'm going to have a go at it.

randysmojo · #13 01-21-2012, 03:46 PM

Quote:

Originally Posted by Phobic

not 100% sure if this is the right spot for this type of thing, but Ive got an Ampeg SVT 3 Pro head and it just keeps cutting out, or dropping the output level to near zero so I can just barely hear anything coming from my cab.

Any suggestions from you guys would be great!

I had the same problem. Is the peak light over by the input lighting up when it cuts out? If so, it might be bad solder joints on the back jack for the mute footswitch. That was my issue. Had to the peak light lights up when you mute it. The bad solder joints made it basically mute it because it wasn't letting the signal pass. This jack could also cause the same problems if it is dirty as well. I basically resoldered all the jacks for the various inputs and outputs on the back and fixed my issue.