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Ampeg SVT 3 pro cutting out? any help appreciated!

P

Phobic

Nov 3, 2011
4
0
boston, ma
#1
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!
 
#7
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.
 
#8
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 :help:
 
#9
The OPs problem DOES sound like dirty jacks or a bad solder issue.

Wasi said:
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 :help:
Click to expand...

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.
 
#11
Wasi said:
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
Click to expand...

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.
 
#13
Phobic said:
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!
Click to expand...

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.
 
#14
Noticed similar issues mentioned on TB about the SVT 7 and thought I would add my recent experience to the pile... I was using a 1 year old Ampeg SVT Pro 7 at the Casino recently and was hugely disappointed. The tone is incredible, but the reliability was horrific. The power LED would fade out as the sound also faded out. I repeatedly experienced the "oh crap" feeling... as the cycle from on to off continued into the last two songs of the set... The failure was not user induced or environmental as I have had many working gigs and sessions thought the decades.

Last week using the same head and gear -I had session work without any issues. The session did not require high volume. The casino on the other hand was loud, so I was pushing the amp. I was providing the appropriate signal strength to the amp. With new 12ga cables, constant power voltage and Acrugroove cab's - the weak point was the amp.

IMO, I would attribute this to a heat related issue and circuit protection. I plan on contacting Ampeg Customer Service to ask why the amp craps out when its properly pushed.
 

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