ampeg svt-3 pro bass problem

[amptech]

Hello all,

Has anyone had this fault before with this amp, oscillation so bad it blow's replacement o/p stage!
this amps problem was very weak and distorted audio o/p signal, and after correct replacement componets had been fitted the above fault happen!
Our service tech with over 30yr's in the trade has checked the board so many times now it's driving him nut's. Every time he bring's up the the supply voltage slowly or direct power up it start's to oscillate and the out put stage over heat's and then blow's fet's....
any help would most greatful thanks.
amptech.

[Enzo]

First, is he putting a load on the amp? It is a solid state amop and does not require a load. I'd work without one until it seems stable.

If the amp goes into RF, then is the output stabilization network intact?

Are you working on it too? Or are you just looking out for him? I mean to welcome you here of course, but if he is the one working on it, invite your tech to sign on here as well. Eliminate the middle man, so to speak.

It is easy to spot fried semiconductors, but when amps blow up, I start looking for open resistors. Like ones in the stability networks and maybe those between bases or gates on the + and - sides.

[amptech]

ampeg svt-3 problem

Enzo,

Thanks for your fast reply, we're both working on this model now! you asked if my tech was loading the amp,no and also the bias circuit componets have been replaced. So with out any loads or in signal connected it's very unstabe indeed My tech as got got the circuit diagram at home to read over it and to see if he missed anything.

Have you yourself came across this fault with this unit before?
Maybe with a fresh out look on things may show the problem or it's back to the drawing board

[Enzo]

Have I seen unstable amps that blow fuses and otherwise cause trouble? Yes. Do I recall which models they were? No.

[Steve Conner]

Did you check the Zobel network like Enzo said? Maybe one of the components in it failed open. I once built a MOSFET amp and forgot to put it in, and it suffered exactly the same symptoms: vicious oscillations in the MHz range, leading to explosion on the launch pad.

Also, if this is one of the amps that uses FETs meant for switchmode use, did you use the proper replacements? Ampeg grade and select them, and ungraded FETs of a different brand might not cut it.

What frequency are the oscillations? MHz means local instability in the output stage, probably due to the reasons I explained above: wrong FETs or bad Zobel network.

100s of kHz is Nyquist oscillations: ie the whole feedback loop has gone unstable due to something wrong in the small-signal stages. You may be able to break the feedback loop: if this is Ampeg's contraption with the TL071 driving a bunch of transistors that drive the FETs, what happens when you remove the op-amp?

Is there a DC offset along with the RF? If you can't keep the FETs intact for long enough to find out, remove all but one pair of them, and temporarily make the source ballast resistors 10 times bigger. The amp will be gutless, but it should function with no load.

Of course, with 30 years in the trade, you've probably tried all of this already.

[amptech]

ampeg svt-3 problem

Yep, he's tried all sorts of tackful testing and still unable to bring it's supply voltage up with out the o/p fet's blowing ,yet still it's pointing to the output stage being unstable. .
We did change the 10k bias pot due to a burnt up and with the tlo74 chip that's part of the bias feed voltage.
Thing about this amp there's no zobel resistor ie 10 ohm 100n cap! but, adding one in it's still the bloom same

[Enzo]

OK, does pulling the drive tube - 12AU7 V2 in J12 - make any difference. In the whole SVT series, I find this one tube the most common failure, causing all manner of symptoms. And pulling it breaks up potential oscillatory paths.

WHat does the circuit do WITHOUT ANY output MOSFETs? Is the string of bias zeners all zening OK? Verify those four zeners and the 15k across each pair.

If the 10k pot burnt, then R10 3.3k, R11 22k are suspect along with MPSA06 Q2.

The current limiters are probably not involved, but lift D28, D29 anyway to find out.

[Steve Conner]

Hey Enzo, thanks for posting the schematics.

Having seen them I agree with the suggestion to pull the 12AU7. With that tube gone, we expect the op-amp DC servo (IC1A) will park the speaker output neatly at 0V and everything will idle fine. This should even happen with no power MOSFETs, as there's a DC path to the output through R16, R17.

With MOSFETs in, we should be able to get output by driving TP4 with a signal generator. Low output, since the whole power stage is just a big source follower, with unity gain from TP4 to the speaker lug. You can think of it as a big pair of boots that give the 12AU7 enough grunt to move speakers.

If it doesn't bias up, I'd suspect IC1A, Q3, Q4. Maybe one of Q3, Q4 is blown short and is trying to turn the FETs hard on. The DC servo would turn the unshorted transistor hard on to try to bring the output back to 0, and the result would be carnage. That would also explain the combustion of the 10k pot that someone mentioned.

If the TL074 exploded, then you need to check its power supplies, as that's the only thing that could hurt it. The resistors connecting it to the rest of the circuit are too big to let dangerous current through. If the power supplies had lost regulation, that could be making the servo loop unstable.

The DC servo is a feedback loop, so all the tricks for debugging feedback loops apply. And all the hair loss, too.

[Enzo]

And while the basic output runs on +/-65VDC, make sure the +/-100VDC rails are OK for Q3, Q4.

[amptech]

ampeg svt-3 pro

It's +/-65v and 100v rails are reading correctly, if we float the gate resistors to o/p fet's it will hold it's supply voltage ok and the fet's dont blow up but reconnecting them you can't bring supply volt up with out the them blowing up.

the tl074 has been replaced along with surrounding componets and it's still the same problem! this thing is driving us round the bend......

[Enzo]

And are all the voltages around all that stuff between the output FETs and the tube making sense? Are the gate lines sitting at reasonable voltages one per polarity? WHile leaving the FET gates out.

[algudkov]

Thanx for scheme!

[amptech]

ampeg svt 450 bass amp.

This one drive us nuts and in the end we did get it to stay working!

Because of time and money etc, in the end we built a replacement power amp stage in to it and all was stable after lots of mod's.
We soak tested the amp at full power for four hours straight and all was well.
ok, some times you may have to mod things to get it to work and there come's a point where you have say let's use option 'B' or cut your loss!

[hearlittlelis]

SVT III Pro

Hey Folks,

I've been reading the posts for this amp and I think I have the same or similiar issue. Although I'm good with a soldering iron, I'm not one who can read scematics and therefore am having trouble understanding the posts.

Anyway, I hear RF or just simply a high pitched hissing sound. Also, with any frequency, there is a slight osscilating sound on the sustain...sort of a 'woo woo woo' if you will. In addition, whenever I touch my pick ups, there's a buzzing sound until I ground myself on the strings. I've tested my bass on other rigs and this doesn't happen. I've also tested other basses on my rig and it does...obviously a grounding issue amungst others.

Any insight for the laymen?

[ImEX82]

Lack of Volume in a Ampeg SVT 3 pro

yeah im sure this is slightly off topic, but im just seeing anyone out there can offer some advice!
i just recently got a SVT 3 PRO off ebay for a decent price wit gator flght case, and managed to try it out proparly last sunday!
when i fired it up i was happy with the tone and when it came to have full practise with band i wasent happy, dammm lack of volume or what!!
so i got a mate to check it out for me and to run some test on it to see how much exactly it was throwing out.
It states in the manual 8 ohms 275 and 4 ohms 450
what he got it on the scope was 8 ohms 125 or there about and 4 ohms 250 ect
as u can imagine i wasent best pleased. Bin for ever reading frew most of the forums on the net, and alot of people have stated this problem!!

To add insult to injury as well the speakon connector is fucked and one of the 3 1/4 inch conectors isnt workin to good either, so wht i need to know is how to "mod" it or just get it to wht it "says" on the actual amp

the cabs i use are 4'10 ashdown 8Ohm and 1 '15 beringher 8 ohm
( i knw not the best cabs in the world)
5 string Spector (cheap spector)

so hopefully this thread is still open and someone can suggest some cheap options in what i can do!
and sellin it isnt a option, cos i blagged big time for help with the funds to get it lol

cheers

Sam