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served 23rd November 2012 08:54 PM

Ampeg svt 3 pro Power Amp

I have a problem with ampeg svt 3 pro Power Amp.
There is anoying distortion and its coming from the power amp, as its not there if I take the signal from "PreAmp Out" socket.
Its like my speakers are broken and it occures on higher notes (on bass) low strings are sounding good.

I switched to another cabinet, but this problem is still there and its not the speakers.

Can you suggest, what could be the problem with it?

I changed the two tubes there, but no use.
Could it be the FETs?
What could be measured there without taking it all apart?

Here is the schematic for the power amp

ampeg svt 3 proPowerAmp.pdf download - 2shared

indianajo 23rd November 2012 09:37 PM

I suspect you have power supply sag if is only the output stage. You need to clip lead a DVM to the power supply rails, and play a loud note, and see if the voltage sags. Warning, the note will have to be rather sustained, DVM's average the signal over about 4 seconds.
You could also have a coupler cap going bad, particularly if there are any electrolytic caps over 20 years old. These are aluminum cans wrapped in cardboard or plastic, with a plus on one lead or a minus in balls pointing at the other. Or, it can look like a peanut M&M, those are tantalum electrolytic caps.
Once my amps start sounding funny, and the caps are over 20 years old, I replace all the e-caps before I do anything else. Oddly, Salas and Enzo said something about that process happening at the incoming inspection desk of their amp shops. I date the new ones I install with a sharpie, the Nipponese don't date their caps with more than a one digit year and a week. I guess no consumer electronic device is supposed to last more than 10 years. Oh, BTW, Sony and Panasonic bonds went to Junk status this week. Funny thing, if you're going to buy trash may as well buy directly from the source of trash 100 miles to the west. You bought an Ampeg, worth buying and worth fixing IMHO.
Okay, looking at the schematic, C3 and and C5 are old electrolytic caps. Not a lot of electrolytic coupler caps, but the plate couplers c6 and c7 are in the position that a lot of caps fail, even plastic ones. This amp also has 1/4 phone plug to the speakers, which leads to a lot of blown output transistors if a phono plug gets pulled a little bit out. No protection circuit. If the Power Supply rails aren't collapsing, you might have to remove these FETs and test them one at a time by shorting gate to source and seeing if they will hold off 18v through 47kohms or something. (I use a car battery charger with a cap on it to produce the 18v, with a 47k resistor on a wire coming off to the amps scale of the DVM and a clip lead going back. Any amps, the transistor is leaking). Lots of shops bid all new output transistors on any guitar amp job.

served 23rd November 2012 09:51 PM

Yes, I have to agree, The quality and structure of this amp is really amayzing.

So you bet on caps.
It could be one of them yes. C18 doesn't look that healthy. Not like MM but not flat top also.
I will parallel something with it to see if anything happens if I rise the value.

indianajo 23rd November 2012 10:03 PM

The parallel value of a capacitor and a short is a short.Bulging top electrolytic caps are caps that have failed already, and are ready to leak slime water. You need to buy a DVM (not autoranging) and measure the voltage on them. Hot performance is more important than any measurement you get cold.
I take back what I said about no short protection, you have a speaker disconnect relay and something going on on page 2 and 3.
The print I've got doesn't have the power supply drawings and doesn't show C18. The download only has page 1. But, that is possibly where your problem is.

served 24th November 2012 12:48 PM


So I measured some voltages.
Max drop on thouse caps was around 1V.
Between 0 (GND) and +Vcc

Between GND and -Vcc

I fed the input with a signal generator and this I got with the full sweep.
My ears are killing me now.

Full schematic for power amp

indianajo 24th November 2012 04:20 PM

Do you have DC coming out of that amp speaker terminals? That is a bad sign you have output transistor problems.
I use 5 ohm 200 watt resistors for a load when I am power checking my amp. I got them from a surplus house for $5 apiece. I have 4. I put two in series on each channel (on a stereo PA amp) and put a 4 ohm car radio speaker from the charity resale shop in series with two back to back >1000 uf caps across 5 of the 10 ohm ( on high powered amps). The speakers play music right through the DC problems if I have DC problems, or most recently, made a big click when the DC jumped out of control. You can get new 10 ohm resistors from farnell for about $20 each. The tapped ones can be set for 8 ohms, I have two of those also I bought 25 years ago.
If your power supply is not sagging at full power, then the quality check of the output transistors might be in order. Particularly if you have DC out the speaker terminals. If you don't have a 47k resistor and a wall transformer that will put out 12 VDC or 18 VDC (I have a race car set transformer at 18 VDC) to do the leakage tests listed above, then a first test is to short g to s with a clip lead and use a DVM ds. However, this is at 2 v and is not much of a test. Watch static on the gate, it can blow them up. I suggest a humidifier in the room, or a hot bath.
If you have a meter with a 2 vac scale, you can poke around looking for AC signals, but most DVM's have a 200 VAC scale which is useless.

Zero Cool 24th November 2012 04:51 PM

I have found that sometimes when these amps get pushed real hard the gates of the mosfet's get stressed and this creates a sort of "fuzzy" torn speaker sound. if you look at the output with a scope often times the amp is oscillating and or ringing!

Also, be very careful with Ampeg mosfet bias pots. one way too far causes high bias, the other way too far causes oscillation!


served 25th November 2012 12:50 PM


But could it also be because some of the fets are bad?
I am thinking that maybe the amp is uneven and thats why there is this distortion.

served 25th November 2012 03:49 PM


I messed around with the Bias trimpot, and if I max this out, the sound will clear out.
And I can repeat this again and again. Signal generator Frequenzy is 586Hz sine.
With guitar attached I also didn't hear any problems with it.

Is it possible that the bias has changed by time?
I only had to turn the trimpot a very tiny bit to fix it.
I am worried that this is not the real problem and it will come up again in the near future.

But I have to say that thouse Electrolytes have to be changed also, they are going any time.

indianajo 25th November 2012 09:18 PM

Potentiometers have been known to lose contact with the wiper after a long time not adjusted. Perhaps that is what happened to yours. Zero Cool sounds like he knows what he is talking about. One way on the pot- too much ringing. Other way on the pot- not enough crossover distortion eliminating bias current. Be sure to look on the schematic of how to measure the bias current and make sure it is right. usually voltage across an emitter resistor, but this has source resistors. A little contact cleaner on a pot can't hurt. Warning, contact cleaner is flammable, no smoking, open flame, electricity turned on or off within 10 m while using.

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