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Ampeg SVT 3 pro tube preamp section troubleshooting

#41
Paul Szegedin said:
Hey thanks man. Mine was the ST24A3. Glad to know that a new one will be quieter.

Mine was pushing back, as in over the transformers/resistors, which is how it should be -- that or pushing straight down over them from the top also works :wacky::woot::wacky::woot::wacky:. If your fan was installed backwards sucking air in, I guess that means that it doesn't need a heck of a lot of cooling at moderate volumes. I've yet to see mine speed up at all. Some SS amps don't have fans at all.
Click to expand...
Oh, check it out.
My fan
https://youtube.com/shorts/1_4hiwf6ulk?feature=share
 
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#42
Paul Szegedin said:
Hello all -- I picked up an Ampeg svt 3 that was barely producing sound and followed @ejschultz fantastic instructions for biasing the amp in this thread; Mine was reading less than 1mv and I successfully biased it up to spec around 25mv.
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Hi to all... biasing an amp svt3pro??? Can someone explain me why that? It's the first time I hear the biasing of a solid state amp... thanks
 
#43
mickbellinello said:
Hi to all... biasing an amp svt3pro??? Can someone explain me why that? It's the first time I hear the biasing of a solid state amp... thanks
Click to expand...

You bias the mosfets in the same way you would tubes.

I played my SVT3PRO for 8 years and never had an issue. Seems to me this bass has problems way beyond the OP's ability to fix, so trying to carry on into the unknown is not going to end well. Take it to a repair man and get a bass that can be played for many years to come.
 
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#44
mickbellinello said:
Hi to all... biasing an amp svt3pro??? Can someone explain me why that? It's the first time I hear the biasing of a solid state amp... thanks
Click to expand...
There are two sets of output transistors. One set is connected to a positive power supply and supplies current when the output signal is positive; the other is connected to a negative power supply to drive the output signal when it is negative. When the output signal is changing between positive and negative, or when there is no signal (output voltage is zero), both sets need to be conducting a small amount of current.

The bias adjustment controls how much current passes through the output transistors under these conditions. If there's not enough, there will be "crossover" distortion: the waveform is distorted near the change between positive and negative output voltage when one set of transistors turns completely off before the other set turns on. This is the "fuzz" you can hear from these amps if the bias is set too low.

If the bias is too high, too much current flows through the output transistors directly from the positive power supply to negative even when there's no signal. This creates a lot of heat - it's how people blow up an SVT-3Pro with a careless bias adjustment. That control really is as sensitive as people say. It's supposed to be set at the factory (to suit the characteristics of the particular set of transistors in the amp) and not need further adjustment.

@JeezyMcNuggles, I hope you have some sort of current limiting device for the AC power into the amp when you change out that trimpot. Or a spare set of matched MOSFETs....
 
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#45
mickbellinello said:
Hi to all... biasing an amp svt3pro??? Can someone explain me why that? It's the first time I hear the biasing of a solid state amp... thanks
Click to expand...
Actually, all tubes and transistors need to be biased. Some are biased as part of the circuit design, and are inherently stable. Some must be trimmed because the bias is not defined by the circuit but by an additional circuit.

Bias is a voltage or current that is used to place the control element (grid, gate or base) at a state that allows the device to operate around a specific point on the operating curve.
 
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#46
rickdog said:
There are two sets of output transistors. One set is connected to a positive power supply and supplies current when the output signal is positive; the other is connected to a negative power supply to drive the output signal when it is negative. When the output signal is changing between positive and negative, or when there is no signal (output voltage is zero), both sets need to be conducting a small amount of current.

The bias adjustment controls how much current passes through the output transistors under these conditions. If there's not enough, there will be "crossover" distortion: the waveform is distorted near the change between positive and negative output voltage when one set of transistors turns completely off before the other set turns on. This is the "fuzz" you can hear from these amps if the bias is set too low.

If the bias is too high, too much current flows through the output transistors directly from the positive power supply to negative even when there's no signal. This creates a lot of heat - it's how people blow up an SVT-3Pro with a careless bias adjustment. That control really is as sensitive as people say. It's supposed to be set at the factory (to suit the characteristics of the particular set of transistors in the amp) and not need further adjustment.

@JeezyMcNuggles, I hope you have some sort of current limiting device for the AC power into the amp when you change out that trimpot. Or a spare set of matched MOSFETs....
Click to expand...
Yeah, turn it all the way down...limited.
 
#47
JeezyMcNuggles said:
Oh, check it out.
My fan
https://youtube.com/shorts/1_4hiwf6ulk?feature=share
Click to expand...

Thanks much, that's useful. Mine has a little bit of a vibration to it that makes it louder -- it's 20 years old -- so it's got to go and a new fan should help a lot. That's what I mean when I say it's quieter pointing down - on account of the vibration. But still, I guess even with your amount of fan noise, you wouldn't record with the mic 12 or 18 inches below that, would you? Maybe put the amp in the next room? My background is in recording engineering, not electrical. My GK, no fan, you can hear a pin drop between notes.

chris_b said:
Seems to me this bass has problems way beyond the OP's ability to fix, so trying to carry on into the unknown is not going to end well. Take it to a repair man and get a bass that can be played for many years to come.
Click to expand...

The amp is worth maybe $400 tops used. I got mine for free. I fixed it in a half hour and it's playing well. If it drifts out of bias again, I will re-bias. If it does a second time, I will part it out. I got the amp for FREE. A trip to a tech would exceed the value of the amp, and the problem is just as likely to return. The PO just installed five brand new Sovtek tubes in it, trying to fix it. I can use these tubes in my Fender guitar amps. This whole thing has already ended well. I have fixed many amps and guitars, and have a wonderful collection of them, all of which combined I paid well under $1,000 for. When it comes to material possessions, I reckon there is a recipe for happiness and one for unhappiness.
 
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#48
Paul Szegedin said:
Thanks much, that's useful. Mine has a little bit of a vibration to it that makes it louder -- it's 20 years old -- so it's got to go and a new fan should help a lot. That's what I mean when I say it's quieter pointing down - on account of the vibration. But still, I guess even with your amount of fan noise, you wouldn't record with the mic 12 or 18 inches below that, would you? Maybe put the amp in the next room? My background is in recording engineering, not electrical. My GK, no fan, you can hear a pin drop between notes.



The amp is worth maybe $400 tops used. I got mine for free. I fixed it in a half hour and it's playing well. If it drifts out of bias again, I will re-bias. If it does a second time, I will part it out. I got the amp for FREE. A trip to a tech would exceed the value of the amp, and the problem is just as likely to return. The PO just installed five brand new Sovtek tubes in it, trying to fix it. I can use these tubes in my Fender guitar amps. This whole thing has already ended well. I have fixed many amps and guitars, and have a wonderful collection of them, all of which combined I paid well under $1,000 for. When it comes to material possessions, I reckon there is a recipe for happiness and one for unhappiness.
Click to expand...
Yeah, I wouldn't use a mic to record anyway. You always get such a stronger, cleaner, more malleable signal off the di.
 
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#50
Just note that some fans do not do well in the horizontal position, their lifespan can be 90% shorter and can also be very noisy depending on the bearing type. Be sure to check the manufacturer’s documentation.
 
#52
I've seen a few reports here of SVT-3Pros coming out of the factory biased on the low side. With that said, if your head seems to be operating okay, don't touch it. If there's a constant low-level distortion (audible only at low volume levels), take it to an Ampeg authorized service center for warranty service.
 

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