Ampeg SVT-3 Pro Output Issue

[bongostealth]

I have an Ampeg SVT-3 Pro that is in like-new condition. I have never gigged with it. The DI signal works great and the amp sounds really good through it. However, today when I connected it to my Ampeg Classic 4x10, I get no output. After a few seconds I might get an audible hum noise followed by some light and random crackling. But that's it. I don't understand what the issue could be. It's not my cabinet or my cables because I connected three other different amplifiers and they all work great through my cab.

Any thoughts??

 

[agedhorse]

Try inserting a plug into the effects return jack a few times and see if normal operation is restored.

There are a lot of possible causes, this either confirms or eliminates one of the most common ones.

 

[bongostealth]

Try inserting a plug into the effects return jack a few times and see if normal operation is restored.

There are a lot of possible causes, this either confirms or eliminates one of the most common ones.

I just tried this and it unfortunately did not work. The output problem is still the same...

 

[agedhorse]

What happens when you send a signal from your bass directly into the effects return jack?

 

[bongostealth]

Nothing... I tried that and same results...

 

[agedhorse]

That narrows down where the problem is and where it isn’t.

I will have to look at the service docs to see if this model has a tube driven mosfet output or not, I don’t remember.

 

[bongostealth]

That narrows down where the problem is and where it isn’t.

I will have to look at the service docs to see if this model has a tube driven mosfet output or not, I don’t remember.

Would a serial number provide clues?

 

[agedhorse]

Would a serial number provide clues?

No, I wouldn't have access to anything useful from that since I never worked for Ampeg (though I did work under the late Bill Hughes, who designed most of the SVT amp, while at a different company)

 

[agedhorse]

I looked at the service docs, and this amp does have a tube driven MOSFET power stage. It's possible that either V1 or V2 on the power amp board has failed BUT the first thing to listen for is a click about 4 or 5 seconds after power-up. If you don't hear that click, it indicates a failure in the output stage or in the protection circuit itself. If you don't hear the click, don't do anything more. Contact qualified service tech who is experienced in this type of amp, it's not something for a tube amp only guy to be messing with.

 

[bongostealth]

I looked at the service docs, and this amp does have a tube driven MOSFET power stage. It's possible that either V1 or V2 on the power amp board has failed BUT the first thing to listen for is a click about 4 or 5 seconds after power-up. If you don't hear that click, it indicates a failure in the output stage or in the protection circuit itself. If you don't hear the click, don't do anything more. Contact qualified service tech who is experienced in this type of amp, it's not something for a tube amp only guy to be messing with.

I do hear a click. After the click is when it either starts to hum or I get some slight crackling or both... some slight popping at times, too.

 

[agedhorse]

I do hear a click. After the click is when it either starts to hum or I get some slight crackling or both... some slight popping at times, too.

Ok, that's good news. The most likely cause it either V1 or V2 on the POWER AMP board. V1 is a 12AX7A and V2 is a 12AU7, you must use the same tube types in this circuit. I recommend changing one at a time, starting with V1. It's likely that only one is defective/failed.

If you are unsure of any of this, find a tech that's experienced with this particular output section. It's a fairly unusual topology.

 

[bongostealth]

Ok, that's good news. The most likely cause it either V1 or V2 on the POWER AMP board. V1 is a 12AX7A and V2 is a 12AU7, you must use the same tube types in this circuit. I recommend changing one at a time, starting with V1. It's likely that only one is defective/failed.

If you are unsure of any of this, find a tech that's experienced with this particular output section. It's a fairly unusual topology.

Is there a certain brand that you recommend?

 

[bongostealth]

Also, is this picture correct as to where to find the power amp tubes?

 

[agedhorse]

Yes.

 

[bongostealth]

Yes.

So I found the culprit. The 12AX7 power amp tube was blown. I replaced it with another exact JJ 12AX7 and the amp works like a complete champ! And another benefit was that my fan was always loud. But now that I replaced the blown tube, my fan is near silent!

Thank you so much for your expertise and help, agedhorse! You saved me time AND money!

You're a true legend!

 

[agedhorse]

Glad this worked out.

For those members who guess and then argue when presented with why their guesses are implausible, this is a good example of how structured troubleshooting works based on understanding how the circuits work, analysis and process of elimination.

When a successful qualified, experienced service tech approaches a problem like this, following a structured path to first identify the area(s) where the failure occurred, then narrow it down by first excluding everything it can’t be (differential diagnostics). Then, analyzing what it might be and coming to the most likely causes based on the results.

Fortunately, this was a simple cause with a simple solution, but the same method (plus test equipment) applies with more complex cases.

I also appreciated your respectful communication without arguments based on AI and internet expert search results (which are mostly wild ass guesses anyway).

 

[SactoBass]

What an enjoyable thread to read! Here's a guy (bongostealth) asking TB for help with his malfunctioning amp. Andy chimes in and, using proper diagnostic methods, works with bongo step by step to solve the issue. Andy, you are a rock star. Thank you for your willingness to share your knowledge and experience in this forum!

 

[bongostealth]

Glad this worked out.

For those members who guess and then argue when presented with why their guesses are implausible, this is a good example of how structured troubleshooting works based on understanding how the circuits work, analysis and process of elimination.

When a successful qualified, experienced service tech approaches a problem like this, following a structured path to first identify the area(s) where the failure occurred, then narrow it down by first excluding everything it can’t be (differential diagnostics). Then, analyzing what it might be and coming to the most likely causes based on the results.

Fortunately, this was a simple cause with a simple solution, but the same method (plus test equipment) applies with more complex cases.

I also appreciated your respectful communication without arguments based on AI and internet expert search results (which are mostly wild ass guesses anyway).

So now I checked my other Ampeg amp, my SVT-4 Pro, to make sure I don't have any problems with that one, and low and behold, my volume is extremely low. The amp turns on, the fan is silent, there is no clicking or popping. The EQ sliders and graphic eq section work, the preamp section works, but I have to crank the volume nearly all the way up just to get some "normal" volume. And when I do this, the amp starts to fart. This amp is also like new and I have never gigged it.

Any thoughts, doc?? I tried all of my other amps and they all work like champs. This SVT-4 Pro is the last one giving me issues....

I seem to be on a roll here! But I prefer these issues rearing their ugly heads now and NOT during a live situation!

 

[agedhorse]

The signal routing in the 4 Pro is much more complicated, and it's easy to set the routing and mode switches incorrectly.

The first step here is to verify the proper setup, it may not appear obvious (even to me).

Start with the amp configured for stereo (left and right), send an input to the left input and see if you have output on the left speaker output. Do the same for the right channel. If you have signal, then it's almost certain that there's a configuration issue with the signal routing switches (common).

What's concerning is the lack of relay click engagement which can indicate a major internal fault. Depending on the design of the fan's proportional controller, it may not start up when the amp's cold (or it may be the symptom of yet another problem).

Start with the configuration switches... you may want to double-check how the amp is supposed to work by looking at a copy of the owner's manual.

 

[bongostealth]

The signal routing in the 4 Pro is much more complicated, and it's easy to set the routing and mode switches incorrectly.

The first step here is to verify the proper setup, it may not appear obvious (even to me).

Start with the amp configured for stereo (left and right), send an input to the left input and see if you have output on the left speaker output. Do the same for the right channel. If you have signal, then it's almost certain that there's a configuration issue with the signal routing switches (common).

What's concerning is the lack of relay click engagement which can indicate a major internal fault. Depending on the design of the fan's proportional controller, it may not start up when the amp's cold (or it may be the symptom of yet another problem).

Start with the configuration switches... you may want to double-check how the amp is supposed to work by looking at a copy of the owner's manual.

So just as this morning, I was once again able to figure out the problem thanks to your insight, specifically here being regarding stereo configuration. I plugged my Speakon cable into the mono input, as I usually would. However, I had failed to notice that there is a small square button for stereo or mono bridge mode. This button was in stereo mode. I pushed on it and it pushed inwards for mono bridge mode.

Problem solved. This is all it was. Amp works as intended.

Thanks again, agedhorse!!