Help! Unidentified buzzing in Ampeg SVT 3 Pro

[Wgtroutman]

hello all,

I've been charged with the task of figuring out what's wrong with this amplifier and determining what needs to be done to repair it. There is some weird buzzing that apparently happens in the higher register of the amp at random times (I haven't played through it yet). I've perused the forum and the possibilities seem to be bad biasing or failing components inside of the amp. Below is a low quality sound clip that the guy who asked me to figure it out made for our local music shop. They couldn't find the problem.

One tube has been replaced (not sure if it was the 12ax7 or the 12au7) since the problem has started and it didn't fix it. As far as I know, the buzzing is heard even from the direct out.

Thanks a ton in advance. I know if anybody can figure this out it's the experts here on the forum. I'm hoping to use this as a learning experience as well as do a favor for a friend.

 

[sincity]

Subscribed. I get a very faint fuzz from the high register notes as well. Mainly above the 12th fret. I know, we should never be that high up the fret board anyway.

 

[Wgtroutman]

Bump

 

[frankenp]

Can you isolate the problem at all?

- Do you still have a problem with the EQ on vs. off?
- Is the noise heard on the preamp out?
- Is the noise heard on the DI... pre vs post settings?
- Is the noise present if you use another preamp into the power amp in?
- Noise present on all frequency knob settings?
- Is the peak LED lighting up?
- How is the sound from the tuner output? This is buffered directly from the input jack it should be nice and clean.

If it seems like a tube, try swapping one at a time.

Use the block diagram on page 13 to help you isolate the problem: http://www.ampeg.com/pdf/SVT-3PRO_OM.pdf

I had a similar problem on a Trace Elliot amp. It turned out to be a cracked solder joint on the input jack and a dirty tube socket. The cabinet vibrating the amp caused the amp to fail intermittently with a slight loss of volume and a fuzzy tone.

Best of Luck.

 

[Jim C]

I had a 3 Pro that went to the shop twice for bad solder traces.
It worked for months after the first repair and then had another noise problem a few months later; it was in a different spot.
Not sure that is your problem but the shop told me this was not uncommon.

 

[Jerrold Tiers]

If you have it in the tuner out, it;'s right at the front end. if in the line out when on "post", but not tuner out, it's later, but still in the pre.

Look around the mute circuit... the mute FET can be bad, possibly from a very strong static zap to the input.

if it seems to be later in the circuit, check all the points where the clip light has sensing.... if something goes bad there, it will clip the signal.

 

[pop13]

Hello, I have almost the same symptoms as Wgtroutman
a bit of fuzz, especially on the mids and highs, crackling, a lot of hum especially when the equalizer is on, and when the mids and highs pots go beyond the mid position and even more so when they are in the max position.
I looked into where this could be coming from by isolating the inputs and outputs, so :
- When I send the line out to another power amp (hot rod deluxe guitar in this case because I didn't have any other bass amps on hand) the sound is clear with no fuzz, no crackle, but with lots of hum when I turn the master to max.
- When I output from the tuner output to another power amp (hot rod deluxe guitar in this case again): no sound at all.
- And when I connect the output of the pre to the input of the power amp with a jack on the rear panel: Miracle!!!! The sound is clean, powerful and dynamic, no hum, no fuzz, no crackle...???

Do you have any idea where it might have come from?
thanks

 

[pop13]

ha I almost forgot a noise like “clac” or “pop” is heard when I press the mute button

 

[anderbass]

Have you already tested if patching the effects loop with a guitar cord instantly solves the issue?

 

[pop13]

Not yet, but the issue is solved by patching with a cord "pre out" "power in"
It's the same thing not ? Perhaps not the same routing…

 

[anderbass]

Not yet, but the issue is solved by patching with a cord "pre out" "power in"
It's the same thing not ? Perhaps not the same routing…

Cool, then you've diagnosed that your problem was being caused by the shunt inside your "power in" jack having a poor connection between your pre-amp & power-amp sections. (it's fairly common for effects-loop return jacks to develop the same problem)

Sometimes repeatedly inserting a plug in and out of these return jacks will restore a good connection.

Some guys help that process along by putting a little Deoxit D5 on the plug before repeatedly inserting.

I've had this problem with a few of my older amps, and just solved the problem by permanently patching the loop with a short angled cord made for guitar pedals.

I've circled the shunt in this pic, that little shunt is what has to make a perfect connection In-between the pre-amp and power-amp sections, anytime your NOT patching any external effects devices into those loop jacks... and those little shunts can fail to make a good connection because of dirt or corrosion/oxidation issues.

PS: I don't recomend spraying any type of electrical cleaner into these jacks, because it can result in damage to other nearby componets. That's why i solve the problem by permanently leaving a short angled guitar pedal in-between the send & return jack.

 

[pop13]

Actually no, I made an error in judgment
The drop in hum and hiss was due to the “ground lift” when I used a jack cord to connect the “pre out” to the “power in”.
I retested everything and it's not working, I still have saturation and also a lot of hiss and hum when I set the medium potentiometer outside the midi position and I also have a lot more hiss and noise when I engage the equalizer even if I leave it flat.
I'm leaning towards a bias adjustment as a priority, but also towards tired components.
Anyway, I'll take it to a technician.
Thanks

Translated with DeepL.com (free version)

 

[Marcosail]

Ciao a tutti,

Mi è stato affidato il compito di capire cosa c'è che non va in questo amplificatore e di stabilire cosa bisogna fare per ripararlo. A quanto pare, si sente uno strano ronzio nel registro acuto dell'amplificatore in momenti casuali (non l'ho ancora provato). Ho esaminato il forum e le possibilità sembrano essere dovute a un bias errato o a componenti difettosi all'interno dell'amplificatore. Di seguito è riportato un clip audio di bassa qualità che il tizio che mi ha chiesto di risolvere il problema ha realizzato per il nostro negozio di musica locale. Non sono riusciti a trovare il problema.

Ho sostituito una valvola (non so se la 12ax7 o la 12au7) da quando il problema è iniziato, ma non è stato risolto. Per quanto ne so, il ronzio si sente anche dall'uscita diretta.

Grazie mille in anticipo. So che se c'è qualcuno che può risolvere questo problema, sono gli esperti qui sul forum. Spero di poter usare questa esperienza come lezione e anche come favore a un amico.

Ho esattamente lo stesso problema con il mio Ampeg SVT 3 Pro. Come l'hai risolto?