fizzy static through AC30 TB

[frank]

Hi all! I'm new here, and I've searched a bit for an answer, but haven't found one yet. So here it goes:

I have a 90's era AC30 TB, and when I play it at a pretty good volume, (about a third or more) I get a fizzy kind of buzz through both speakers. Because it's coming out of both speakers, I'm guessing that it's not a blown cone. I also would hazard a guess that it's not mechanical.

Could a bad preamp tube cause this? If so, I wonder which would be the first one to swap out.

Can an amp with bad wiring eat preamp tubes.

Do fish cry? just kidding.

Thanks to any and all who can shed a little light on this!

[PRNDL]

Could a bad preamp tube cause this? If so, I wonder which would be the first one to swap out.

Usually the first preamp tube is the culprit, although bad power supply electrolytics are the next on the list.

[acorkos]

it's possible its your speakers. try a different cab to rule them out.

also, make sure all the cabinet screws and chassis mounting bolts are tight. I've encountered more than one of those ac30ri amps which were rattling crazy from loose screws.

[frank]

Thank you guys for responding. I don't think it's a mechanical rattle, as I have put a microphone on both speakers to record, and the same -fizzy sound comes out of both at the same level. (I'll still tighten up all the loosies, though.) Since the annoying sound comes through multiple channels, I changed the PI tube, since that tube is common to all. The sound seems to have gotten better, but it's still there. Maybe a power amp tube?

I'm a little intrigued by what PRNDL said, about the PS electrolytics. I am pretty sure they have never been swapped out. There's markings on the inside the chassis put the date of manufacture in 2000. Could a cap go bad that quickly?

Thanks again. I am sure this is probably a very newb-question, but any help is muy bueno!

-J

[Dave Curtis, dB AudioTech]

Thank you guys for responding. I don't think it's a mechanical rattle, as I have put a microphone on both speakers to record, and the same -fizzy sound comes out of both at the same level. (I'll still tighten up all the loosies, though.) Since the annoying sound comes through multiple channels, I changed the PI tube, since that tube is common to all. The sound seems to have gotten better, but it's still there. Maybe a power amp tube?

I'm a little intrigued by what PRNDL said, about the PS electrolytics. I am pretty sure they have never been swapped out. There's markings on the inside the chassis put the date of manufacture in 2000. Could a cap go bad that quickly?

Thanks again. I am sure this is probably a very newb-question, but any help is muy bueno!

-J

Careful not to overtighten the speaker mounts; you don't want to warp them...

Doesn't sound like bad PS caps to me; is the "fizzy" sound constant, or only when playing? Rule out the tubes first. That complaint sounds like a bad tube, especially if the noise is heard only when playing.

[Enzo]

DOn't rule out the speakers either, PLEASE play through different ones to see.

SPeakers in the same cab couple acoustically. Wire the amp to just one speaker and watch the one with no wires move anyway. Not only that, they are electrically wired together, so anomalies in one can affect the other, especially if they are wired in series.

And it certainly is not a large chore to swap out a preamp tube and find out about it.

[Robert M. Martinelli]

Bad/Cold solder joint?

Hi Frank,
I think yours could be a combination of a mechanical/electronic issue : If your amp is not handwired ( limited run models/reissues ) but a standard AC30 from the 90s, some bad solder on the PCB could be the reason for this, as the volume increases the mechanical vibration inducted by the speakers could cause a bad contact on that solder joint which in turn makes the amp crackle.

This usually happens with wave soldering, ( this is one of the reasons I love handwired amps! ) - To track the bad solder joint down you could run the amp at a volume just below the level at which the problem occurs and tap the PCB with a plastic stick or pen or something similar-chances are that you will be able to identify the "bad spot" by inducting the fault with this procedure.

Another simple check to run is to (lightly) tap the tubes one by one starting with the preamp's input tube to check for microphonics.

Hope this helps - Bob

[frank]

Thanks all. Just wanted to catch up on this thread.

It was a bad power tube. Don't ask me how, since it was new, but a replacement solved the problem....hopefully!

[Dave Curtis, dB AudioTech]

Ah yes. Tubes are generally the first thing to rule out (they're the easiest). New power tubes bad out of the box? Say it isn't so! That's why I get them from someone who burns them in at operating voltages, and then matches them and tests them in an actual amp. They cost a bit more, but I'm buying a little peace of mind, too.

-DC

[Enzo]

I don't care who sold them, power tubes are always the first thing to suspect. Even from the most reliable seller, the tube still had to bounce around the back of a UPS truck across the country to get to you.

[Dave Curtis, dB AudioTech]

Just sayin'. I've been dealing with this guy for over 5 years, now (7?). Not *one* case of infant mortality. And he packs well and ships fast, too.

[Enzo]

I am not disagreeing with you. I am just pointing out that one should never say "Oh it CAN'T be the tube, it's new." A cardinal shop rule is: never think of reasons not to check something.

[frank]

agreed on all points here. Even decent tubes sold at decent shops do go on delivery trucks with not-so-decent shocks.

Also, I am beginning to suspect the tubes went bad because my roomate may have hooked up the AC30 to an 8 ohm speaker I've got.

man, ya can't trust anybody.

[Dave Curtis, dB AudioTech]

I am not disagreeing with you. I am just pointing out that one should never say "Oh it CAN'T be the tube, it's new."

Exactly. New doesn't always mean "good". Someday I may get bit by having so much confidance in my supplier.

A cardinal shop rule is: never think of reasons not to check something.

Another one: Start with the easiest and work towards the more difficult. Tube amp? Rule out the tubes first.

[Gtr_tech]

Combos are very hard on power tubes.....EL84s especially. They get rattled to death in *no* time at high playing volumes. Treat them like strings and replace them when they get rattley. Preamp tubes are not quite as suspect since the structure is smaller and they tend to take the abuse a little better, but they can get rattley too.