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Do I have a microphonic tube?


Dan T.

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Actually what does it even mean when a tube goes microphonic?

 

My problem is I have an Ampeg SVT 4 Pro and when I run it in the Monobridged setting with a correctly wired speakon cable to my ampeg 8x10 I am getting a loud almost "gunshot" like noise when I play through my amp. Has anyone ever encountered something like this? The ampeg board which sucks and has no answers for me had a thread where someone was talking about the tubes being microphonic in a similar case.

 

Anyone got any help/advice before I take my amp to the service guy and sound like a total idiot?

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exactly today the guitar player for my band took his amp to the shop because he was getting a weird farty noise with every E he played, and it turned out to be a tube problem. So i guess yours is too.

 

Good luck.

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Originally posted by Dan T.:

anybody?

Microphonic means that the tube is acting like a microphone, i.e. it's picking up vibrations and amplifying them as well the amplifying the input signal. If you turn the amp up without plugging in your bass, and hear sounds from the speakers when you tap (or bang) on the amp casing then the tubes have gone microphonic. The more gain you use, the more obvious it'll be - with overdriven guitar amps it's very obvious as the amp can end up feeding back without having a guitar attached.

 

Alex

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c. alex, your wealth of knowledge always impresses me. If there were you bass guitar/music query category on jeopardy (american quiz show) you would definitely sweep the category.

 

dan t.

 

Diagnosing and fixing the problem yourself way void your warranty. I'd recommend just bringing it to a certified repairshop as well as calling ampeg customer service.

 

jason

2cor5:21

Soli Deo Gloria

 

"it's the beauty of a community. it takes a village to raise a[n] [LLroomtempJ]." -robb

 

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Speaking as a "wise ass" :D with a tainted background in things electrical, let me take a "shot" :D at this... (get those darts ready, guys, I'm about to get stupid again..)

 

Originally posted by Dan T.:

Actually what does it even mean when a tube goes microphonic?

Over time, one of the voltages, usually the one feeding the "plate", changes in relation to the voltage on the "anode/cathode" path, so that more current passes through the tube than it was designed to handle. The impedance drops while the tube become more efficient to the point that it can take stray magnetic fields and amplify them where it wasn't intended to in the first place - I think the phenom is referred to as "stray capacitance". Too much current going through essentially a non-shielded electronic device, you get a microphone, or a tube in the process of breaking down.

 

Something similar happens when the magnetic containment field breaks down in a warp-core engine, allowing matter and anti-matter streams to integrate dangerously, creating the famous "warp-core breach" introduced in "Star Trek: The Next Generation" .

(are we laughing yet?) :D

 

Originally posted by Dan T.:

...I am getting a loud almost "gunshot" like noise when I play through my amp. Has anyone ever encountered something like this?

The "gunshot" is an excess amount of current being introduced into the amp through the power cord in the first half-second before the output power capacitors are able to regulate the load by storing the excess energy. In the first half-second when you hit the switch, nearly all the power produced by the amp can surge right through into the speakers. This is why they tell you to make sure the power/gain is down to 0 or that the standby switch is engaged before turning the amp on. In your case, it's happening when your playing is combined with some component failure (probably capacitors but also likely to be a rectifier or worse) to allow more-than-enough power to hit your speakers. There's also the possibility that certain resonance frequencies get to pass through the amp at full power, accounting for an intermittent "gunshot" to happen. Here, bro, you be talkin' a good 2-4 hrs. parts and labor to fix it, so check your local service rates and that's the estimate it'll come out to.

 

Originally posted by Dan T.:

Anyone got any help/advice before I take my amp to the service guy and sound like a total idiot?

Basically, if you walked into a shop and talked like me, they'd probably throw you out or overcharge you on principle just to see if you really know what you're talking about. Wise ass that I am, I've learned not to talk like this to a service person. Treat him like a professional, because he IS. Likewise, he should be treating you the same, because you is the man who's gonna feed his family or make his car payment next month.

 

What I do is describe the symptoms within a minute or two and allow the service guy to ask me his own set of questions. By the questions he asks me and the amount of information he's willing to share with me, I have a clue as to whether (a) he knows jack, (b) he respects I'm not Paris Hilton bringing her poodle in for a manicure. If I can get him to give me a ballpark estimate as to time in shop and potential cost, or if he's willing to call me after looking at the amp to give me the full story and ask my OK to fix it (instead of making me sign on the dotted line that I agree to pay whatever he decides will be the final cost of repairs no matter what) that's a must.

 

OK, crew, I'm ready for my online spanking!

I can only hope some of this will save you time and money at the repair shop.

:D

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Originally posted by Fred the bass player:

The "gunshot" is an excess amount of current being introduced into the amp through the power cord in the first half-second before the output power capacitors are able to regulate the load by storing the excess energy. In the first half-second when you hit the switch, nearly all the power produced by the amp can surge right through into the speakers. This is why they tell you to make sure the power/gain is down to 0 or that the standby switch is engaged before turning the amp on. In your case, it's happening when your playing is combined with some component failure (probably capacitors but also likely to be a rectifier or worse) to allow more-than-enough power to hit your speakers. There's also the possibility that certain resonance frequencies get to pass through the amp at full power, accounting for an intermittent "gunshot" to happen. Here, bro, you be talkin' a good 2-4 hrs. parts and labor to fix it, so check your local service rates and that's the estimate it'll come out to.

thanks for the lesson fred. I think that gk has addressed this phenomena in the last two series of their amplifiers by putting the amp through a self diagnostic stage before a signal is sent to the cabs. I would guess that other amps that have a long power up time (aguilar) have something similar as well.

 

jason

2cor5:21

Soli Deo Gloria

 

"it's the beauty of a community. it takes a village to raise a[n] [LLroomtempJ]." -robb

 

My YouTube Channel

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Originally posted by Gospel5theZealot:

thanks for the lesson fred. I think that gk has addressed this phenomena in the last two series of their amplifiers by putting the amp through a self diagnostic stage before a signal is sent to the cabs. I would guess that other amps that have a long power up time (aguilar) have something similar as well.

Thank you, G5tZ! I learned all that back in the late 70s, but it's good to know they're building overload protection like that in amps nowdays. I'm gonna have to spend more time browsing these sites, but it helps when a fellow forumite enlightens you. Arigato!

:wave:

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