Signs that a tube has gone bad?

Spaceduck,

Preamp tubes should last a long time (years) but can become microphonic from shaking, etc. It will sound a bit noisey even if the tone is OK. The best way to determine if it is bad is to switch it with another tube you know is good. What type of tube is it? 12AX7? If you need to replace it, there are a bunch of options other than the Groove Tubes you will find at the local music store.

Scott

In this case it's a chicken egg type thing... something else is making the tube melt down... but by now the tube is probably not so much either.

It's a current repair project I'm messing with in my free time.

best,
mike

I don't think it's the tube. Those mic tubes don't even run a full plate voltage and should last forever. Might be the capsule going bad or has dust or something on it.

If you have another mic of the same model also try swaping the power supply to see if that's what's bad.

Look for discoloration (like silvering) at the top or bottom. Over time, the electron stream will knock a lot of material from the anode which shows up as a silver like coating (not the kind added already).

However, from this pic, it looks pretty clean.

Look for solders that may have become loosened, look for filaments in the tube that may have broken.

Well, I had a look at the capsule, and although it could use a good shot of windex and a scrubbin' , I didn't see anything that could be causing any real trouble. Nifty stuff, though, if you've never seen one of these up close:



So if the tube looks ok, the solder joints look ok and the capsule looks ok, what's left? A capacitor (or whatever those funny chicklet looking things are)?

What's so weird is that the noise gradually appeared one day, getting louder and louder by the end of the session. Wouldn't a short cause immediate noticeable problems? Same thing with a bad tube... don't they just suddenly go bad all at once, like a lightbulb?

Btw, yah I checked the cables, the input jacks on my mixer & everything. I was using two identical mics in a stereo x-y setup, and the right ear developed the noise. So I swapped mics (just the mics, nothing else), and suddenly the noise was in the left ear. So whatever it is, it's in the mic. Although I'd sure love to blame it on a $10 cable at this point.

ORIGINAL: foxwolfen

Maybe the connector has gone bad? Is it XLR?

Hey... that's is something to check. In fact I had an AKG SolidTube that developed a bad hum and it turned out that the case was not fitting tight enough together. The screen (head) of the mic was not making good connection with the shell. I had to take it apart and scrape some paint off the joint and tighted the screws real tight to get the hum to go away. Even that didn't really work all the time so I ended up soldering a wire to the screen and running it down to pin 1 of the XLR. It was VERY quiet after that.

ORIGINAL: Spaceduck

Well, I had a look at the capsule, and although it could use a good shot of windex and a scrubbin' , I didn't see anything that could be causing any real trouble. Nifty stuff, though, if you've never seen one of these up close:



So if the tube looks ok, the solder joints look ok and the capsule looks ok, what's left? A capacitor (or whatever those funny chicklet looking things are)?

What's so weird is that the noise gradually appeared one day, getting louder and louder by the end of the session. Wouldn't a short cause immediate noticeable problems? Same thing with a bad tube... don't they just suddenly go bad all at once, like a lightbulb?

Btw, yah I checked the cables, the input jacks on my mixer & everything. I was using two identical mics in a stereo x-y setup, and the right ear developed the noise. So I swapped mics (just the mics, nothing else), and suddenly the noise was in the left ear. So whatever it is, it's in the mic. Although I'd sure love to blame it on a $10 cable at this point.

What is that over at the right were the hole with no screw in it is ? I see some material sticking out over the edge. It even makes a shadow near the edge in this photo.

ORIGINAL: ohhey
What is that over at the right were the hole with no screw in it is ? I see some material sticking out over the edge. It even makes a shadow near the edge in this photo.

Good eyes. That was actually a stray piece of plastic that was hanging over the diaphragm. I used a nail clipper and cut it off. Alas, no difference

ORIGINAL: Roflcopter

In your first picture I see that black wire arcing over the blue white and green there, and connect to the board on a soldering point. Somehow I'd expected that red cable there to be connected just to the left of the black one, but it's not visible if it's connected at all to the board. Is it?

I traced that red wire, and it loops under the circuit board and connects on the other side. The solder connections all look pretty solid (on both sides). Even the resistors look good (don't they usually turn black down the middle if they bun out?)

Jiggled the connector as much as I could, but that didn't seem to affect anything. One thing worth noting... when I switched on the -10dB pad switch, the rumble got quieter. If the problem were in the connector (right at the output), then it shouldn't make a difference if the -10dB pad were on or off, right? The noise would be the same regardless? If so, then I think we can rule out the possibility of a bad connector. Whatever is causing the problem is either at the capsule or the amp stage.

Crazy stuff, I'm actually having fun tinkering around in there, even though it's been fruitless so far. And I appreciate all the suggestions & advice. Thanks guys. But tomorrow I guess I'll send it off to the experts (unless the "tube fairy" magically fixes it overnight...)

ORIGINAL: Roflcopter

One thing worth noting... when I switched on the -10dB pad switch, the rumble got quieter.

Not trying to be facetious, but I think it's supposed to do that. -10dB is quite a bit.

Right, but my point is if the noise were being generated in the connector (or the cable or the mixer, etc) then the mic's -10dB pad would have no effect at all. The fact that the -10dB attenuates the noise proves that the problem happens in the capsule or amplification stage (inside the mic). At least that's what it seems like theoretically...

In fact I'm guessing that the -10dB pad is introduced right at the first stage of the signal path, so wouldn't that tell us that the noise is being generated right at the capsule and not further down the line?