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Old 15th October 2005, 06:30 PM   #1
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Default Rattling Tube

I have a pair of valve art 300b tubes in my amps right now. One of them, when tapped gently (or when moved around) seems to rattle a little bit. It is not microphonic, and otherwise seems to work fine. Is this something to be concerned about or should I simply not tap on the side? The other one does not do this.
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Old 15th October 2005, 09:56 PM   #2
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Unlike transistors and ICs, the internals of hollow state aren't embedded in anything solid. So, therefore, all those delicate metal parts will rattle a bit in just about every VT. That's why microphonics is a bigger problem than with solid state.

So long as the VT's working correctly, don't worry about it, and don't whack on it.
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Old 16th October 2005, 07:46 PM   #3
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Default I shouldn't have dropped it.

You should worry! I have a 12J5GT with a dirty great lump of loose glass plus some smaller fragments rattling around inside. Despite that, it's measurably one of my best valves!
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Old 18th October 2005, 05:01 PM   #4
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I bet if you did a signal/noise ratio test, the output tubes will
< sing quite nicely >......... I get this quite often with 7199's (lovely little versatile tube) but the microphonïcs can be annoying. Unfortunately the only remedy is replacem't.

Some time past I had a batch of EI KT90's which had bits of loose swarf inside; at the time I was puzzled why the B+ fuse was continually popping I couldn't believe myself it was the tubes.
So we have it.


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Old 18th October 2005, 06:06 PM   #5
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The manufacturing quality of some current production (and later vintage NOS) is not what it used to be. It is possible that some debris was left inside the envelope when the tube was made. If the debris is not metallic, it should not cause a problem. If the debris IS metallic, it could cause fireworks if it gets near the electrodes inside the tube. I have seen tubes with bits of mica or glass inside, and I have seen tubes with loose metal inside.

I have a old 45 where the getter ring is loose inside the tube. Since it works good, I am not going to toss it, but extreme care is needed to use it. I also have a 45 that has an intermittent short between the filament and the grid, the whole grid structure is loose. I hook up an ohm meter, and whack the tube until the short goes away, and then use it!

It is also possible for the glue that holds the base to the glass to get loose, this type of rattle is harmless.

If you can't tell what is loose, be nice to the tube. A short can blow up some expensive parts.
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