Posts: 3156
Location: Winnipeg, Manitoba
Joined: September 1, 2005
Contributor Since: March 1, 2018
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A tube either "is" or "isn't" microphonic (*see note at end of post). However, after it has been warmed up, a tube may physically seat itself better, literally "dampening", or "reducing", the amount of microphony.
Plus it is not just the tube that has warmed up, but the entire component. So now everything has tightened up. It's like a rough idle in a car that goes away after warm up. The tube is dampened, and the entire component is less prone to transfer mechanical vibrations to the tube. All good.
The dilemma is, did the tube noise disappear because the warmup/dampening reduced the microphonic, or did a better connection allow the tube to work as it should?
Once the component has warmed up, and the noise has disappeared, there is a test you can try. Gently tap the suspect tube with the eraser end of a pencil. If it "rings", it is microphonic. If it "thuds", it is normal.
Neither is a huge problem if you are happy with the overall sound in the end.
Some of the finest signal tubes are inherently microphonic. Yet they sound wonderful. Some, like Siemens, will even send a "ping" through your system when turning on/off a line selector or mono-stereo switch. Some dealers will even warn you of this, and that it is normal. In fact these tubes depend on their microphonics in a good way (not to the extent of whistling...:) Dampening them with rings does reduce the "ping", but it also takes away some of the magic.
If tapping with the eraser end you hear a "thud" after warm up, it means the tube is operating normally, not microphonic, but something was impeding the connection upon cold start. "Fixing" itself after warm up. You can leave it as such, or you could continue to try and address the situation so it is fine both on cold start or warm up.
My entire system remains on mute for a minimum of 30 minutes. It is dead silent upon unmuting in both the phono and line stages. If I didn't allow it to warm up, I would likely hear a cacophony of ticks, pops, hisses etc. Exaggerating a bit, but whenever I have not allowed a warm up, you can hear the equipment stretching before the race.
Sorry to be so wordy and hope some of this helps.
Enjoy those tubes!
Jonesy
(*all tubes are microphonic to some degree. For the sake of this discussion, we are using microphonic in the "extreme" sense.)
"I know just enough to get into trouble. But not enough to get out of it."
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