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Yet another MP-3 question

71.186.199.100
Posted on August 29, 2009 at 21:44:55
Frank
 I read (either in the archives or the manual) that after awhile, the 12AT7s start to become "noisy" and need to be replaced.

Is this a steady-state noise that can be heard without a signal, or is it noise that is added when a signal is present?

Also, how many hours use is "a while"?

If anyone could give me a heads up on what to listen for, it would be appreciated.

Thanks, Lew and Ralph!, posted on August 31, 2009 at 16:52:19
Frank
 If Ralph's 3K hour estimate is in the ballpark, I had better start looking for new 12AT7s -
In about five years!

RE: Yet another MP-3 question, posted on August 31, 2009 at 08:17:46
Ralph
Manufacturer

Posts: 1157
Location: Minnesota
Joined: April 24, 2002
Contributor
  Since: April 1, 2002
 We find that 12AT7s in general don't go much more than about 3000 hours- about 4 months continuous. For that reason I don't recommend running anything that uses them on a 24/7 basis, unless you're not concerned about tube replacement.

Lew is right- the tubes slowly loose transconductance, the effect is that of more noise/less signal. The Chinese tube is the most consistently quiet, so much so that we use the MP-3 with the ZYX Universe here at the shop, and noise is not a problem. We suspect that the grid of the tube gets contaminated as it gets noisy. I've given noisy 12AT7s are hard thump (and only once) on the side and had them be quiet for a good year longer- almost as if the contaminants fall to the bottom of the tube where they are harmless.

We **don't** recommend any NOS tubes for the phono- 99% of what is available seems to be way too noisy (the really quiet NOS stuff was sold off the shelves long ago). OTOH, JJ and EI both make better sounding 12AT7s than the Chinese, but they have to be selected for low noise and microphonics, which is not that hard (usually you find 5 good tubes out of 10 or so). I would stay away from the EH tubes- short life, hard microphonics and noise have meant in the past that they are nothing but trouble!

RE: Yet another MP-3 question, posted on August 31, 2009 at 07:34:52
Lew
Audiophile

Posts: 2686
Location: Bethesda, Maryland
Joined: December 11, 2000
 As tubes age, they tend to lose transconductance. This alters their gain characteristic such that the signal to noise ratio is likely to narrow a bit. So to that extent, all tubes become more noisy as they age. I don't know exactly how to describe the "noise" part of this equation. It is not the same as "hum", and you can live with it much easier than with hum. The number of hours of use before noise becomes noticeable will depend upon many things, not just the type of tube. It will also depend upon how many Watts are dissipated across the tube, a function of the plate voltage and current. Here you are in luck, because Ralph runs the tubes VERY conservatively. And it will depend upon brand. The NOS tubes from days of yore are probably made better than modern ones and might be expected to last longer. Chinese-made 12AT7s can sound great but don't last long, in my own experience.

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