2. amps-preamps
  3. 186105-184156

Bad Tube?

Hello,

I am new to tubes so forgive this question if it seems silly.  Last month I picked up a pre-owned Muzishare x7 integrated tube amp.  I enjoy the sound very much, and it is functioning fine, but I read that upgrading the stock tubes could improve the amp even more.  I purchased a matched quad of Shuguang kt88_Z black treasures power tubes  as well as a pair of electro-harmonix 12ux7's and a Gold Lion 12ax7 (I haven't touched the rectifier tube).  I dropped my bias to zero on all the power tubes, replaced the tubes, and re-biased  the power tubes.  They sound much better than the originals immediately.  Still detailed but weightier, if that makes sense.  My problem is, after 1/2 hr, I noticed audible static from my right channel.  Checking the V4 bias meter showed that the voltage was not settling at 40 (needle is fluctuating between 30 and 60).  I zeroed out the bias on all the tubes again and after letting everything cool off for a bit, swapped my V4 tube into the V1 position (left channel).  Starting the tube up and biasing them all again, I now hear static in the left channel and the V1 bias meter is not stable.  I have only 1/2 hr on the tubes.   Is that tube bad?  or will it settle in? Or did I do something wrong?

Thanks so much for helping a new tube enthusiast out!

Ed

 

tritube
newbee
4,767 posts
 

Sounds like a bad tube to me. Your vendor should give you a new one.

ghdprentice
6,965 posts
 

@newbee 

 

+1

This happens occasionally, most often with new tubes. Likely once replaced, all will be well for the rest of the normal tube life.

tritube OP
5 posts
 

@ghdprentice  Do I need do ask the vendor for at least a matched pair?

 

 

ghdprentice
6,965 posts
 

Typically, they will know what  they sent you and will send you a matched single tube. Or the voltage (?) of your set will be written on the box your tubes came in… and you can send them the parameter to match.

newbee
4,767 posts
 

In case ghdprentice has gone to bed, if your tubes were bought as a match pair your vendor should be able to match the tube he sold to you. Each tube (or the box in which it came) should have the tested #. But, even if he can't/won't give you a matching tube, setting an identical bias should get you most of the way. Thing about a match pair is that in use the tubes won't remain matched forever any way, ergo the need to check your tubes bias regularly depending on hours of use.

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