Bad Tube?


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!







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

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.

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.

ghdprentice, sorry about that. For all I knew you were from the East Coast. :-) Good to know you are not another New Yorker!

Thank you both for the responses. Much appreciated.  Question about the matched voltage of the tubes. The amp manufacturer says to bias the tubes to 40 mA and the tubes I received were a matched set with 30 written on them. What should I bias them to (I biased them to 40 on my meter)

Stop using that tube immediately!    Not worth taking out a bias resistor.   

I am going through nearly this very problem with some premium Linlai KT-88s, one power tube causing very low volume in whichever channel it is placed (Willsenton R8 with Skunkie mods.) whenever in ultralinear but not in triode. Bought as a matched quad, I am hoping for a matching replacement.

tritube, You did it correctly, biasing the tube to 40ma as recommended by the manufacturer. You would have done that with any voltage designation on the tube/box. Tubes can be too 'hot' or too 'cold' and be unable to meet the biasing requirement but this rarely occurs. That has happened to me a couple of times with tubes 'tested' at the factory. You should be OK.

















Sometimes new tubes are lemons it happens as suggested by newbee and ghdprentice contact your seller and get the bad tube replaced.