Biasing signal tubes on a tube power amp/Quicksilver M120
Took possession of a new pair of Quicksilver M120 power amps with the KT150 tube upgrade a few weeks ago, am in the process of break in with about 70 hours so far. About 30 hours in I noticed that a 12AU7 tube had started to flare up when that amp was turned on so swapped that tube to the other amp and it followed the tube, I contacted Quicksilver and he told me that this is not a concern but to please switch that tube back to it's origional amp as the amp was biased for that specific tube, now my question is if I wanted to tube roll these 12AU7 or 12AT7 tubes am I out of luck and what happens when it is time to replace them as they wear out, are my tube rolling days over or are they just limited? What say you?
Thanks for your replies, Ghasley and Oddiofyl. The pedigree on these tubes is uncertain. I bought them a few years ago from an ebay seller with a very good feedback score, but he is no longer reachable via ebay. Hmmm. Here’s a description of the tubes, please see my two questions at the end:
The tubes are printed with "General EL84" in yellow. Each tube is also printed with the number 973 in smaller type. Inside the glass, they have a saucer-like pan beneath the top getter flash. Beneath the saucer they have a piece of metal bent into a horseshoe shape that hovers just above the top mica. I think I have seen Russian tubes with that gizmo. The "Generals" came in oldish-looking yellow boxes printed in black with "General" and "Rohren," the latter of which makes me wonder if they were made in (East?) Germany. Also must say that in a very brief try-out, they sounded superb, but I am reluctant to use them due to the biasing issue.
My 2 questions: 1) Do new tubes often deteriorate over time in the box? 2) Does a tube’s ability to reach or hold a particular bias level deteriorate with use?
I’d be grateful for any info about what might limit a tube's ability to accept and hold a particular bias level -- thank you!
Would be grateful if someone could answer a question about dubious biasing behavior in EL84 power tubes. An octet of EL84s purchased as "NOS" could not be properly biased in my Manley Labs Mahi monoblock amps. Some of them biased easily at the mfgr’s recommended 250 millivolts, but others would not. On those tubes, the biasing screw would max out while the multimeter read only in the high 100s or low 200s. (Swapping a tube to a different socket did not make a difference.) Does this mean those "NOS" tubes were probably already heavily used? Thanks for any info about this.
@tooblue Did you figure out if you need to balance the 12AU7 on the QS-120 ? I just purchased a pair of QS Mono 60 from QuickSilver ( not on the Website yet) I was wondering if i could tube role the 12AU7 without balancing ? A_shah
@seikosha , @yogiboy , For clarity on my original post, I used the word bias when referring to the 12AU7 and 12AT7 tubes on my newly acquired Quicksilver M120 amps when the term balanced should have been used.
There is nothing in the manual for that amp about having to bias the input tubes. Perhaps the output tubes would need to be biased when the input tubes changed and that's what Mike was referring to.
You may want to double check as I do believe i read in another thread that the QuickSilver amps do need the small signal tubes biased when replaced, also if quicksilver told you to put the original tube back into the original spot in the original amp because they biased it for that spot then it would indicate to me that they are biased. I do know many(most) tube amps do not need the small signal tubes biased but you may want to double check, and i'm sure your dealer can do this work for you.
@tubegroover , I am replacing a pair of VTL MB125s that are 21 years old, would have replaced with a like set but were out of my pay grade and took a chance on these, am very happy, just really hate break in time but so far have already showed themselves to be keepers.
Enjoy those amps tooblue, if they have the same sonic characteristics, with more power being the difference as the silver 88’s I am happily listening to, you are in for a real long term treat. I have owned many tube amps through the years but the Quicksilvers REALLY hit all the right notes, very special, especially in light of what you get for your money.
@audioconnection , I am really liking the amps but still a bit confused as to the biasing of the smaller tubes, the power tubes are no problem biasing but I am not aware of the process to bias the smaller tubes.
Many 12Au7 types have this flash on turn on. No concern. Just make sure you check the bias when you swap in any new tubes. Quicksilver Great company you should enjoy for many years.
You must have a verified phone number and physical address in order to post in the Audiogon Forums. Please return to Audiogon.com and complete this step. If you have any questions please contact Support.
