2. amps-preamps
  3. 131894-1659

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?
tooblue
tcdandy
7 posts
 

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.

ghasley
2,487 posts
 

Or it could mean that they degarded over the preceding 50 years. Did you buy them from a reputable person/business? Tested?

oddiofyl's avatar
oddiofyl
3,778 posts
 

Not all EL84 are the same.  

http://www.audiotubes.com/6bq5.htm

tcdandy
7 posts
 

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!

 

 

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