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VT-231 tube hum

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Posted on January 6, 2017 at 11:20:05
akolegov
Audiophile

Posts: 979
Location: Russia
Joined: July 1, 2010
 I installed a pair of Sylvania 6SN7GT/VT-231 tubes in my amp. The tubes were purchased as NOS from a reputable dealer several years ago. One of the tubes instantaneously produced a very loud low frequency hum, which means that it is faulty. I wonder what could be the reason for such failure, if the tube was kept in a drawer for several years. The same thing happened earlier with one of JAN GE 6550 tubes and one of Siemens/RCA 6922 tubes.

 

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Just out of curiosity, posted on January 6, 2017 at 17:42:33
FenderLover
 Did you ever play with the global negative feedback in this amp?

 

RE: Just out of curiosity, posted on January 6, 2017 at 22:36:15
akolegov
Audiophile

Posts: 979
Location: Russia
Joined: July 1, 2010
 No, I didn't. I just don't know how to do it.

Only three or four of approx. 50 tubes in my stash behaved like that. I wonder if they might have lost vacuum during storage - I mean, may it happen in principle?

 

Dunno about that, posted on January 7, 2017 at 00:57:15
FenderLover
 I can't remember any which have lost vacuum sitting in storage.

 

RE: Dunno about that, posted on January 7, 2017 at 02:24:48
akolegov
Audiophile

Posts: 979
Location: Russia
Joined: July 1, 2010
 I used GE 6550s from time to time in Cary SLI-80, then put them into storage, and when I installed them again later, one of them produced hum (GE 6550). I threw it away yesterday.

One of VT-231s was probably faulty from the start, though I am sure that the seller tested all of them before shipment.

 

RE: Dunno about that, posted on January 7, 2017 at 05:52:59
sony6060
Audiophile

Posts: 1465
Location: USA
Joined: August 8, 2014
 Likely a cathode to filament short.

 

RE: Dunno about that, posted on January 7, 2017 at 11:35:44
Jim McShane
Dealer

Posts: 5910
Location: Chicago
Joined: January 13, 2003
 The likely cause of your hum (based on your description) is heater to cathode leakage. If you have AC powered heaters the leakage can induce a hum in the tube.

Try running the tube for a few hours then let it cool and check it again. And be sure the base and pin area is VERY clean.

Go see if the GE you tossed out is retrievable too - it's definitely worth saving!

 

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