Hum from 6U8 tube.....defective tube???

[Audible Nectar]

I suspect that I have a problem with a 6U8 tube.

I recently took possession of a newly updated McIntosh MX-110; in preparation for such, I purchased a pair of Telefunken 6U8's for the unit.

After rolling the Tele 6U8's into the unit, the system exhibits a hum from one channel. I suspected a bad tube, so I powered down the preamp then reversed the two 6U8's - the hum moves to the other speaker.

Also of note, the 6U8's that came with it are also hum free.

All of this looks, smells, and sounds like a bad tube. Before I begin asking the seller of the tubes to resolve this, I thought it best to run this by the forum to see if I am missing anything obvious (except for not buying tubes from ebay )

Should this have been obvious to a tube seller with a tester, or would the tube need to be tested in gear to know this would be a problem?

 

[ron-c]

If it looks like a duck... You might want to wire brush the tube pins to make sure they are clean.
If it is determined that the tube causes the hum the way to fix it is to put the tube on a cement surface and step on it. Then it is fixed.
Ron-C

 

[dshoaf]

Right you are, Ron

If there's any question about a tube's stableness, its outta here. While I don't necessarily apply excess force with the foot, I do *not* return suspected marginal tubes to by bin of known good ones.

Cheers,

David

 

[Audible Nectar]

In fact, it is outta here! Seller is refunding on the bad tube, so at least that turned out OK.

Now finding replacement Tele's might be a tad more difficult, without paying out the wazoo for 'em

I appreciate the help, as always

 

[WhiteSE]

Originally posted by ron-c

If it is determined that the tube causes the hum the way to fix it is to put the tube on a cement surface and step on it. Then it is fixed.
Ron-C

LOL..that sounds like an jewish audiophile wedding...maybe I will do that at mine!

 

[dewickt]

Had a 110 on the bench last week, had .8% THD in one channel and .05 % THD in the other using the aux input, gave it a tube swap and found a bad 6U8, tested with .05 % THD in both channels when done, closer exam showed that the tube had heater to cathode leakage - Hummm, seems that the 6U8 is sensitive to this problem, try another.