tube failure...curious

[greiswig]

I have heard this type of failure twice now in a new-manufacture Tung Sol 12AX7. The tube works fine, other than after you strike a note, there will be a point at which the note volume drops abruptly by just a little bit, and there will be a residual buzz behind the note at the same frequency as the note that trails off into static. The static almost sounds like when you turn standby to the "Stand by" position with a note playing.

Just kind of curious about what causes this, and also whether other people have had this happen with TS tubes in particular?

[Firestorm]

Are you certain it's the tube? This sounds exactly like the type of parasitic oscillation that occurs in most silverface and some blackface Fenders. Assuming you are sure it is the tube, what position in the amp is the tube in? Does it happen right out of the box or does it appear after time? Does the tube exhibit unusual current readings (and transconductance readings if you have access to a tester)?

[selloutrr]

Personally I have not experienced this with Tung Sol or any other brand.

If it's happened to you twice in the same amp I'd take a closer look at the amp.

[tubeswell]

How do you know it was the tube? Did you try swapping tubes?

[greiswig]

Sorry, yes: replacement of that tube (V1) with another Tung-Sol fixed the issue. That's why I think it's the tube, not the amp.

[tubeswell]

What operating conditions (plate, grid and cathode voltages) are you running the tube in? That might give a few clues as to possible failure modes. Some modern production tubes have poor h-k insulation and don't like high cathode voltages

[David Root]

Good point. NOS tubes don't like real high h-k voltages either I recently screwed up a nice '62 (real) Tungsol 12AX7 in a CF at 180 Vk. Took no more than 2-3 hours operation.

[iknowjohnny]

It's sad really because to many like myself they are the best sounding new production tube, but from my experience they also share a ridiculous failure rate along with the GT mullard reissue of maybe 8 years ago or so. They are the only ax7's i have ever used that i had to stop using because they fail so often. I had 4 and all 4 either went south or hummed real bad, and no, i wasn't using it in a cathode follower slot. I've been wanting to buy another lately because i really miss that tone. But why get used to a tube that will just leave me disappointed and broke all the time. really a shame, i love those things. The only other new production ax7 i like is the russian mullard reissue, and wouldn't you know it....of 3 i bought all 3 hum fairly loud ! Why is it only the tube I happen to feel are above the rest are junk?!

[greiswig]

Interesting...I'm running about 200V on the plates; 100k/1.5k plate/cathode pair. Thanks for the input!