Strange problem with 6CA7

[gamato]

Hi all
I had a strange problem with EH 6CA7 tubes on my hi-fi amplifier: the internal wire connecting the cathode pin to the cathode sleeve become unwelded. Giving a more careful look at the tube, I noted that the problem is due to the cathode slightly moving toward the top of the tube itself, up to the point where the wire connecting the cathode to the pin gets broken.

This can be clearly seen in the attached picture, where you can see that the cathode is exposed about 0.5 cm from the top of the tube, while it disappeared from the bottom (where the wire connecting to the pin is welded). It seems that, for some reasons, the cathode sleeve got loose and mysteriously started moving vertically. In the picture you can see that there are white marks on the top of the cathode sleeve indicating that it shifted up and down several times.

The amplifier where the tubes are used is an hi-fi amplifier, and never moves, so I exclude that this might be due to vibrations or knocking the tubes during operation.

This problem happened to me twice with different sets of the same tubes types. Does anybody experienced similar issues with EH 6CA7 tubes, or does anybody has hints or suggestions on how this can happen, and especially how it can be avoided?
Thanks for any feedback
Giuseppe

[sluckey]

Nothing you can do about that manufacturing defect. If I had more than one EH tube with the same defect I would switch brands.

[SILVERGUN]

Not sure if this is applicable to your situation , so just posting for reference...
I remember reading this when I was considering buying a bunch of these in a deal.
Basically, what is being said is that the EH's are not internally wired exactly like an original 6CA7
 
http://www.marshallforum.com/showthread.php?t=12639

[jojokeo]

Good link SG, thanks.

[gamato]

Thanks SILVERGUN.
I read that thread. I am not sure it is related to my problem, which I believe has to do with some mechanical issue: the cathode sleeve, for same reason, free to move up and down.
However, I have not clue if this is originated from some electrical defect in the amp, or from some manufacturing defect in the tubes (as sluckey suggests).

Strange it happened twice on two different quartets.
I have no idea of what can cause the cathode to get loose and move. I considered the possibility that the filament becomes too hot, but 6.3 Volts are correctly applied to it.

[HotBluePlates]

... the internal wire connecting the cathode pin to the cathode sleeve become unwelded. Giving a more careful look at the tube, I noted that the problem is due to the cathode slightly moving toward the top of the tube itself, up to the point where the wire connecting the cathode to the pin gets broken. ...

I would think the cause-and-effect is the other way around: the pin-to-cathode weld breaks (or was never made), which then allows the cathode sleeve to move up/down in the mica spacer.

I'd call that a manufacturing defect, and contact the retailer for a refund/replacement. Let the retailer deal with the issue with their supplier/wholesaler.