Tubes Asylum Questions about tubes and gear that glows. FAQ

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RE: Out of curiosity

Posted by unclestu (D) on January 28, 2011 at 15:40:38

In Reply to: RE: Out of curiosity posted by Mendel on January 28, 2011 at 14:00:26:

It does sound like you have three different versions of the Mullard Long Plate!

In combining the two different Mullards (if both are in different channels), the excessive warmth of the mC1 will dominate and cloud the sound (IMHE). One of those Mullards you have, if the construction is identical to the f91 except that it has a mica piece "spring" loaded against the cathodes, would be an f92. It does not have the openness of the f91 although more extended on the top end than an mC1 which normally has a true D getter.

If you got them used, and suspect some aging on the tubes, you can rejuvenate them by doing a bake out. I place the tubes in a toaster oven at the lowest temperature setting and bake them for 30 minutes and then raise the temperature by 50 degrees every half hour to about 300 degrees, and just leave them there for as long as I feel comfortable (overnight some times). Then I simply unplug the oven to allow them to cool to room temperature slowly. Usually the sound is restored to like new conditions, although there is no guarantee how long this bake out will last.

Try the procedure on some older tubes first. It will NOT change any readings on a tube tester, but will absorb a tremendous amount of the gasses which have been purged by running the tubes ( emitted from the metal structure itself as well as the glass) and thus restore the tube closer to the original factory vacuum. I've noticed in using this procedure that that blue glow that many tubes acquire over time will disappear, particularly on power tubes.

Stu

Edits: 01/28/11

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