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In Reply to: RE: Out of curiosity posted by unclestu on January 28, 2011 at 12:57:31
Ha--learn something new every day. It appears they are not a perfect pair. One (B8J) has the staple with a small white wire running from it. The other has a slightly raised section on the top mica--no wires. Both actually have halo getter's supported by a "bridge" getter holder that cuts across the circle getter, making it look like a D getter. Wait--I found another one. This one has the Mullard shield logo code B7A--the same long grey plates and a bent square getter with "bridge" double getter holder.
Given that they are not a perfect pair my judgement of these tubes may be harsh (but I doubt it). Hopefully soon I will find a match to one of these three a try them again.
Follow Ups:
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
Wow--great tip Stu. I think I have heard of that one before, but never tried it.
Thats what these forums are suppose to be about! Will definitely give it a try.
This is probably a stupid question,but are the new re-issue 12ax7 tubes any good?How about NOS Amperex 12ax7's?Thanks,
Dave
Hi Dave:
The only new issue 12ax7 I have tried is the sovtec LPS---a pretty good tube--very quiet, good extension at both ends. But it does not have the "midrange magic" of the best NOS tubes IMO.
Amperex made many 12ax7's. They are all pretty good, the best are probably the early long plate(17mm) D getter tubes. The bugle boys are sweet and warm sounding while the orange globe tubes are good too. But as in all things tube, it depends on the equipment. In my amp the Amperex is not my favorite, the three tubes I mention in my original post and most 5751's easily best them. In another amp or pre-amp the Amperex's could be killer. You just have too spend your money and try it. That is why people end up with huge tube collections worth far more than their amp's. Collecting tubes is fun!
Ahhhhhh!
The Betty Crocker tweak?
Best from Tucson
Bob
"He (R.M. Nixon) was a foul caricature of himself, a man with no soul, no inner convictions, with the integrity of a hyena, and the style of a poison toad." H. S. Thompson
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