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In Reply to: RE: Anybody checked out this 12ax7-hd?? posted by DAK on August 26, 2015 at 17:17:34
This may very well be a Russian 5751 with some plating on the pins, a socket wafer, some o-rings, and a piece of cosmetic metal jewelry glued to the top. (Which, if it's too rigid, may actually damage the tube long term)
Since everything is covered up by a sticker, nobody will really know without peeling one off.
The "Maximum balance differential voltage" specification is laughable, as this specification depends greatly on the circuit used, and it isn't hard to get better than 0.3V with a dissimilar triode like a 12DW7. I also like the 10,999 hour rating.
I'm kind of hoping this is an April Fools Day joke that hasn't been taken down.
Follow Ups:
You can see from the bottom that it was originally a 9 pin tube with some of the pins moved to different locations (pin two to pin three, and pin eight to pin six) and all the pins moved CCW slightly so they fall between the old pin locations. There may be some Russian tube that has similar specs. to a 12AX7 but a different base. Someone was very clever is coming up with this hybrid. I did a quick scan of all the 9 pin bases looking for a tube with pin 8 as plate #2 and pin 3 as grid #1 and couldn't find one. Maybe it is an obscure Russina or Chinese tube.
It just occurred to me that the 5755 is a similar to the 12ax7 with a amplification factor of 100. I checked the pin out and it is as you described. The use of the small PCB reassigns the pins to the ax7 pinout. Western Electric made some 5755 maybe he has a big stash?? But cynical me wonders if it could be the Raytheon 5755 which are quite cheap. It would explain the mumbo jumbo as way to confuse the buyer. Caveat Emptor, Dak
The 5755! That's the one I was trying to remember; it's not in my GE 'Essential Characteristics' tube manual which is why I didn't find the base listed. Yes, these were going for a dollar or so each in cases of a thousand, although now the price has risen some. The little adapter PC boards are even available on epay; the exact same ones used in these "12AX7-HD" tubes.
In the end though they are NOT a replacement for a 12AX7 since they have only 40% of the transconductance of a 12AX7 which is already a wimpy tube and almost twice the plate resistance. That means much more noise and much less ability to drive a following stage. I would steer well clear of these tubes for audio.
Has to be the 5755, likely Raytheon manufacture as said. I believe that most of the Western Electric manufactured 420A's were clear tops, actually all clear glass, appearing to not have a getter at all. IIRC, they used some sort of powder as getter material.
I know that W.E. subcontracted companies to manufacture their types, so technically, these could really be "W.E. tubes".
I have used the 420A in place of 12AX7's in a preamp (re-wired), and the sound was harmonically rich and initially pleasing, but not accurate. Likely, for the reasons you have stated. They sounded like a really fat Mullard or old RCA black plate 12AX7. It was a fun 'roll' anyway.
Not exactly what I'd call an "HD" 12AX7 sub, if anything, the exact opposite.
Cory
Seriously, I have a whole box of Raytheon 5755. I thought many times about using it in a circuit but never got around to it. I bought them a long time ago. And seriously if you want a 5 pack of Raytheon 5755 shoot me an email and we can work out a good price. cheers, Dak
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