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In Reply to: RE:IT might be a 5755 posted by DAK on August 30, 2015 at 08:52:22
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.
Follow Ups:
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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