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I was recently shopping for replacement tubes for my universal player, specifically E182CC/7119's. One offer included a pair of NOS tubes that were made in Holland over 40 years ago. They were describes as "pinched tubes", and upon further inquiry, I was told that pinched tubes would only protect against noise caused by vibration in the glass (which I presume could also be controlled by dampers), but more important would also control vibration in the metal element contained in the tube. Since these pinched versions were significantly more expensive, I would appreciate your input on whether this claim for pinched tubes is accurate or not. Interestingly, I don't see many tubes, even some well know names, that bear this designation, so I'm wondering why there aren't more manufactured if they are truly superior.
Thanks
Pete
Follow Ups:
In my experience, the pinch does nothing of what this guy described. And if you think about other companies like GE and RCA, they NEVER had a pinched finish. So why wasn't this technique never used across the board?
What we have to imagine is where the pinch comes into play during the manufacturing process. In the video linked below, it's not exactly obvious, but we get an idea as to how all the parts are put together. For small signal tubes in the automated machines, it essentially follows like this: the tube's internal parts are assembled and soldered to the base, these are inserted into the tube's glass envelope, their glass is fused by a torch, the tube is evacuated of its gas, the torch-pinch-and-seal occurs, and the getter is activated at the end.
What I think what may happen is there are 4 preheated quarters that come in and clasp the tube by the base of the evacuator stem/top of the tube envelope. It allows the tube to be secured during the automated process, probably reduces the chance of distorting the envelope, and maybe even reduces cracking from too-sudden a heat change after being hit by the torch.
So, does it reduce microphonics? I don't think so. I think the pinch is more to help the manufacturer to reduce production losses in this automated machine. What will prevent microphonics are things like extra-thick micas, a 3rd mica, 2 support rods for the getter, 2 cross-structure support rods, welded plates instead of crimped plates, notched control grid rods, and welded grids.
I hope that helped! Cheerio!
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May your tubes be warm, long-lasting, and always lively. Holy be thy heater.
Kevin Deal has said that the Amperex pinched waist 6922 are the most microphonic tubes of that type he has collected despite the "pinch" that was supposed to reduce this problem. They were only produced for a few years AFAIK and the technique was later abandoned by tube manufacturers, whether for costs reasons or for ineffectiveness is unknown.
I will say that the Amperex pinched waist 6922 is the best 6922 I have ever used in my SP14 phono stage, but can't say what roll the pinched waist has in that.
No idea either but what you report is what I've heard, too. Different application but until it went noisy, definitely the best in terms of sound stage and frequency response.
Joe
It might have been a fad. I have a few "pinched waist" Amperex 6922s. They only made them for about one year, 1959. What's rare gets elevated to cult status.
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
I have a quad of Amperex 6dj8 (not 6922) pinched waist tubes, apparently quite rare.
I sometimes see vendors who decide I think in an arbitrary way to call any Amperex with a crooked bottle shape "a rare Pinched Waste..." I see them do something similar with 6SN7s They call all bottom getter Sylvanis GTs Bad Boys, either purposely or not knowing it's easy to tell which are and which aren't true Bad Boys.
Steve
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