KT66 Pin 1 variations - which one is accurate?

[tubeswell]

There seems to be variations in pin-outs shown on different datasheets for KT66s.

JJ's KT66 datasheet schematic labels Pin 1 as going to "g3"/suppressor grid (but their actual schematic drawing suggests 'no connection'?)

Marconi's KT66 datasheet says 'no connection' at Pin 1

GEC's KT66 datasheet says Pin 1 is "IC" (whatever that means?)

I take it that different manufacturers have their own way of dealing with Pin 1 in this regard, and that if you're in doubt, then you should treat Pin 1 as being connected to ...

(TIA)

[Enzo]

Well, that's the question. innit?

Over the years, I have several tube manuals of various brands. From back in the days when the factories like RCA published one. And certain tubes had the "unused" pin problem. And the way to deal with it is to assume there is a connection and don;t use that socket pin for anything. I'd look up the particular tube in the book from its maker. yeah, I know, those days are gone.

In the book, NC means no connection. IC means internal connection. What this means is that the pin inside is bonded to some part. it really isn;t intended as a functioning circuit connection, but for example a screen grid might be made of two parallel vertical posts with grid wires looped between them surrounding the cathode. One of those posts is connected to the screen grid pin on the base. Just for mechanical support, the other vertical post might be welded to that IC pin. SO the pin is hot at screen voltage, but that pin is not a legitimate screen connection in that tube type.


More common would be something less threatening than a screen. SOmetimes it is a shield element inside. Some are in fact marked IS for internal shield.


SOme of the fat tubes with the metal ring around the base get that pin connected to the metal ring. Guys then get the big surprise when they touch the ring when the amp is going and find a voltage there because that socket pin is not grounded.

[Steve Conner]

It is easy enough to devise a socket wiring scheme that works with all octal power tubes. So just use that. OK, I forgot what it is.