vac tubes / was Re: Schematics of Atanasoff-Berry

[William Blair]

Brent Hilpert wrote:

> It has occurred to me that if one were setting out to build
> a (small?) tube
> computer today for fun, to look into selecting the primary
> tube(s) from that
> class; the question being if one would do any better than
> just using
> duo-triodes, which are in higher demand.

That's exactly what I'm doing.  The advantage of going with tubes currently used for audio is that they are still in production, mostly in China and Russia.  The disadvantage is that they cost quite a bit more.  For small projects like mine, I have no worries about the availability of tubes that are no longer produced.  For that matter, I suspect that even very large projects wouldn't have problems with tube availability considering how many of these out of production tubes are apparently still around.  For instance, I can buy OOP 5BK7A dual triodes for US$0.78 each (I have ten on hand) or OOP 4BS8 dual triodes (to be purchased on my next order) for $0.38 each.  The 5BK7A has a 4.7V heater and the 4BS8 a 4.5V heater.  Either of these would work fine at 5V being outside of the typical filament voltage +/-10% rating only very slightly in the case of the 4BS8.  For a power supply, I plan to use a modified PC power supply. I have three used, unmodified ones
 just sitting around from previous PC builds:

http://web2.murraystate.edu/andy.batts/ps/powersupply.htm

The +12V will be used for the B+ and the +5V for the filaments.  Both +12V and -12V could be used for the plate voltage giving a total of 24V as long as the project is small enough to not exceed the 500mA rating (in my case) of the PC power supply -12V output.  And, of course, the modified PC power supply will be useful for other projects, too.  I've got several homemade variable DC supplies, but they are all limited to 1.5A output which would be exceeded with just three 5BK7A filaments @ 600mA each.