21 Aug
2008
21 Aug
'08
2:15 p.m.
William Blair wrote:
I'm not very familiar with vacuum tube circuit
design, but I have a PDF copy
of the 1952 USAF technical order TO16-1-255 "Basic Theory and Application of
Electron Tubes" which looks like it might be sufficient. I've built two
different 12AU7 dual triode vacuum tube stereo headphone amps from plans
which use much lower and, therefore, safer and easier-to-produce voltages
(24 - 60V) than were used historically in such circuits. The required plate
voltages for these low-power circuits are produced using simple voltage
multipliers attached to commonly available and inexpensive low-voltage
transformer secondaries. Since, in the case of digital logic circuits
linearity of operation is not a requirement as it is with audio circuits,
even lower voltages might be usable although 24 volts seems to be plenty low
enough.
Man, do I know that now. I posted my comment before
I'd fully digested the
ABC web page.
A multi-stage direct coupled vacuum tube circuit. Fun.
Yes, be prepared for some fun, in both senses of the word: it is fun to play
around with, but the implemention of tube logic can be problematic or
unreliable, at least in the way the ABC tried to implement both NAND and NOR
gates with resistive input circuitry.
Atanasoff makes it sounds easy in his paper, but if one reads it closely
it's not quite so, at least as measured by modern standards where one comes
up with a gate design and then simply repeats it ad infinitum.
The ABC reconstruction and the original required (at least some) hand-picked
resistors in the gate circuits.
Some of the issues:
1. Tubes can be insufficiently non-linear. Driving between saturation and cutoff
can take a bit of swing and saturation is soft (curved).
2. Plate circuit impedance is high. If the difference between the plate circuit
impedance (lower better) and the grid circuit impedance (higher better) is
insufficient, then different fan-outs in the logic circuit (loading), and
varying tube/component characteristics, can pull the logic levels away from
the design targets and upset everything.
Going to the low plate voltage is a nice idea from a practical view but I
wonder if it may compound issues of point 2 above. It will reduce the voltage
shift between the plate and grid circuits that needs to be accomplished, but it
may also reduce the voltage swing between the 0/1 logic levels. How it works
out in the balance will be interesting.
It may depend on how close one wants to stay to the original design. I was
working with the original tube types (6C8) and voltages, trying to target the
original design and working with the constraint that the gate input resistors
were all of the same value
regardless of the gate type, changing only the bias resistor to achieve
a particular type of gate (NOR,NAND). The task may be easier if one is willing
to step outside that constraint.
(With all that said, I didn't spend all that much time experimenting around the
gate resistors issue, was something I intend(ed) to get back to.)
That's strange that they'd be so restrictive
about the distribution of schematics for a machine that has no current value beyond that
of a historical curiosity. I know that there was a legal battle between Mauchly with his
ENIAC and Atanasoff with his ABC, but that was decided against Mauchly and claimed that
his ENIAC plagiarized concepts used by Atanasoff's ABC which then invalidated
Mauchly's earlier patent claim to the first electronic computer. Since Atanasoff won
the case 35 years ago, why would they not willingly release all schematics now? Strange.
No, I don't understand it either.
Regarding the schematic, the Burks' book "The First Electronic Computer"
contains some more tidbits of schematics, particularly relating to the
capacitive-memory refresh of the drums, and another version of calculations of
tables for the gate resistors in an appendix (IIRC, it's been a few years since
I've seen the book).
I was mostly interested in examining the control section to see the split
between electromechanical relay circuitry and electronic vacuum tube circuitry
(as well as those resistor values).
--
Straying completely away from technical issues, and I'm somewhat loath to
mention this, but the animosity engendered by the early-70s court battle
continues decades later. The somewhat nasty inter-personal battle
found it's way into the Amazon book reviews as recently as 2004.
Another bizarre twist in the ENIAC patent saga on the legal/social side.