Schematics of Atanasoff-Berry Computer logic circuits? - test-drb@ccmp.vtda.org

30 Aug 2008

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  Message: 8
 Date: Fri, 29 Aug 2008 13:47:07 -0800
 From: Brent Hilpert <hilpert at cs.ubc.ca>
 Subject: Re: Schematics of Atanasoff-Berry Computer logic circuits?
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 William Donzelli wrote:

  I think, rather, that that's a good portion
of the explanation of why they
  were so slow.
  Ballpark example, take a 12AU7: the sum of the grid-to-plate and
  grid-to-cathode capacitance is around 3 pF. Suppose the network resistance
  feeding the grid circuit is 250 KOhm, that's an RC time constant of 0.75uS,
  a little better than just 1 MHz. (R can be reduced of course but power
  consumption is then on the climb.)  
 Well, OK, i could buy that, but from what I have seen from the
 circuits and construction of tube logic circuits is that the 3 pF is
 not much compared to all the stray capacitance kicking around due to
 construction techniques.  
 I'd say point-to-point wiring becomes problematic when you start getting going
 into the higher shortwave / low VHF, and granted it becomes an issue in a
 physically large system, but it may not be bad as you might think. The 10MHz
 stage of the HP 524 counter is done with 'rats-nest' point to point wiring with
 components on tag boards separated by wires from the tube sockets, even for the
 grid circuits. The key was they went to high-frequency pentodes instead of
 duo-triodes, low-R circuits, diode clamps, 1% resistors: hi-speed was
 'possible' but very expensive to accomplish.
 Looking up the plate to grid Direct Interelectrode Capacitance,
    grid to plate is 1.5 pf
However , the Miller effect makes that quite interesting -
   depending on the effective stage gain,
the effective capacity is increased by the effective stage gain -
   which is a big frequency limiting (and stability) factor -
basically why pentodes and tetrodes were developed,
  (reduce the Miller effect and move the cut-off frequency up a way.

The grid to plate capacitance (and the accompaning Miller effect)
  make trying to get stability in a tuned plate, tuned grid circuit
"interesting".
(Interesting enough  with proper tetrodes  ;-))

I ain't a curcuits guy, but remember stuff, the wrong stuff -

Ed Thelen