[vcf-midatlantic] vacuum tube computers

Herb Johnson hjohnson at retrotechnology.info
Mon Dec 4 12:09:07 EST 2017

The discussion below concluded in late November, but I have some 
comments and considerations.

First, I'll say that there's any number of explanations available, for 
the history of computing technology and use. The person or institution 
doing the explaining, has their own objectives. The item has its time 
and place, and has levels of technology in it. And, there's sometimes 
consideration of the audience (child, expert in X but not Y, etc.).

So - no right and singular answers to questions of presentation. I'll 
give or support a view for my own reasons.

  Jeffrey Brace via vcf-midatlantic wrote:
>> How would you explain the George Philbrick machine to an 8 year old?
>   Well, as you know it's critically important to tune your presentation
> to the level of the guest.  When starting a more personal tour of LSSM,
> we usually find a non-intrusive way, starting out with reading body
> language etc, to ask a person what their levels of technical knowledge
> and interest are.  Then we explain things in different ways, on
> different levels, based on that.
>   For a total technical neophyte, our explanations are more geared
> toward talking about how what we have now (holding up my smartphone)
> wouldn't exist without having first passed through all of these
> (gesturing toward the exhibit floor) phases of development, then go to
> some specific stories as to how and why.
>   For an EE or CS person with a background in electronics and/or
> processor architecture, that's a waste of time.  We go into things like
> "this machine is a 12-bit system implemented using all 7400-series TTL
> chips" when I fire up the PDP-8/e.  I talk about "no chips at all, all
> discrete transistor logic" when we walk past the table of Straight-8s.
>   While the LSSM doesn't have anything like the Philbrick system, we do
> have a Heath H-1, another late-1950s tube-based analog computer.  To a
> young person, we'd explain that it was used by scientists and engineers
> to perform calculations for designing planes and big buildings.  It's
> complicated, look at all those knobs!
>   But to an EE/CS person who maybe doesn't know about analog computers
> (most haven't), we explain that it's a system with no bits, just
> voltages representing physical quantities, and most importantly it's not
> a discrete system, but a continuous system.  These operational
> amplifiers (gesturing toward the op-amps) and resistors can be wired up
> to perform most any mathematical primitive, and wired together with
> capacitors to perform integration and differentiation, to solve the
> differential equations that most every real-world process is based on.
>> How do you explain what a vacuum tube actually does to an 8 year old?
>   I wouldn't, as there's essentially no chance that he/she would be
> interested or have the background to be able to understand it.  See
> above. ;)
> -- 
> Dave McGuire, AK4HZ
> New Kensington, PA

I have to agree with earlier posts by David Gesswein, who stressed the 
point that analog computers solve "differential equations". And all have 
mentioned in various ways, that analog computers use voltage-levels, not 
"bits", to represent the values computed. Those are fundamental 
considerations from my view. Other people have other views, I won't 
argue much with them - I'm arguing with people who are still choosing 
what views to consider.

My view is, as a BSEE educated in the 1970's about both analog and 
digital circuits, who saw analog computers in use in academia and in the 
factory, who worked on one or two of them in my working lifetime. The 
hardest thing about explaining analog computing, is that it's almost 
TOTALLY EXTINCT today. It's hard to think or talk about something, 
outside your experiences. And preservation of extinct computing - and 
explaining it in it's own context - matters to me.

One can explain "voltages not bits", in any number of ways, appropriate 
to audiences and settings. "Temperature" might be a good example. 
There's still mercury thermometers, mechanical thermostats and 
mechanical thermometers. They use the expansion of metals in an ANALOG 
way, to represent "temperature". The Weather Channel, gives it some 
number. But we feel "hot" or "cold" without numbers in our heads. A 
point to consider: the physical world is not lists of numbers, it's 
events and materials and the flow of time.

"Differential equations" is jargon used by engineers and scientists. But 
it comes down to actions in systems over time. How fast does water boil 
on the stove? Or freeze if put outside in the winter? How long does it 
take your car to go from zero to 60? How long does it take a 
baseball/football in flight to be caught? These are ANALOG values that 
change with time. And, there are analog circuits (components wired up) 
which can represent these rates-of-change calculations. Then you feed 
those circuits the analog values as voltages, and watch what they do 
over TIME. *That* is what an analog computer is about. Circuits and 
components, wired for one computation, using time and voltages to 
represent values - no "bits", no digital (except for numeric results or 

I myself, don't tell people "this is too complicated to explain". I give 
them *something*; it will provide some impression and place to start; 
further consideration will be their choice to make.

So. I hope the two explanations above, of "voltages not bits" and 
"differential equations", are helpful. I hope the notion of "extinct 
computer preservation" gets some attention. Otherwise, I'm not going to 
debate some ultimate description or how-to-talk-to methods. I've 
explained why, and I've offered my own views.

Herb Johnson

PS: a vacuum tube, controls electrons going from a hot metal to a metal 
plate. The flow of electrons is a current, like water from a faucet. The 
current is controlled by the electrons going through a grid. a grid is 
wires strung across the current-flow, which repel the electrons like 
magnets do. Just as you can control water by turning a faucet, you can 
control electric currents with a vacuum tube, by changing voltages on 
its grid. Old radios and TV's, very old computers, used vacuum tubes.

Herbert R. Johnson, New Jersey in the USA
http://www.retrotechnology.com OR .net

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