29 Aug
2008
29 Aug
'08
11:55 a.m.
(Picking up where we left off.. a few messages sent to the list last week seem
to have been dropped during the outage.)
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.)
(Not to say there weren't other reasons they were slow..)
Transisters were probably adapted much more
quickly in switching networks than in signal processing for that
reason and because no extra work had to be done to handle the stray
capacitances that tubes suffer from.
Ultimately, yes, but in the 1950s, the machines were slow enough that
the tiny capacitances in the structure of tube elements did not matter
much.