29 Aug
2008
29 Aug
'08
2:47 p.m.
William Donzelli wrote:
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.
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.