27 Oct
2025
27 Oct
'25
12:17 p.m.
On 2025Oct 27,, at 9:37 AM, Christian Corti via cctalk <cctalk(a)classiccmp.org>
wrote:
On Mon, 27 Oct 2025, Paul Koning wrote:
Cob, fT not quite as good, but close:
2N5400: Cob=6p0, fT=400MHz
2N4916: Cob=4p5, fT=400MHz
vs 2N3639: Cob=3p5, fT=500MHz
Perhaps amongst those you tried (?).
I see from the schematic they were going to quite the effort to maintain speed while still
using capacitive triggering.
Have you tried with an ordinary switching
transistor that's readily available? I'm surprised at that storage time, why
would it need to be that low for a machine that runs at just a couple of MHz?
The storage time is very crucial. We have tested many different switching transistors.
The point is, the PDP-8 uses needle impulses (around 30ns) to switch a flip-flop. They
just don't work with today's transistors. And on modules like the R211/S211, they
introduce a too long delay.