On 10/27/25 14:17, Brent Hilpert via cctalk wrote:
On 2025Oct 27,, at 9:37 AM, Christian Corti via cctalk <[email protected]>
wrote:
On Mon, 27 Oct 2025, Paul Koning wrote:
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.
Cob, fT not quite as good, but close:
2N5400: Cob=6p0, fT=400MHz
2N4916: Cob=4p5, fT=400MHz
vs 2N3639: Cob=3p5, fT=500MHz
The MPSH81 shows Ccb = 0.85 pF and Cce = 0.65 pF and fT 600
MHz. They don't give a Cob spec.
Jon
