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
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.
