Den mån 27 okt. 2025 kl 23:03 skrev Jon Elson via cctalk <
cctalk(a)classiccmp.org>gt;:
  On 10/27/25 14:17, Brent Hilpert via cctalk wrote:
  On 2025Oct 27,, at 9:37 AM, Christian Corti via
cctalk <
 
 cctalk(a)classiccmp.org> 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.
 
They actually show the output capacitance in a diagram as a function of
reverse bias voltage. Between 0.8 pF and 1.8pF depending on voltage.
/Mattis