I would try a 2N5401 which is my go-to high voltage small signal PNP
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just to see if it works well enough; Tektronix used these in z-axis
amplifiers. After that and sticking with common transistors, maybe
the KSA992 and BC556 which have lower capacitance.
On Thu, 9 Nov 2017 15:27:24 +0000, you wrote:
The second criterion is speed. Q373 has to switch a 63V swing as fast as possible.
The main limit on speed in this case is collector capacitance rather than transition frequency. Storage and fall time are not factors since D372 prevents Q373 from saturating.
So you want a 70V (or more) transistor with small Cob.
According to the Common Design Parts Catalog, Cob for the 2N2207 is 2.3pF . The replacement doesn't have to be Germanium, but you will have a hard time finding a Silicon part with a collector that nimble.