[sdiy] Transistor questions

[David G Dixon]

Hey Synth DIY Team,
 
I have built sawcore VCOs with two-transistor cores based on the Korg
PS-3100.  I initially used 2N3904 and 2N3906, and they worked great, but
simulations suggested that the saw reset time would be around 30ns, which
was more than I wanted, because I want perfect tracking up to about 6kHz
without high-frequency compensation.  Hence, I tried many other transistor
models, and settled on 2SC945 and 2SA733.  My simulations suggested that the
reset time would only be 2 or 3ns, and this is fast enough.
 
My question is this: given that the 2N390X transistors have higher
transition frequencies and lower collector capacitances than the old
Japanese transistors (according to All Transistors), why should the latter
give significantly faster saw reset times?  What is the actual figure of
merit for this?
 
Cheers,
Dave Dixon
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