On Sat, 8 Nov 2025, Tom Hunter wrote:
I got hold of some MPSH81. Unfortunately they are not a suitable substitute for a 2N3639 or 2N3640.
[...]
Well, the datasheet say that this is an RF transistor, it is per se unsuitable for on-off switching into saturation. "This device is designed for general RF amplifier and mixer applications to 250 MHz"
The datasheet for the MPSH81 doesn't even specify tON, tOFF and tS, but the fT of 600 MHz appeared promising.
fT is not really important. That tON and tOFF are not specified is normal for RF-only transistors. You need to look out for "high-speed staturated switching" applications in the datasheet.
Christian
