[Radio Wrangler]

With the switching in bypass mode, you'll find that the relays used and the circuit layout will be good enough for 2m, but may give a fair bit of loss and a poor match on 70cm. As you said, it pre-dates dual-band radios and the designer will have been thinking of 2m only.

The preamp has tuned circuits in its input to protect it from overload from out of bad signals like pagers. In the 70cm band it will give you a lot of loss and not gain.

If the preamp is in the bypass path and you transmit on 70cm, with the amplifier in bypass mode, you could be transmitting into the output of the preamp.

It's safest to stick with just what it was intended to do and connect your antennae directly to the FT817 when using any other band.

With an SSB/FM switched mode amplifier, the bias conditions are changed. SSB is set for lower efficiency, higher linearity. FM doesn't need good linearity, so the bias will change to something more like class-C to put the efficiency up. CW should work best with FM mode provided the rf detecting auto change-over doesn't set the relays clattering. A solid transmit/receive connection or control may be needed.

David