Looks like it could be a JCM schematic re-drawn for Jfets.
Trimmers to bias the Fet's, otherwise the same values as the preamp ?
That simple [if it compares to amp schem].
Or...see Fetzer Valve and other reads for a deeper look into Jfet/Tube conversions.
Whether Simple or Deep, 'it' should be able to be made to work, if 'Deep' is chosen, some study and design modifications might be good before starting to populate.
Not being particularly familiar with JCM, but suspecting that much of "JCM sound" is constituted to a large degree by a volume control being turned CW to a point where speaker/output tubes contribute to the distortion.
I read ''they use diodes in JCM'' [to contribute to getting the distortion]...about JCM's, but didn't investigate it.
It looks to be a 'boost the source to the next Jfet input' distortion, a number of Jfets boosting signal [similar to the way tubes prepare signal for an output amplifier stage]. The source signal peaks boosted can reach a 9volt supply very quickly [compared to 100+V, whatever the triode tube stages use]...essentially making this a very different circuit than a tube-preamp.
