it is simply using the first section of the 6SN7 as a grounded cathode volt-amp DC coupled to the second triode and then capacitor coupled to the 300B grid. This amp does not do the 300B justice (I’ve built it before).
As you know a circuit like this will result in a higher output impedance so wouldn't be the best application for driving a grid that has high capacitance and needs a lot of swing. The 300b has nearly the same mu as the power tubes we use in our OTLs. We also built an OTL using 300bs for a demonstration so I'm not speaking idly.
We don't run the tube with a lot of current either, despite that, direct coupled to the grid of a 300b will have no problem driving it to saturation. At that point the 6SN7 section will saturate too; when replaced by a much gutsier tube the difference was less than a 1% change so its very obvious that the 6SN7 can do this job quite well.
We use the 6SN7 driving the 6AS7G; if you compare specs to a 300b you see that the former has slightly higher grid capacitance and slightly lower mu; the 300b is easier to drive. Despite that we can get one 6SN7 section to drive as many a 14 6AS7G grids with no worries. One grid of a 300b would be like a walk in the park.
If you like the 6SN7 that much, if I were you I'd be looking a better topologies to make it happen.
