For some strange reason, I got locked into thinking "2A3" rather than "6B4G"
I corrected my previous posting , for future reference, and to eliminate confusion
The Sovtek single plate 6B4Gs (and 6A3s, and 2A3s) can take more power dissipation than the earlier vintage USA and Russian made
two-plate versions of the 6B4G / 6A3 / 2A3, these were basically two parallel connected 45s inside the globe, and 15W was their maximum safe PD (thypcally they were operated at 250VDC across the tube, at 60 mA, but the Russian Sovtek "single plates" can go up to at least 18W, or even 20W, with grater P-K voltage.
So the SS rectification should be OK, though you should watch the increased plate current, the value of the cathode resistors may have to be increased from 900 ohms total, to 1100-1200 ohms total, , in order to keep the plate current within safe limits.
There is no 40 watt PD version of the 6B4G that I know of. 300Bs with a voltage dropping resistor would work, as explained earlier up the topic.
With the triode strapped 6AV5GAs and SS rectification, I am pushing the 6AV5GAs to between 18 and 20 watts PD with no ill efects (they are published at 11 watts, which is really timid, and overly conservative, depending on the brand and interior structures, some of these can be pushed up to nearly 40W PD, without melting down.
A triode strapped 6AV5GA behaves very similarlyto a 6B4G, just needs a different connection scheme to its socket, in order to work in this circuit.
/ed B