When I first developed the Paramount circuit, I thought about that. You can raise the filament voltage by installing a capacitor at the choke input - I suppose 10000uF/16v would be adequate. The cathode resistance should increase to 1500 ohms - replace the paralleled 2000 ohm 25W resistors with 3000 ohms 25W each.
My calculations show a voltage close to 7.5vDC, but I have not tested this myself nor have I heard of anyone who has, so some experimentation would be in order at first to be sure the filament voltage is right.
The original old-stock type 50 specified a low grid resistance of 10K ohms, presumably because it had a poor vacuum and was prone to grid current. This would require a different driver circuit and coupling cap - I don't personally think it's practical. But the EML specifies a 250K maximum grid resistor and should work. The Type 50, like the 45, has a fairly high plate resistance so the damping factor is poor which will make a difference with some speakers. Be aware that you won't get much more power than you would from a 2A3.
