I will add to this, although many on the internet are not a fan ( I do wonder if they have even tried it tbh) due to "noise" caused by them - I only needed to use this method on my separate 12.6V filament PT, and there is no noise difference between using that and my 6.3V filament PT. Anti-parallel diodes to drop heater voltages, which would remain constant despite heater requirements of the tube in use.
Keep in mind this is the B+ rectifier, meaning any noise introduced by those diodes is now in the B+ supply. I haven't tested this as there are better solutions, but I wouldn't recommend it for a rectifier filament winding. For a cathode heater, it matters much less and is probably fine.