The diode/dropper resistor is definitely an option but be aware that a diode failure in many cases will result in a in a short and should that happen it could take out other tubes in the string. You may want to investigate the use of a capacitor dropper. Have a look at this write-up on droppers https://www.vintage-radio.com/repair-restore-information/valve_dropper-calcs.html
There are several technical articles available on the use of capacitors as droppers but few mention that you cannot use electrolytics or DC capacitors. You have to use an AC motor runtime capacitor. Good luck.