According to my RCA Tube Manual a 6BA6 is rated 6.3 V at 300 mA, so 2 tubes in series 12.6 V at 600 mA. Supply voltage is 25.2 V so you want to drop 12.6 V with 600 mA of current 12.6/0.6 = 21 ohms 21x0.36 = 7.6 watts A resistor ( 21 ohms at 10 watts would seem right David C. Hallam KC2JD