I looked it up. The cost is modest, $54 to $75 each, but I like over-design and 10 Amps is the limit which pushes it to the limit. I run my cathode heaters at about 9 Volts when the data sheet says 9 to 11 Volts.9 Volts is supposed to cause the tube to last longer. So does 120 ma, unlike making a spot in the plate glow red hot. Also, if I got someone to hire me to build them a custom amplifier or 833A output stage they could drive preferably with a 45, 2A3 ( which is the same thing as two 45s in parallel ) or a 300B, and a transformer such as a Lundahhl LL2765 or a British made Sowter 5k to 600 Ohms I would consider saving them money by letting them drive it with two car batteries and a dropping resistor of 0.3 Ohms and over-designed 25 Watt rating. Such an arrangement could run two or three hours and be recharged when the amplifier is turned off.