The "B" version of the tube is what the factory built it with also called 7034. The heater is rated at six point zero Volts, not 6.3. There are variations. The 4CX250R/7580 will deliver more power, but requires more negative-bias voltage to control it properly. The DX300's blower really isn't big enough to remove the additional heat efficiently, so I don't recommend that type. The 8930 is the same as the "R" on the inside, but has a 350 Watt heat radiator ring. Likewise, it would need more air to deliver its full potential safely, but wattmeter worshippers want what they want. Amperex made that tube and called it a DX393. It also requires a chimney that accomodates the larger-diameter radiator ring.
The 4X150 is the same tube as the 4CX250, but with a ring of glass insulation. It was developed first and has a lower upper temperature limit. It's fine in a Pride on low side. A bit of a risk on High. The 4X150D and the 4CX250F are meant for airborne use and have heaters rated for 26.5 Volts, not six.
During the design's progression from the original glass/metal 150 tube, Eimac sold a "4X250". Had a ceramic ring but with glass on the bottom where the pins protrude. Eventually that bottom layer also became ceramic and the "C" was added to the type number.
Just stay away from "4CX350" tubes. Yeah, 350 sounds like more than 250, but it has a totally different drive characteristic. Won't tolerate even a tiny bit of excess drive power. The DX300's design throws 90 percent of the radio's drive power into resistors. The tube only needs the remaining 10 percent. The 250 tubes' grids are rated at 2 Watts. The 350's grid is rated at zero Watts. Honest. Not a wise choice for this model.
The DX300 was only built in 1977, 1978 and for a short while in 1979. That's when the FCC tightened the rules on what's a "legal" ham linear. Closed the loophole that prompted D&A and Palomar to put a bandswitch on their CB linears.
The DX300 is probably not any harder to find that a driveable Camaro or Mustang made those years. Just gotta wait until the right owner dies, moves to a condo or a care home.
Every time I buy a derelict and rehabilitate it there's a waiting list for it by the time it's ready for a new home. Last time I had to advertise one to sell it was six or seven years ago.
I'm always on the lookout for a basket case, cheap. The more stuff broken/missing the better. Worst one I ever rebuilt is here:
https://www.worldwidedx.com/threads/pride-dx300-basket-case-no-maybe-more-like-body-bag-case.264657/
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