[DaleHCook]

Newer radios usually have chassis ground as the most negative point and resistors from that point to the cathodes of those tubes that require bias.

Yes, and that is called self-bias or cathode bias. What the 650 has is fixed bias, derived from a point in the power supply which is negative with respect to the grounded cathodes.

Some variation in the fixed bias voltage always occurs in the ways in which it is normally implemented in consumer equipment such as the 650. That is why commercial equipment using fixed bias (such as the broadcast equipment and instrumentation) that I have worked on for more than 50 years generally uses a separate bias supply.

Self-bias (aka cathode bias) has its own drawbacks where varying plate current causes varying bias.

I do not expect rock-steady bias in any consumer equipment - providing it costs too much.