value and grounding the cathode results in realizing the full spread of difference between tubes of the same type in idle current variations. For example, three EL34's might bias up to 45, 50, and 55 mA's in a test rig with 400 volts of B+ voltage and -30 volts of grid voltage, but these same tubes placed in a test rig with 430 volts of B+ voltage and a cathode resistor of 600 ohms would probably bias up to 47, 50, 53 mA's. In an SE amplifier, this degree of auto-biasing might prove sufficient; in a push-pull amplifier, probably not, as even a 6 mA imbalance is not good for the output transformer.

Tightening the idle current draw requires using at least one active device. The easiest would be one of the two terminal current limiting diodes, such as the CR0-470. This device consists of a FET whose gate is attached to its source. By using only FET's with an IDSS of 4.7 mA, this device constitutes a fixed current source of 4.7 mA. Replacing a cathode resistor with a CRO-470 would work with a 12BH7, but not with a 6DJ8, as the 12BH7 lower mu requires a greater cathode-to-grid voltage than does the 6DJ8, and as these current sources have a fairly high dropout voltage of 4 or 5 volts, the 6DJ8 required 3 volt bias would fail to turn the FET on. Additionally, these  devices are available only up to 10 mA's, not nearly

enough to bias up a 6550. Of course they could be wired up in parallel, which might work, albeit at great expense, if the dissipation and voltage limits (1W and 100 volts) for the devices were not exceeded.
If a more complex circuit is permissible, then more options become available.  The next level of complexity would be the commonly available 3-pin adjustable voltage regulators. An LM317 would make an excellent current source. It contains a fairly precise, fairly low noise voltage reference which is used to define a fixed voltage differential between its output and its adjustment pin. If a resistor is placed in between these two pins, the current limit will equal the reference voltage divided by the resistor value:  Current = 1.25v / R .

For example, a resistor value of 125 ohms will yield 10 mA's of current, 0.01 = 1.25 / 125; a 25 ohm resistor, 50 mA's of current, 0.05 = 1.25 / 25. The disadvantage to this device is that it might require a heat sink if the dissipation is too great, and that it has a fairly low voltage differential limit of 40 volts. This last limit is eased up (50 volts) in the high-voltage version, the LM317HV. For even greater voltage headroom, the Texas Instruments TI783 works, as its voltage limit is 125 VDC. For an even greater voltage limit, a cascoding of any of these regulators with a high voltage MOSFET or tube would do the trick. Alternatively, a zener or zener string could be placed in series with the regulator,

Circuit for up to 40v of bias

Circuit for up to 100v of bias

Circuit for up to 400v of bias