It depends of operative voltage. For EL34 I have used 2,5k for about 300VDC and 5k for about 360VDC. Perhaps about 3,5k is optimum for RL34? Often we first get OT and can double or half its impedance changing loudspeaker and where it is connected.
Basically OT impedance, voltage and bias current define operating point on loadline and it define output power and distortion.
Open Universal Loadline Calculator page and input data there and you can see the loadline and test different biases. On it the V+ voltage is voltage between Anode and Cathode so input value which is deducted from your B+1 what voltage loss comes on OT primary coil and carhode resistor. Now when EL34 uses higher current there comes more voltage loss to OT as well. Screen voltage effect to g1 voltage. You can start using your current 6V6 values first.
When you input headroom there comes also distortion boxes. For single ended I set headroom so that 2nd and 3rd distortion are about the same so comparing different setups is easier. Cathode resistor value comes: Grid Bias Voltage : divided by : Quiescent Operating Point.
thank you! is there a short version for the math-impaired? Like just stick a 4 ohms or 16 ohm speaker??
I have changed OT impedances both ways "one step" and plan to use 4 ohm speaker on 16 ohm output for very high tube load on future push pull build.
I always use Universal Loadline Calculator to set operating point I want which means I use lower B+1 when I increase load. I think it is great when there is no need to search tube datasheets and draw loadlines when it takes half a minute to set a new tube and few seconds to change setting. Giuseppe Amato has done great job there!!! His e-book is good and was only $5.
So far I have not burn any OT although I have dimed all my builds running them to reactive attenuators. I don't have a tube tester so I can't say anything of tube wear yet when my amps do not get enough use.