CCS mean Constant Current Source, and you can imagine it like very high AC load resistance (hundreds of kOhms, even MegOhms), connected at the very high B+ supply (kiloVolts). Then our load line becomes horizontal, and in combination with linear tube like 5687 we have very little distortion and amplification about tube mu. Output resistance of the common cathode line stage is load resistor in parallel with tube anode resistance. For 5687, rp