The following is an excerpt from some of the recommendations I received from Mats on tuning up the amplifier. He did such a good job of tuning the cavity before sending it to me, I found that the only adjustments I needed to make were to peak the cathode and anode tuners. I did not experience the large variations mentioned here relating to reverse screen current, but the caution is certainly valid.
Naturally, if you experiment a bit and change the plate voltage, you'll need to reset the idling current (I set mine at 50ma); and re-peak the anode tuner, which is why it's a good idea to have a front panel control for it. The cathode tuner can be set and forgotten unless the tube is replaced.