I'd guess that it's not normal with tubes.
Without looking-up the circuit diagram of preamp it's hard to realy make a conclusion why it takes place or poised to take place and how often.
6922 is a dual-triode very often can be used as a differencial input(if you're familiar with transistors that will be similar to the differential stage on two identical transistors) and in regular voltage-gain stage that can use such dual triode in number of different ways. The gain stage is where the circuit stability is most on demand that pretty much depend on power supply and regime the tube is being operated.
Without looking-up the circuit diagram of preamp it's hard to realy make a conclusion why it takes place or poised to take place and how often.
6922 is a dual-triode very often can be used as a differencial input(if you're familiar with transistors that will be similar to the differential stage on two identical transistors) and in regular voltage-gain stage that can use such dual triode in number of different ways. The gain stage is where the circuit stability is most on demand that pretty much depend on power supply and regime the tube is being operated.