Yaqin MC10 to MC13 Phase splitter problem

I noticed this phenomenon on a MC10T whilst testing using a four meter plate current monitor box. I thought it was just an odd amplifier as it was giving such high B+ (HT) but I was to come across it again when servicing a MC13S.
The cure was the same but was it over reaction on my part?
Incidentally, the main fault on the MC13 was an intermittent short inside the Psvane KT90, causing Red plating and blowing the B+ fuse.