G7 tube mic weird frequency response depending on pattern

[Gachet]

Hi

I've built a gyraf G7 tube mic with tim campbell capsule.


When I switch directivities I have:

Figure 8: Lots of lo end , a bit woofy
Cardio: A great sound pretty well balanced
Omni: No lo end at all, like a 500Hz Hi pass...

I've made some measurement:

Fig 8: Front 0V, center 0.7V,  Back 32V
Cardio: Front 0V, Center 0.7V,  Back 16.5V
Omni: Front 0V,  Center 0.7V,  Back 0V

Is that normal?