Mellotronists

I finally tried a volume pedal to my Mk VI Mellotron.

It is a Roland EV-5 Expression pedal with a balanced 3-pole cable. 

At low volume(pedal up) the pedal causes distortion on the signal. 
The volume does not disappear completely when the pedal is in up 
position.

There is not much alternation in the volume level. It is roughly 
quiet, distorted in up position, goes first quickly louder but 
distorted when pressing the pedal downwards, and changes the loud, 
clean when going more down. The rest is clean, loud, no change 
(increase) in the volume level.

It appears to me that this pedal is not compatible with the mellotron.

Is here anybody who knows which volume pedals are compatible with Mk 
VI mellotron.

Thanks
Mikko