Just wondering if a Russian near equivalent of the Ac701 tube exsists , Identical characteristics wouldnt be required , grid exiting the top of the valve would be great . 5840 /EF732 is a superb tube for cathode follower mics ,as used by Bruel & Kjaer , and Ive had success building mics with them ,but the top cap grid would be really sweet as it should have very low capacitance and fit perfectly into a pencil mic, solder directly to 7 pin xlr , only the grid going up to the capsule with no wires crossing over , of course the 5840 can be placed inverted and the leads appart from grid bent back around the tube and down to the connector ,but you probably need to screen the tube in that case .I have seen Russian sub mini's with plate top cap connection ,but havent been able to find grid top cap connection .