Would Vb=18V be enough and the control grid floating? (Attention to any voltage for safety) (a suitable fuse must always be used for current safety too). And then perhaps if everything sounds fine increasing cassette magnetic tape speed to 7.5ips. Or a nice portable reel to reel tape recorder running at 7.5ips or more.
According to some previous tests on Pleiades V6 used as a mic booster. Using the EF183 grid at space potential and anode supply of 9V gives an anode current of 25μA. Doubling Vb increases anode current many times as there is the penetration of the anode field to grid [Deketh]. See previous posts.
If for example a current of 400μA is desired but is not achieved by Vb=18V then either the anode supply must be increased or a suitable megohms value of anode to grid resistor chosen.
Or using other electron tubes. The 1H4 seems to have a large anode field penetration affecting grid space potential.
See also:
https://euroelectron.blogspot.com/2018/10/connecting-directly-tape-recording-head.html
Fundamentals of radio valve technique - Deketh - Philips
Technical Library
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