I read somewhere 6AK5 similar to 5840 so I made a little point to point verion of the Oliver whats his name TLM149 conversion used values he suggested for the 5840..... sounds great dont no if I should change any componet values....I A B ed a Ela circuit I also made with 6072A using Tim CT12 capsule same power supply & out put capacitor 1U pio......the mic sound the same also very quiet.