Adi, it would be worthwhile trying an ECL80 as the output valve in a design like this. These valves are rattling around in almost any valve junk box - they were a mainstay of 50s 405 line TVs. You don't need to worry about the triode section, just use the output pentode. The heater is only 300ma so would help your battery life. I'm sure lots of fourum members would be happy to send you a couple without charge.
http://www.r-type.org/exhib/aaa0227.htm
Another low powered output valve is the EL42, which is electrically the same as an EL85 but with a B8A base.
http://www.mif.pg.gda.pl/homepages/f...030/e/EL42.pdf
Good luck, Paul