RD50 self-biasing - how to tweak?

Anyone out there done any work on bias in the RD50's, which appear to be fixed- or self-biasing?
I measure 28/38 mv across the 6.8 ( actually 7.5)cathode resistors, which is about 4-6 ma per tube - much colder than other models. I'd like to increase static current draw to about 12ma, but the RD bias circuit is quite different to other models. Anyone looked at this?