i found some working point for the 6h30:
Va (volts) Ia(ma) Vg1(volts) Gm (ma/V, see note) Mu Ra (Rp)(ohms)
50 5 -2.8 7.1 15.2 2100
50 10 -2.3 9.8 15.9 1600
50 20 -1.5 12.3 16.3 1300
90 10 -4.7 8.4 15.3 1800
90 20 -3.9 11.7 16.0 1400
90 30 -3.1 13.3 16.2 1200
120 10 -6.5 7.6 14.9 2000
120 20 -5.6 10.9 15.7 1400
120 30 -4.8 12.9 16.1 1300
120 40 -3.9 13.3 16.3 1200
180 10 -10 6.5 14.5 2200
180 20 -8.9 10.0 15.3 1500
180 40 -7.1 13.3 16.0 1200
250 10 -14.4 5.8 14.4 2500
250 20 -13.2 8.7 15.0 1700
250 30 -12.1 11.0 15.4 1400
250 40 -10.5 12.5 15.8 1300
IMHO the working point:
180V/20mA Vg1~9V (battery bias!!!) gives a mu of 15 and 1500 Ohm.
What are your favorites working point with that tube?
Kind Regards
Barossi