Andy Evans wrote:
Re. conversation with Nick G, dug up this bit of calc. from Paul Joppa:
For current, I generally like to keep the "operating resistance"
(plate-cathode voltage divided by current) to 3 to 5 times the plate
resistance. At 4-5mA, plate resistance of an SN7 is around 10k. So the current
would be 130v/(30-50k ohms) or 2.6 to 4.3 mA with a 30k to 50k plate load on
the first stage and the same value load on the second stage cathode. Grid bias
will be 0.5 to 0.7 times plate-cathode voltage, divided by mu, or 3.25 to
4.55v. You want to keep the grid negative by 2v at all times if possible, to
minimize the effect of tiny leakage grid currents.
30k-50k anode, 2.6-4.3mA Joppa

So, for 100v A-K, and 10k rA for the 6SN7, a ratio of 3 to 5 would suggest
current of 2ma to 3.3mA


Interesting, I am at the lower end of that at 2ma, I will try increasing
the current by a tad and see what effect it has.

--
Nick