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

=== Andy Evans ===
Visit our Website:- http://www.artsandmedia.com
Audio, music and health pages and interesting links.

"About the smallest cathode bias resistor you can get away with is rp/mu"

Found the above statement also - this implies that for 6SN7, 10,000/20 gives
500 ohms as 'smallest cathode resistor"


=== Andy Evans ===
Visit our Website:- http://www.artsandmedia.com
Audio, music and health pages and interesting links.

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

"Andy Evans" wrote in message
...
"About the smallest cathode bias resistor you can get away with is rp/mu"

Found the above statement also - this implies that for 6SN7, 10,000/20
gives
500 ohms as 'smallest cathode resistor"


=== Andy Evans ===


Hi Andy,

If you haven't already done so, you might like to try the
6CG7 which is said to be the nine-pin equivalent of the
6SN7. These are plentiful as NOS, and there is also an
8CG7 (8V heater version) which can be bought in the
UK at a very reasonable cost. It is an excellent valve,
and had the advantage of a lower heater current than
its octal sibling. The heater is between pins 4 and 5,
and pin 9 is a shield between the two halves of the valve.


In most circuits where I have seen it used, it draws between
2.2 and 3mA, and so one could expect a very long life.


Cordially,

Iain

If you haven't already done so, you might like to try the 6CG7

I tried a couple of versions of that when I had a CJ Prem 4. Obviously couldn't
compare directly to a 6SN7 because different sockets. I did also try it in a
Leak Stereo 20. I never got any magic out of it - maybe some brands are better
than others. On the other hand, some of the octals are quite exciting. I have
some old round plate 7N7s which are lovely (early 40s and very rare), and the
2C22/7193 is terrific. So are the 6P5GT and CV1932. I bought some 76, 56, 37
and 27 so they're next up. Different sockets again. Morgan Jones tested a lot
of octal valves in his latest book, and generally found them better than
novals, and there are also other distortion tests on the web which confirm low
distortion for the above valves.

=== Andy Evans ===
Visit our Website:- http://www.artsandmedia.com
Audio, music and health pages and interesting links.