Hi, Vacuumlanders,
I'm refurbishing the Bendix radios and intercom ("interphone") from a
WW2 Lancaster bomber that we are rebuilding for static display at the
Canadian Air & Space museum, Toronto.
See http://casmuseum.org/avro_683_lancaster_x.shtml
I have the interphone amplifier working on the bench using a Heathkit
variable, regulated B+ supply and a 26 volt supply for the heaters
(there are internal ww resistors to get about 6.3 volts on each
tube.) The tubes are one 6V6GT and one 6SJ7. I'd appreciate the
opinion of the experts on the 6SJ7 operating point found in this unit.
The plate load resistor is 470 Kohms, the cathode bias resistor is 470
ohms, the screen is fed from the tap between an 82 Kohms to B+ and a
10 Kohms to cathode (a 2 mA drain) and sits at +22.5 VDC to chassis
(ground.)
For an external 220 VDC applied, the B+ rail for the 6SJ7 is about 185
VDC (after a B+ RC filter.) The plate voltage is only +30 VDC for a
0.33 mA plate current. Cathode bias is 1.0 VDC. While this tube is
use as a low-signal microphone amplifier, the plate and screen
voltages and plate current seem rather low to me. While the gain,
given by (gm x RP) x RL/(RP + RL) seems good enough (not measured
other than by a "wet finger"!) and RL is certainly nice and high, does
the tube gain (gm x RP) hold up under such low current and voltage?
This operating point is so low down on the left of the tube
characteristic curves that its far below the knee of the pentode
family of curves, where they all combine, so you can't read it.
6SJ7 tube data (for normal operating points): RP = 700 Kohm, gm =
1.575 mA/volt, Thus, mu =~ 1100
In the spirit of "museum quality" refurbishing, regardless of any
better design, I certainly do not propose to change the values!
Thanks for all replies.
Cheers,
Roger
PS. I've also cross-posted in "rec.audio.tubes".