On Mar 17, 6:37*pm, Engineer wrote:
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".

Interesting. hadn't realized that perhaps later versions of the
Lanchester, perhaps those built in Canada? had North American (Bendix)
electronics. Always thought that they had British style electronics
based on the T1154 plus R1155 etc. Plus the later H2S and either
British or US versions of the IFF with their explosive charges in a
tube in the middle.
I do recall some USA Bendix 'crew intercom' units on sale war
surplus.Had a bright aluminum case IIRC; whereas British stuff was
generally black (sometimes black crackle?). Two tubes, but I thought
one of them was 28D7 or something and the other was a 6.3 volt tube
such as the 6SJ7 mentioned with a dropping resistor for it's heater.
Was also under impression those units operated totally at 28 volts,
both for plate and heater voltage. probably a different unit?
Another note; later in WWII both USA and UK aircraft probably both
used SCR522, or equivalent VHF radios?