[Hallicrafters] Headphone connection for S-40A

[Richard Knoppow]

     Hallicrafters used the same arrangement for headphones from the 
S-20R to I think the S-40B. The jack has a "normal" that disconnects the 
speaker from the speaker winding of the transformer and connects the 
phones to the grid of the output tube. This results in a low level 
signal needing high impedance phones. I think it leaves the output 
transformer without a load but have not confirmed this yet. The 
arrangement was changed in the S-120, about the last of this series, to 
the more common one of switching from the speaker to the phones on the 
output with a minimum load resistor on the speaker output winding. One 
the earlier sets the phones work fine with the output tube removed.
    The handbooks say the tone control works for both speakers and 
phones but since its just a capacitor or capacitor and series resistor 
across the speaker it no effect on the phones at all. This error was 
repeated in all the handbooks from the S-20R through the S-40B. One of 
these books, I think actually a Service Bulletin also says the phones 
should be low impedance, that's wrong. I think the S-20R book says 
standard magnetic or crystal phones, meaning high impedance.
     I think it would not be difficult to modify the connection so that 
the phones were fed from the speaker output. That way you could use 
modern high fidelity phones if you wanted.
     I just had my S-40A open to clean the switches and touch up the 
alignment. The case on mine requires some struggling to get the screws 
back, something not quite in line. I don't want to open it again just to 
check if there is still speaker drive on the jack when phones are 
connected but did confirm the wiring is as shown in the schematic.
    Curiously enough the S-77, which is the AC/DC version of the S-40, 
has the regular headphone connection in place of the speaker.
    Hallicrafters sure did some funny things.
-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL