Since all the filaments in this Airline radio already add up to 118v why did they need to use R-15 a 39 ohm resistor in the middle of the string? Especially since the print shows the input AC at 117v?
http://www.nostalgiaair.org/PagesByModel/713/M0011713.pdf

Possibly to limit cold filament current surge.
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:Since all the filaments in this Airline radio already add up to 118v why did they need to use R-15 a 39 ohm resistor in the middle of the string? Especially since the print shows the input AC at 117v?
:http://www.nostalgiaair.org/PagesByModel/713/M0011713.pdf

Peter: The schematic shows voltages with 117V on the line. But the specs show the power supply rated for 105-125V, a pretty wide range. If 125V were actually the line voltage, then the heaters would draw too much current?
Doug

:Since all the filaments in this Airline radio already add up to 118v why did they need to use R-15 a 39 ohm resistor in the middle of the string? Especially since the print shows the input AC at 117v?
:http://www.nostalgiaair.org/PagesByModel/713/M0011713.pdf

You also have R16 to limit the surge current into the B+ line. Probably to keep the pilot light bulb from burning out if the radio was turned off and right back on again with discharged filter caps and hot filaments. They sometimes would do that.

Lewis

:Peter: The schematic shows voltages with 117V on the line. But the specs show the power supply rated for 105-125V, a pretty wide range. If 125V were actually the line voltage, then the heaters would draw too much current?
:Doug
:
:
::Since all the filaments in this Airline radio already add up to 118v why did they need to use R-15 a 39 ohm resistor in the middle of the string? Especially since the print shows the input AC at 117v?
::http://www.nostalgiaair.org/PagesByModel/713/M0011713.pdf

:Peter: The schematic shows voltages with 117V on the line. But the specs show the power supply rated for 105-125V, a pretty wide range. If 125V were actually the line voltage, then the heaters would draw too much current?
:Doug
:
Yes Doug I saw that but .... in the real world that doesn't seem to really make a difference.... most homes today are living with about 125vAC anyway.. and these radio seem to be quite fine.... no?

Yes, I agree. It's a little odd.

But if, for example, a 120-V incandescent lamp is run at, say, 125V, it's life will drop quite significantly. (It's not a linear function.) I imagine that tube heaters behave similarly?

I've got my books packed away for a move, so I can't research this. I recall that the Radiotron Designer's Handbook has some curves showing heater life vs. voltage.

As long as tubes have good cathode emission, they usually work well with lower-than-rated voltage. And with higher voltage, they work very well - until they burn out.
Doug

::
:Yes Doug I saw that but .... in the real world that doesn't seem to really make a difference.... most homes today are living with about 125vAC anyway.. and these radio seem to be quite fine.... no?

http://www.nostalgiaair.org/PagesByModel/076/M0015076.pdf

The above radio uses the same filament string, but without the resistor. I had this radio (the case broke, and I used the chassis for years, but finally parted it out recently). My sister also has this radio in good condition. Both radios work well, but both of them did have at least one burned out tube, though that seems to be typical of any AC/DC radio, since the tubes usually stress a bit more at turn-on than in AC radios.

Running the heaters a bit on the hot side can also weaken the cathode. Excess heat drives out impurities, which poison the oxide. At any rate, the radio will function well without an extra resistor in the string. The resistor is a nice idea, though.

T.

Peter

Anon gave the asnwer. Resistor takes the initial sure when voltage is first applied to filaments. Philco wrote this up but not sure where? In some of their radios Philco used a 35L6 instead of 50L6 and added the surge resistor. They could have used a 50L6 and no resistor.

Norm

:Since all the filaments in this Airline radio already add up to 118v why did they need to use R-15 a 39 ohm resistor in the middle of the string? Especially since the print shows the input AC at 117v?
:http://www.nostalgiaair.org/PagesByModel/713/M0011713.pdf