hallicrafters: S-107; 6AL5 reduced filament heating

Our member Sinisa Trlin has the following problem:

Why is the Twin Diode 6AL5 filament heating reduced by a 6.8 ohm series resistor?
Specified for 6.3 V / 0.3 amp it is connected via said resistor to the common 6.3 V filament supply.

Such is found in many (all?) post-war models, e.g. S-40, S-53, S-76, 8R40, SX-90,
SX-130, etc.
Pre-war models S-19R and SX-24 are not equipped wich such a series resistor.

Even VTVMs are using the heater reduction.
Same is found for other Twin Diodes like 6H6 or EAA91.

What is the technical reason to reduce emission in such tubes?
To reduce the residual current at the characteristic bend?

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Maybe that a possible reason for reducing the cathode temperature could be the attempt to linearize the small signal diode behavior. From Spangenberg we see that, due to the space charge caused by the initial velocity of electrons, the potential distribution in a diode goes someway negative in the region from the actual cathode to the virtual cathode.

Average velocities are in the order of -0.5 volts at normal operating temperatures, but decrease at lower temperatures.

By the way, for the Hallicrafters S-107 I see a 6H6 listed, not a 6AL5. Maybe there were two versions of the same receiver.

Emilio

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From bama.edebris.com/manuals:

Hallicrafters S-107 Notes from Tom, N4TL.

There are three types of S-107 radios, all use 8 tubes.
S-107, S-107 Mark 1, S-107 Mark 2.

Tubes used in the three versions are as follows.

Mixer, IF, IF, ANL/DET, OSC, AUDIO/BFO,AUDIO OUT, RECTIFIER
S-107 :6BA6, 6BA6, 6BA6, 6H6, 6C4, 6SC7, 6K6, 5Y3
S-107 M1:6BA6, 6BA6, 6BA6, 6AL5, 6C4, 12AX7, 6K6, 5Y3
S-107 M2:6BA6, 6BA6, 6BA6, 6AL5, 6C4, 12AX7, 6AQ5, 5Y3
 

Götz Linß
 

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