[HBR] HBR -- Part 8

[Garey Barrell]

Dan -

 A little better description.....

There is a phenomenon in vacuum tubes whereby the heater (filament) and 
cathode of the tube form a "parasitic diode". One side of the filament 
is grounded, and so on negative half-cycles of the filament supply, 
rectified 60 Hz pulses are superimposed on the cathode, resulting in hum 
in the audio.

The "cure" is to reduce the filament voltage slightly, since the 
parasitic diode action falls off much faster than the indirectly heated 
"real" diode of the tube's cathode and plate. This is not a problem in 
most conventional uses of tube diodes, since the cathode is usually 
operated at or near ground and/or at a fairly low impedance. The culprit 
in the 2-A/B is the NL diode, since it's cathode is connected directly 
to the Hi-Z audio input.

73, Garey - K4OAH
St Charles, IL



Hopperdhh at aol.com wrote:
> Walt,
>  
> I know that some receivers reduce the filament voltage on just the 
> detector with a series resistor.  One example is the Hallicrafters 
> SX-100 which uses two 6.8 ohm resistor in parallel for the 6BJ7 triple 
> diode.  Also, the Knight R-100 used a 3.3 ohm resistor to the triple 
> diode 6BC7.  I wonder if this may have been for the benefit of the 
> noise limiter section because the Drake 2-B used an 8BN8 for the noise 
> limiter and first audio amp powered from 6.3 volts.  I always wondered 
> what benefit that the lower filament voltage gave.  Maybe you can shed 
> a little more light on the subject.
>  
> Thanks,
>  
> Dan
>  
>
>
>
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