[HBR] HBR -- Part 8

[Garey Barrell]

Dan -

This is because of the parasitic diode effect that Walt mentioned 
between the filament and the cathode.  It's not a very efficient diode, 
and the effect falls off rapidly with lower filament voltage.  It is 
only a factor where the cathode is in the audio path, and at a high 
impedance point.  The Drake spot was the series Noise Limiter diode in 
the 8BN8.  Early 2-A receivers had a 6BN8 tube that was changed to an 
8BN8 in later versions.  Older receiver designers didn't have all those 
series TV tubes to choose from, and just put a resistor in series with 
the filament.  I have an article on the Drake at

<http://www.wb4hfn.com/DRAKE/DrakeArticles/Why_8BN8-2B.htm>

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
>