[Hallicrafters] SX-101A Question

[Roger D Johnson]

Mark K3MSB wrote:
> Hi gang --
> 
> Recapping going nicely,  with some "re-resistoring" needed too.....
> 
> I've a question concening the AM Detector/AVC/ANL tube V7, with is the 
> 6BJ7.
> 
> Why are there 2 parallel 6.8 1/2 Resistors connected to the filament?
> 
> Those resistors go to "x" on the schemtic, which I can't find (my
> schematic is a copy of a copy, so maybe it's not clear....), but since
> other tube's filaments go to "x",  and those other tubes have 6.3V AC
> filament voltages,   "x" has to go to 6.3V AC somewhere.   But, the
> 6BJ7 requires 6.3V AC anyway, so why put those dropping resistors in
> there in the first place?
> 
> I was going to replace them with a 3.3 ohm 1W resistor, but I kept
> wondering why the designers might have chosen to parallel a pair of
> 6.8 ohm resistors in the first place?  Perhaps they didn't have 3.3
> ohms resistors as a standard value back then?
> 
> Thanks and 73
> 
> Mark K3MSB

Hi Mark...

I found this on the web:

 >There is a tube phenomenon whereby the heater (filament) and cathode
 >of the tube form a "parasitic diode".  One side of the filament is
 >grounded, and so when the filament AC goes negative, 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. The culprit in
 >the 2-B is the NL diode, since it's cathode is connected directly to
 >the Hi-Z audio input.
 >
 >Hallicrafter's used a series resistor in the filament circuit of
 >their 6H6 / 6AL5 detectors, but Drake had the option of an 8 volt
 >tube available for series string TVs and could eliminate the resistor.
 >
 >73, Garey - K4OAH
 >Atlanta

73, Roger N1RJ

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