[Hallicrafters] SX-101A Question

[Peter Markavage]

They're on my SX-101A schematic.

Pete, wa2cwa

On Sat, 23 Dec 2006 17:39:16 -0500 Roger D Johnson <n1rj at adelphia.net>
writes:
> 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