[Hallicrafters] SX-71 question

[Art Lebermann]

Does anyone know why the two diodes would be more sensitive to hum than the
other tubes?  Any other possible reasons for running reduced voltage?  I
suspect that there is another answer.

ART
W6REQ


> [Original Message]
> From: Carl <km1h at jeremy.mv.com>
> To: <Hallicrafters at mailman.qth.net>; Asif Khan <hallicraftersx28 at aol.com>
> Date: 12/9/2010 5:24:37 AM
> Subject: Re: [Hallicrafters] SX-71 question
>
> National also reduced the voltage on the 6H6 and later the 6AL5, they
used 
> 4.3 Ohms. Dont know who came first as the SX-71 and HRO-50 both came out
in 
> 1949 but the SX-71 had 4 prodution runs ending in 1954 so was the
resistor 
> in all of them?
>
> Its interesting to note that altho the Radiotron Designers Handbook 4th 
> Edition of 1952 discusses the hum issue and methods to minimize it does
not 
> mention the filament resistor.
>
> Carl
> KM1H
>
>
>
> ----- Original Message ----- 
> From: "Asif Khan" <hallicraftersx28 at aol.com>
> To: <Hallicrafters at mailman.qth.net>
> Sent: Thursday, December 09, 2010 4:37 AM
> Subject: Re: [Hallicrafters] SX-71 question
>
>
> >
> > In the absence of the resistor an objectionable  60 hz hum would appear
in 
> > the audio.The resistor reduces the AC filament  voltage to the ANL tube 
> > thereby also reducing the AC hum .
> > Nick
> >
> >
> >
> >
> >
> >
> >
> > -----Original Message-----
> > From: Art Lebermann <artleb at earthlink.net>
> > To: Hallicrafters <Hallicrafters at mailman.qth.net>
> > Sent: Thu, Dec 9, 2010 1:32 am
> > Subject: [Hallicrafters] SX-71 question
> >
> >
> >
> > he SX-71 schematic shows a 6.8 ohm resistor in the filament connections
to 
> > V11
> > 6H6 - AVC/ANL) and V12 (6AL5 - FM detector).  Does anyone know the
reason 
> > for
> > educing the filament voltage to these tubes?
> > 73,
> > RT
> > 6REQ
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