[Hallicrafters] SX-71 question

[Peter Bertini]

I always thought the filament voltage was dropped to reduce shot
noise generation.

Pete k1zjh

On 12/10/10, Richard Knoppow <1oldlens1 at ix.netcom.com> wrote:
>
> ----- Original Message -----
> From: <wa1kbq at aol.com>
> To: <Hallicrafters at mailman.qth.net>
> Sent: Friday, December 10, 2010 3:21 AM
> Subject: Re: [Hallicrafters] SX-71 question
>
>
>> It seems like if it was hum reduction it would need to be
>> center tapped
>> and connected to B- at the center.
>>
>> Regards, Greg
>>
>>
>> -----Original Message-----
>> From: Carl <km1h at jeremy.mv.com>
>> To: Hallicrafters <Hallicrafters at mailman.qth.net>; Asif
>> Khan
>> <hallicraftersx28 at aol.com>
>> Sent: Thu, Dec 9, 2010 8:24 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
>
>      I remember reading an explanation of the purpose of the
> resistor but no longer remember where. I don't think it has
> anything do to with hum reduction but rather has some effect
> on the diode "law" and perhaps its interaction with the
> detector. Note that series type limiters began to be popular
> in the mid to late 1940's, previous to that most limiters
> were of the shunt type. Series limiters are more effective
> and cause less distortion to the modulation. Reduced heater
> voltage is also used for FM discriminator diodes in some
> receivers.
>
>
> --
> Richard Knoppow
> Los Angeles
> WB6KBL
> dickburk at ix.netcom.com
>
>
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