[Hallicrafters] SX-71 question

[Richard Knoppow]

----- 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