[Hallicrafters] SX-71 question

[Chuck McGregor]

The 6.8 ohm resistor in the 6H6 heater circuit was originally added as 
Revision G to SX-42 schematic drawing # 89D257 on Feb 14, 1948.  The 
purpose was to reduce heater temperature, to reduce the effect of a 
virtual diode between the heater and cathode which was a cause of hum in 
the noise limiter.  This detail, like much else, was copied into the 
designs of their "better" tube radios that followed the SX-42.

Some authors have asserted that the change had something to do with the 
detector side of the tube.  This was not the case for the SX-42, as AM 
detection was not done in the 6H6 for SX-42 radios built to schematic 
89D257.  The 6H6 was used for noise limiter only.

73,
Chuck N7RHU

On 12/9/2010 10:35 AM, rbethman wrote:
> Hallicrafters did the same to the "ANL" tube even in the SX-101 MKIII.
>
> They apparently had their reasons, but did not "detail" them in the
> manuals that they produced.
>
> The instance of the SX-101 MKIII, uses the identical ohm and wattage
> rating/value as you have found in the SX-71.
>
> Bob - N0DGN
>
> On 12/9/2010 12:05 PM, Art Lebermann wrote:
>> 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 reducing the filament voltage to these tubes?
>> 73,
>> RT
>> 6REQ
>