[Hallicrafters] SX-71 question

[Carl]

Thanks Chuck and its very interesting. Makes you wonder if National and 
Halli had spies as the timing is more than a coincidence. The HRO-50 was 
already in design so who really discovered that cure? I doubt we'll ever 
know unless it was written up in one of their QST ads. National used 4.3 
Ohms, Halli 6.8, and Collins woke up for the revised 75A2 in June of 1952 
with 10 Ohms. Anybody see a pattern?

Carl
KM1H




----- Original Message ----- 
From: "Chuck McGregor" <cbmcg at comcast.net>
To: "rbethman" <rbethman at comcast.net>
Cc: "HALLICRAFTERS Group" <hallicrafters at mailman.qth.net>
Sent: Thursday, December 09, 2010 2:20 PM
Subject: Re: [Hallicrafters] SX-71 question


> 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
>>
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