[Boatanchors] [Milsurplus] OK Smart People- Back Bias

[J. Forster]

http://home.netcom.com/~arc5/bias/backbias.jpg

The best way to run the thing is to develop the negative bias voltage in
as similar way as it was originally, essentially aqpplying the bias
between A- and B-.

-John

================



> I do not see that circuit. In the circuit shown, there is no way it would
> work. Of course a battery would, but, my comment that you had to find
> fault with, was on his Zener circuit shown. - Mike
>
> Sent from my iPad
>
> On Jul 31, 2012, at 5:10 PM, "J. Forster" <jfor at quikus.com> wrote:
>
>> Originally, Dave's circuit shows the negtive bias is developed across
>> the
>> 620 Ohm resistor in the B- line.
>>
>> That voltage is essentully filtered and applied acrossa the 20k pot.
>>
>> If a 9V battery replaced the voltage developed across the 620 Ohm, the
>> circuit would be battery, Zener, 20k pot.
>>
>> -John
>>
>> ===============
>>
>>
>>
>>> Well John, even if that were true, where are those micro amps coming
>>> from
>>> in that circuit? - Mike
>>>
>>> Sent from my iPad
>>>
>>> On Jul 31, 2012, at 4:48 PM, "J. Forster" <jfor at quikus.com> wrote:
>>>
>>>> Depends on the Zener. Some have very sharp knees in the microamp
>>>> region.
>>>>
>>>> -John
>>>>
>>>> ================
>>>>
>>>>
>>>>
>>>>
>>>>> Dave -
>>>>>
>>>>> The Zener idea will not work due to the real high resistance values
>>>>> in
>>>>> the
>>>>> existing circuit of the 474. 73 - Mike
>>>>>
>>>>> Sent from my iPad
>>>>>
>>>>> On Jul 31, 2012, at 11:51 AM, "David Stinson" <arc5 at ix.netcom.com>
>>>>> wrote:
>>>>>
>>>>>> I have a BC-474 receiver that, as originally designed,
>>>>>> uses a backbias resistor in the B- lead to provide audio
>>>>>> PA and AVC bias.  R25 and C21 provide the bias.
>>>>>> Here's the diagram:
>>>>>>
>>>>>> http://home.netcom.com/~arc5/bias/backbias.jpg
>>>>>>
>>>>>> I'd like to make a temporary "tacked-in" change so I can
>>>>>> use a supply with B- grounded for a short time.
>>>>>> Will 5-volt zeners connected this way work?
>>>>>>
>>>>>> http://home.netcom.com/~arc5/bias/zenerbias.jpg
>>>>>>
>>>>>> Thanks!
>>>>>>
>>>>>>
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