[Milsurplus] OK Smart People- Back Bias

[J. Forster]

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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>>
>>
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