[Milsurplus] OK Smart People- Back Bias

[Mike Feher]

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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