[Milsurplus] OK Smart People- Back Bias

[Mike Feher]

You can have the last word by stating the obvious. - Mike

Sent from my iPad

On Jul 31, 2012, at 5:39 PM, "J. Forster" <jfor at quikus.com> wrote:

> 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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> 
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