[Milsurplus] OK Smart People- Back Bias

[mac]

Dave,

What bias voltage (i.e., the voltage drop across R25) does the -474  
manual specify?  You mention 5 volts in your e-mail but show 7 volt  
zeners in your proposed circuit.

Looking at the circuit again I would recommend -9 volts (derived from  
a battery or otherwise) which is the bias voltage specified for the  
pentode section of the 1D8GT.  Minus 9 volts bias applied to the  
bottom of R15 would also give a broader range of RF gain control then  
would be the case with -5 or -7 volts.

Dennis D.  W7QHO
Glendale, CA

******************
On Jul 31, 2012, at 8:51 AM, David Stinson 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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