[ARC5] Bias Battery

[Richard Knoppow]

    This is known as back-bias. It can also be done with a 
transformer tap. The RCA AR-88 uses back bias to eliminate the 
need for an electrolytic capacitor to bypass the audio cathodes. 
However, where back-bias is used it must reduce the amount of B+ 
voltage. The idea of using bias batteries is to avoid this where 
B+ is from batteries and is limited. Bias batteries were also 
very steady where it was desired to have very constant fixed bias.
    Not all bias batteries were mercury cells, I have an ancient 
Boonton sweep generator which uses a D cell as a bias cell.

On 2/5/2020 11:38 AM, Hubert Miller wrote:
>
>     I always thought the smartest way to get -bias was the
>     resistor in the B- lead to ground. If you just need say, -4.5,
>
>     this is not a serious loss to the B+.
>
>     This is fine for a civilian type radio with battery snaps
>     but when you have an ex-military radio like the BC-474, some
>
>     ham is going to see 2 two wires for B supply and say, well
>     why not just connect the B- to chassis? And a little later,
>
>     he wonders why the audio output transformer went.
>
>     "Contact bias" via a high R grid resistor is fine but only
>     gives you, I think, up to about -1.5 volt.
>     I saw one trick circuit in a 1950s Radio News that used the
>     electrons on the grid, via "contact bias", to power a
>
>     small oscillator with the same tube, very low power
>     oscillator. And no – the filament was not connected to the
>     oscillator
>
>     circuit, so it wasn't getting its B+ that way.
>
>     -Hue
>
>
>

-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL