Dean Jones
You can wire across the bias battery but don't leave it open. Later radios didn't use a bias cell with 6Q7's. In some cases a larger value grid resistor should be used. Your volume control is 2.2 meg and used as a grid resistor which is ok. I prefer this method.
http://www.nostalgiaair.org/PagesByModel/160/M0015160.pdf
If you want to keep the radio original use a calculator or watch battery. These batteries will last their shelf life but eventually need replacing.
Another way to get a slight bias is adding a resistor in the cathode of the 6Q7. There will be a voltage drop across the resistor giving cathode a slight positive potential. This voltage drop supplies grid bias.
Norm
:Hi all,
: I am restoring an RCA 85BT6 (Rider's vol 9, page 57 et. al.) table radio that uses a bias cell in the 2nd detector & AVC circuit of a 6Q7 tube. Any recommendations for its replacement? Another battery (supposed to be .9V), or perhaps a voltage divider added to the powersupply and voltage tapped off of it? Any thoughts or ideas would be much appreciated.
:Thanks for any help.
:
:Dean Jones
Dean
:Hi Dean
:
: You can wire across the bias battery but don't leave it open. Later radios didn't use a bias cell with 6Q7's. In some cases a larger value grid resistor should be used. Your volume control is 2.2 meg and used as a grid resistor which is ok. I prefer this method.
:
:http://www.nostalgiaair.org/PagesByModel/160/M0015160.pdf
:
: If you want to keep the radio original use a calculator or watch battery. These batteries will last their shelf life but eventually need replacing.
:
: Another way to get a slight bias is adding a resistor in the cathode of the 6Q7. There will be a voltage drop across the resistor giving cathode a slight positive potential. This voltage drop supplies grid bias.
:
:Norm
:
:
:
::Hi all,
:: I am restoring an RCA 85BT6 (Rider's vol 9, page 57 et. al.) table radio that uses a bias cell in the 2nd detector & AVC circuit of a 6Q7 tube. Any recommendations for its replacement? Another battery (supposed to be .9V), or perhaps a voltage divider added to the powersupply and voltage tapped off of it? Any thoughts or ideas would be much appreciated.
::Thanks for any help.
::
::Dean Jones
Question for Norm: I am also working on a set with a bias cell. It is a strange looking battery, apparently designed for long life. Would you or anyone else out there know the composition details? It seems a standard carbon zinc battery wouldn't last that long (The one I have which looks like an acorn still has 0.2Volts after 65 yrs!) Could it be a zinc-air battery? Nick
Here is information on bias cells from Chuck's site:
http://www.philcorepairbench.com/biascell.htm
No current drain on a bias cell so they will last shelf like of the battery. Sometimes they will be found in battery radios to give longer life to the "B" batteries. A bias cell makes grid #1 slightly negative causing a tube to draw less current.
You may measure a slight voltage potential on a digital or other high input resistance meter. Battery won't be able to supply current more than microamps?
Norm
::Hi Dean
::
:: You can wire across the bias battery but don't leave it open. Later radios didn't use a bias cell with 6Q7's. In some cases a larger value grid resistor should be used. Your volume control is 2.2 meg and used as a grid resistor which is ok. I prefer this method.
::
::http://www.nostalgiaair.org/PagesByModel/160/M0015160.pdf
::
:: If you want to keep the radio original use a calculator or watch battery. These batteries will last their shelf life but eventually need replacing.
::
:: Another way to get a slight bias is adding a resistor in the cathode of the 6Q7. There will be a voltage drop across the resistor giving cathode a slight positive potential. This voltage drop supplies grid bias.
::
::Norm
::
::
::
:::Hi all,
::: I am restoring an RCA 85BT6 (Rider's vol 9, page 57 et. al.) table radio that uses a bias cell in the 2nd detector & AVC circuit of a 6Q7 tube. Any recommendations for its replacement? Another battery (supposed to be .9V), or perhaps a voltage divider added to the powersupply and voltage tapped off of it? Any thoughts or ideas would be much appreciated.
:::Thanks for any help.
:::
:::Dean Jones
:
:Question for Norm: I am also working on a set with a bias cell. It is a strange looking battery, apparently designed for long life. Would you or anyone else out there know the composition details? It seems a standard carbon zinc battery wouldn't last that long (The one I have which looks like an acorn still has 0.2Volts after 65 yrs!) Could it be a zinc-air battery? Nick