[AMRadio] Elmac Bias Battery

JAMES HANLON knjhanlon at msn.com
Mon Nov 9 19:56:53 EST 2009

I've long since replaced the bias batteries in my AF-67 and AF-68 with a Zener Diode in series with the 6L6 cathodes.  You can get a 5 watt, 22 volt Zener, type !N5358BG, in an axial leaded package from Mouser Electronics for 32 cents each.  It is Mouser part number 863-1N5358BG.  A 5 watt, 22 volt Zener will handle a continuous current of 227 ma, far more than necessary to carry the entire cathode current of the modulators.  Just put it in between the cathodes and ground, oriented so that the cathodes are more positive than ground, with the diode arrow pointing from ground and toward the cathodes.  Then just ground the center tap of the driver transformer that was formerly attached to the bias battery.

Yes, it does borrow 22 volts from the total modulator plate voltage, but that has been no problem for full modulation in my rigs.  

It is much cheaper than a battery, and it is a permanent solution.  You will never again have to replace a bias battery.  

Mouser is on the web at www.mouser.com<http://www.mouser.com/> . They have no minimum order amount, so they will gladly sell you one Zener diode is you wish.


Jim Hanlon, W8KGI

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