Bias advice for class C

[KA2DZT]

Fred. The Fixed bias is there all the time though right?   The Grid leak just goes over that fixed level.  Unless I am reading this wrong.

C

The part you have to look at is the diode linking the bias supply to the grid circuit.  The fixed bias supply is on all the time but when the grid leak bias is more negative than the fixed bias the diode is biased off, so it's like the bias supply is not there.

Sometimes it's harder to see things when you're dealing with negative voltages.  Another thing you have to consider, with that circuit, is that the grid leak resistor acts as a load resistor on the bias supply when there is no drive.  So that 5K grid leak resistor pulls about 50 ma from the bias supply (at -250V).  If there was some drive (but not enough) the 5k resistor would load the bias supply less.  Once there is enough drive to produce -250V there would be no load on the bias supply from the 5K grid leak resistor.  Once the drive produces more than -250V the diode is reversed biased and it's like the bias supply is not there.

So, the bias supply has to able to supply some current and maintain the -250 volts.  Another thing is that, with no drive, the tube doesn't draw any current from the bias supply.  The grid draws current on the positive peaks of the drive signal.  As the grid becomes more negative the grid is drawing current for a shorter duration of the (pos 1/2) sine wave.  As you can see the class C final is really a pulse generator.  It's the tank circuit that produces the full sine wave output signal.

It was really hot here in NJ today.  My brain is somewhat melted, but I think everything above is correct.  I welcome any corrections from others on the forum.

Fred

[PA4WM]

  The fixed bias supply is on all the time but when the grid leak bias is more negative than the fixed bias the diode is biased off, so it's like the bias supply is not there.
[/quote]

Does that mean the gridleak resistor has to be calculated for the total amount of negative voltage needed, instead of just the part to add to the fixed supply?

Martin

Ups... something went wrong with the quote... sri

[KA2DZT]

  The fixed bias supply is on all the time but when the grid leak bias is more negative than the fixed bias the diode is biased off, so it's like the bias supply is not there.

Does that mean the gridleak resistor has to be calculated for the total amount of negative voltage needed, instead of just the part to add to the fixed supply?

Martin

Ups... something went wrong with the quote... sri
[/quote]

Martin,

You're exactly correct.  The grid leak resistor is calculated for the total negative grid voltage.  Don't forget to figure the power dissipated in the grid resistor.  You can use any combination of resistors to get the power rating and resistance.

Remember I'm referring to circuit A in the original post.

Fred

[AMLOVER]

Yes, exactly.
In the A case the grid leak resistor must be calculated suitable to provide the total bias. I changed a bit the sircuit in order to get the possibility of B or AB class.
I will never use my C class as a linear amp but I have heard that keeping a little idle anode current (100ma in my case) in expense of some power during receiving periods keeps my reactor peaceful when I cut off the driver. I use to keep the anode high voltage on during receiving so handling the reactor's energy calm is a special task.

Stefano

[KM1H]

Its always a good idea to bias any Class amp to about 2X cutoff when in receive/standby.

First it eliminates any diode noise in the receiver and secondly it offers much better protection from a parasitic or neutralization type oscillation.

Carl

[ke7trp]

Thanks for the clarification. I see how this works now with the diode. That might be the system that I was thinking of in the gates transmitter.  If the fixed supply is not there during the time the exciter is on, That sounds like a great system.


C