• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Grid current operation

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Now I'm running them at 6mA each. Is it ok, or should I go higher?

I tend to use 10 to 20 mA. You want the source to drain voltage high enough so that the Crss curve becomes flat. This is known as the reverse transfer capacitance and is the capacitance seen by the driver in a follower circuit. You don't want your driver seing a capacitance that varies with signal voltage. The voltage across the fet and the current through it determines the heat sink needed. I have found no advantage to running more than 10 mA on small output tubes, and 20 mA on big triodes.

The mosfet provides a path from the B+ (or whatever positive voltage is on the drain) to supply grid current that is limited only by the Rds ON of the fet (a few ohms) Idle current is not a factor. The idle current path is needed to discharge the output tubes Miller capacitance in the event of a large negative going musical transient.

There was a somewhat popular design published a few years ago that used a 6V6GT cathode follower tied to the grid of an 811A with no resistor or other path to a negative supply or ground. Apply a large square wave drive to this amplifier and see what happens. It's ugly.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.