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6922 Operating Point Limitations

PostPosted: 29 Jul 2013, 09:48
This post is a continuation of the discussion started in an earlier post concerning a design of an amplifier based on 6922 tube.

1. Grid Bias Voltage

In the circuit below 6922 is biased by applying a bias voltage to the grid via R2.


Returning back to the 6922 load line depicted below, we find that as anode voltage Va rises, the grid curves are getting closer to each other. This is indication of non-linearity. Non-linearity is particularly profound close to the 250 Volt point. The region around 250 Volt point is called cut-off point. Designing a circuit to operate around the cut-off point is not advisable if a good linearity of an amplifier is required.


2. Maximum Allowable Anode Voltage and Anode Dissipation

Moving in the opposite direction along the load line, we use the 6922 harder and harder, until finally there is no voltage across it. If however, we have a voltage across the 6922, and a current flowing through it, we are dissipating power within the tube. The limit of power that the 6922 can dissipate is known as the maximum anode dissipation and it appears on the 6922 datasheet. The 6922's anode dissipation limit is 1.5 Watt for each anode.

The 6922 datasheet also specifies two additional, and connected, restrictions for the choice of the bias point. They are maximum anode voltage Va and Va(b). Maximum Va for the 6922 is the maximum DC voltage at which the 6922 could be continuously operated and is 250VDC. The Va(b) is the maximum voltage to which 6922's anode is allowed to swing under signal or cold conditions and is about 300 VDC for the 6922. Exceeding the Va and Va(b) limits usually results in destruction of the 6922 tube.

3. Maximum Allowable Cathode Current

The final limitation is the maximum allowable cathode current Ic. The maximum allowable cathode current for the 6922 tube is 100 mA.

Proceed to the next chapter.