Cathode and grid Bias

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Electronic Dynamic Bias Systems

 

 

Cathode Bias Operation

Over the years I noticed people being overly fussy about having a certain value resistor for R1 in the circuit below. Allegedly resistor R1 is needed to "prevent the cathode from soaring to B+ voltage" or "establishing cut-off bias". Both claims are generally not true. Let's look at a working example:

Simplified bias switching circuit

 

V1=3CX1200A7 (used for measurement data in this example)

R1 is cathode bias resistor when relay is open

D1 is normal zener diode

This is a simplified bias circuit, but it does electrically represent the function of this system.

 

 

Bias circuit resistor dissipation vs resistance

R1 Bias R1 Pd Ip Amperes
infinite 24 0 0
100,000 21.3 0.0045369 0.000213
50,000 21 0.00882 0.00042
25,000 20.75 0.0172225 0.00083
10,000 19 0.0361 0.0019
5,000 17.9 0.064082 0.00358
2,200 18.018 0.14756742 0.00819
       
3CX1200A7  3650 volts anode

Table to the left is dissipation in watts for various values of R1 resistance. Note dissipation increases as resistance decreases over this range. At some point R1 dissipation will peak and start to decline as resistance goes below that value, but this would be with grossly excessive standby current.

Over a normal range of R1 values, dissipation is insignificant.

 

 

 

 

Self-bias voltage vs. resistance value

R1 Bias R1 Pd Ip Amperes
infinite 24 0 0
100,000 21.3 0.0045369 0.000213
50,000 21 0.00882 0.00042
25,000 20.75 0.0172225 0.00083
10,000 19 0.0361 0.0019
5,000 17.9 0.064082 0.00358
2,200 18.018 0.14756742 0.00819
       
3CX1200A7  3650 volts anode

Table to the left indicates measured cathode voltage with various values of R1 resistance. Note that a completely open circuit only produces ~24 volts cathode bias on the 3cx1200A7 tube! A typical 3-500Z produces about 30-35 volts cathode bias with no resistor.

Note there is very little change in bias voltage over the range of 2000 ohms to infinity! The cathode is an almost perfect constant voltage source at low currents.

 

 

 

 

 

 

Grounded Grid Amps