limits in transient time

[metebalci]

Hi,

Assume we have a fixed-biased triode output stage. If all voltage sources (plate and bias source) are turned on at the same time, is there any possibility of exceeding the rated voltage, current and/or power limits of the valve ? For example, bias source may be delayed by some miliseconds and this may cause high plate current. In other words, do I need to consider transient response ?

Thanks in advance.

MB

[James D.]

In essence - no. Valves are very tolerent of short term over-voltage and will not 'punch through' and destroy themselves like solid state devices

However, it is good practice to engineer the amplifier to start up safetly as unconstrained transients can cause safety problems to the user of the equipment or cause damage to other circuit components. It maybe that a degree of 'soft start' will be provided by the valve heater warm-up time but do not dpened on this - some valves warm-up very quickly, others are engineered to take 20 seconds or so to warm-up.

And remember that a transient overshoot from 200 Volts is quite different from a transient overshoot from 1000 Volts so the circuit context needs to be taken into account.

In my experience the start-up problem is one of over voltage on the powersupply lines until the valve is conducting exceeding capacitor maximum voltage rating and so shortening capacitor lifetime. Always use capacitors of 50%or more higher voltage rating than the static B+ voltage and use valve rectifiers with slow warm-ups too.

I guess one conclusion is that valves are rugged but people (and capacitors etc.) are not...

ciao

James