NE555 Delay Switch Calculator

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Tube amplifiers need some time for the heaters to warm before a tube starts conducting. Depending on the tube type, this can range from a dozen seconds to a minute.

But normally the tube’s high-voltage supply comes on at the same time as the heater voltage, often supplied by the same transformer. That means the tube sits there for up to a minute with high-voltage applied and no conduction. This can appreciably decrease tube life.

It is compounded by the fact that the high voltage in the initial, unloaded situation can have a much higher value than when the amplifier is operating.

To preserve the tube life, a device should delay the high voltage until after the tube heaters have warmed.

Tube amplifiers with a tube rectifier don't need a delay because the tube rectifier is building up the hight voltage slowly...

Calculation Of The Delay Time in Seconds

The duration of the output pulse ti is through the components R1 and C1 specified. In the diagram above, it becomes clear at which points in the circuit and which states within the NE555 have an influence on the charging time of the capacitor.
Do you want the pulse duration ti set, then you set for the resistance R1 a potentiometer. For the example values ​​given here, a potentiometer of 50 kOhm or 100 kOhm is best.


The capacitor C2 ensures that the circuit does not oscillate. Especially the NE555 can easily get into an uncontrolled swing without this capacitor. When operating as a stable multivibrator, it is the worst. The reason is that at each transition edge at the output of pin 3, a current transient is generated on line + VCC. The solution for the NE555 is an electrolytic capacitor with at least 10 μF and, in parallel, a 100 nF ceramic capacitor between + VCC and GND as backup capacitor as close to the IC as possible. Even very small voltage changes to + VCC are transferred to the resistor network. This results in short-term changes in the reference voltages. The consequence is a disturbance behavior. Especially at higher frequencies. With the capacitor C2, the operation becomes more stable.

What is the meaning of the constant 1.1 in the formula for calculating the pulse duration?

The charging process of C1 starts at the GND level (0 V) and ends at the upper trigger threshold (threshold voltage). This relative voltage has a value of 2/3 of +VCC bzw. 67% von VCC (0,67 * +VCC).
The trigger voltage (67% of +VCC) is greater than the tension from the RT/CT-time constant (RT * CT) of 63%(0,63 * +VCC). That's why the RT/CT-Time constant multiplied by a factor of 1.1. However, due to the inaccuracy of the upper trigger voltage (due to the tolerances of the IC internal resistors), there is a timing inaccuracy of ± 10 percent. This means that the real factor is not 1.1. He may vary between 1.0 and 1.2 for specimen spread.
Because the factor can range from 1.0 to 1.2, take the mean of 1.1 and ignore the tolerance in further calculations.

Calculate R1 ( incl. P1 ) or C1

Enter the Delay Time and R1 or C1

Delay Time(s)   Required
R1 + P1 (Ohm)
C1 (µF)


Calculate Delay Time

R1 (Ohm)    Required
P1 (Ohm)    Required
C1 (µF)    Required


Minimal Time Delay seconds
Maximal Time Delay seconds

Is a delay switch (Standby Switch) really necessary? More info what's going on in the warmingup of tubes.

Another Delay Circuit possibility by Elliott Sound Products.

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