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The most common failure mode is heater exhaust, where if not properly handled, it just breaks somewhere, and even if your tube is new, it becomes completely useless. In order to increase the filament lifespan, you can build a simple cycling system, that constantly supplies the heater with some voltage (around 2-3V for a 6.3V filaments for instance) and when an input signal is received on your amplifier, the filament voltage jumps to 6.3V and the amp is immediately started. This will skip some of the heat up time for the amp as well. You must incorporate this system to the HV delay control circuit, so that when the filament voltage falls to 3.3V the HT rail must also be disconnected. This is important, because operating a tube at lower filament voltages with a full HT supply, may poison the cathode and ruin your tubes even faster! The circuit works as it follows:
1. When you enable the amp, the 3.3V heater voltage will enable and keep the tubes semi-turned-on. At this point the HT supply must be blocked and mustn't flow.
2. After an input signal above 2mV is detected (below 2mV it's mostly noise), the comparator starts shaping square waves at the output. A simple detector circuit rectifies this, at the peak voltage of around 11.3V and enables a second comparator, driving the voltage switching relay. After the input signal is disconnected, the detector capacitor starts discharging through the 1Mohm resistor. After it discharges to 2V the output disengages the relay and enters stand-by. The discharge time is around 6minutes with 1Mohm and 220uF. This is the moment to disengage the HT supply as well, with a servo loop to the HT control system.
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