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What should be the cathode resistor value for an EL34 in autobias, seeing 75 mA of current and an anode to cathode voltage of 350 V?

ChatGPT

The cathode resistor value in a self-biasing (also known as autobias or cathode bias) configuration for a vacuum tube like the EL34 is determined by the desired bias current and the tube's characteristics. In this case, you mentioned a bias current of 75 mA and an anode-to-cathode voltage of 350 V for the EL34 tube.

The cathode resistor value (Rk) can be calculated using Ohm's Law and the tube's characteristics. The EL34 typically has a cathode-to-heater voltage (Vk) of around 10-12 volts and a cathode current (Ik) of 75 mA.

Rk = Vk / Ik

Rk = 10 V / 0.075 A
Rk = 133.33 ohms

Rounding up to a standard resistor value, you might choose a 150-ohm resistor for the cathode bias circuit. However, please note that this calculation assumes a simple single-ended configuration and doesn't take into account variations in tube characteristics or potential circuit optimizations. It's also important to ensure that the chosen resistor power rating is sufficient to handle the voltage and current without overheating.

Keep in mind that self-biasing circuits can provide a relatively stable bias point but might not offer the precision or flexibility of other biasing methods, especially for more critical applications or when you want to fine-tune the operating point of the tube. If precision is crucial or if you're designing a more complex circuit, you might want to consider other biasing methods or consulting a tube amplifier design guide or expert.

Avec 350 V Vak, -10 V grille et une Rk de 150R on doit être à 130 mA de courant plaque et une dissipation de plus de 45 watts. Durée de vie de l'EL34 : au mieux quelques heures. Et si on a un peu de chance, le bulbe fondra à certains endroits, donnant un très joli cadavre de tube pour sa collec'.