Discharges in the tubes, bias current regulators and related problems

Almost everybody who built up a PA with a larger tube (especially tetrode) experienced a “slap” in the tube.

What is that “slap” exactly? - It’s a discharge in the diluted gas, because the vacuum in the tube is never perfect. The tube’s ceramic body has microscopic voids through which air molecules can get in. These molecules can get ionized and so excite a discharge. In order to eliminate this effect tubes contain a special substance called getter which is able when warmed to absorb the gas molecules.

What happens exactly if a discharge occurs? - The tube can be compared to a spark gap at this moment. The electrical resistance of the discharge is very low (a couple of ohms). This means, that that a huge current would flow primarily between the anode and the grid (since the grid is closer to the anode), but partly also between the cathode. What are the consequences of this process?

A high danger for the tube. -The energy of the charge in the anode power supply capacitors dissipates in the tube. Most likely the grid would be blown or at least a big hole would be burnt in it. The first case means the total destruction of the tube, the second one means increase of the capacity anode/cathode and suddenly we have an oscillator instead of a stable PA. In addition the cathode portion of the discharge current can blow the bias current regulator.

What can be done to protect the usually expensive tube? - There are basically several ways:

1. The most simple solution is putting a resistor about 15 – 20 ohm between the power supply and the anode. It must be well sized due to big power dissipation at the moment of discharge. Since the discharge resistance is just a couple ohms, the discharge current is virtually limited just by the inner resistance of the anode power supply. At the beginning of the discharge is the inner resistance almost zero due to the charge accumulated in the filter capacitors. It means that at 3000V anode voltage the peak current would be about 150A! It must be taken into account when choosing the right resistor. The best types are the ones on a ceramic body with dick resistance wire with at least 20W power dissipation. Don’t use those in white coat, they could explode.

2. Using an anode power supply with current protection. This type of power supply can discharge the filter capacitors into resistors when the anode current exceeds certain level and so limit the current through the tube. Check description of the power supply by OK1VPZ with such a protection.


3. The bias current and the grid glowing. – A very important though often underestimated factor for the surviving of the tube is the bias current level. If this current is unnecessary low the grid catches (due to its potential) part of the electrons from the cathode. This causes an additional grid current and the grid can even glow under this load. A glowing grid is much more sensitive to the discharge current and a hole is very likely to be burned in it when a discharge occurs.

BIAS circuit robustness - within "slap" in triode PA tube may high peak current destroy usual (zener diode empowered by transistor) BIAS circuit - particularly Base - Emitor  transition of transistor. Use appropriate resistor in series to Base. It will increase a bit internal resistance of this circuit, but for such use it is not critical.