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protection measures for the power amplifier circuit with BJTs

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experimental circuit with BJTs

protection measures for the power amplifier in the power amplifier

In a booster, it can happen that a defective output tube work. To other tubes and the amplifier to protect themselves, protective measures must be taken. These are described here.

grid emission or insulation failure may cause a small grid current. In conventional circuits with Gitterableitwiderstand calls this current a voltage drop on Gitterableitwiderstand, which counteracts the negative bias. As a result, the plate current swings high at first unnoticed, leading to the destruction of the tube and the apparatus. This is one of the most common causes of errors in conventional tube radios and amplifiers and is especially happy after overloading the output tube. The galvanic Coupling avoids this known. The grid would have to complete the rest of the current Katodenfolgers take in order to distort the lattice strain dangerous. In this respect, the electrically to the output tube (s) coupled Katodenfolger a very effective measure against this effect.

course of Katodenfolger can even be dangerous when he has a lattice Katodeschluss or even worse, makes a rollover to the anode. The latter is quite possible if the anode is connected to the anode of the Katodenfolgers pentode. The grid of output tube may be destroyed. To counteract this helps a resistor, which is comparatively small signal not should be recognized as a series resistance. With push-pull circuits, which can very elegantly realized with a common protection for resistance in both push-pull working Katodenfolger. If this resistance is correspondingly high resistance or even a power source, so you get a FREE anode coupled differential amplifier .

A very hot potato is a grid-Katodenschluss in an output tube. This also applies to parallel output tubes. The current flow may not raise enough from the line fuse, which in case of failure of the negative bias may still be possible. A protection circuit that also in case of negative bias or a grid in an output tube Katodenschluss appeals should therefore be missing in any power amplifier. How is the 6 x EL34 amp is made, the diagram shows

Here comes again the galvanically coupled Katodenfolger as rescuers into the game. Without taking into account the need for coupling caps, it makes it relatively low-Sense 56KΩ resistors of the current node P (P = Protect) to lead. If the current at node P has a minimum value, here about 1mA, open collector off the stage in the amplifier existing STBY and anode voltage delay circuit immediately on standby, ie anode-energized when an error in the final stage occurs. The electro-mechanical analogy, would be a relay which drops at low 1mA.

A few additional 0.8 mm holes and the circuit is already accommodated on the board of the amplifier.