Protecting an LM324 Op-Amp in a Vacuum Tube Deflection Amplifier

[Ravaner]

Hello. Here joined is the schematic of a small deflection amplifier. Outputs H0 & H1 are connected to electrodes placed in a vacuum volume. These outputs are protected by varistors. High voltage to collectors is 200 V DC. Sometimes, not very often, the head Op-Amp (LM324) is burnt. The only credible cause is an ionization or ESD in the vacuum tube that 'come back' until LM324. I am surprised by the fact that only this Op-Amp is destroyed and absolutely never the HV transistors. What can I do more to protect this Op-Amp ?

 

[yungman]

It is very common, I worked with a lot of HV in high vacuum and this happen all the time.

If the op-amp is farther away from the transistor and only the output of the op-amp tie to the transistor, then

First put a 2W resistor of either carbon comp or more expensive Caddock type resistor in series with any 1/4W resistors to pin 7 of the op-amp. Then put two transorbs one up and one down in parallel from ground to the junction between the two resistors. The 2W resistor limit the current, the small resistor protect the op-amp output as the break down voltage of the transorb might not be that accurate and might over the supply voltage of the op-amp. Do the same idea to the other opamp.

If the whole board is on the vacuum flange, you might want to protect the input also. You can use sckottky diode for the input.

This kind of stuff is very common. We always protect the amplifier. We do very high voltage, we even protect the transistors also.

 

[Ravaner]

Many many thanks for your answer. At least someone who encountered this kind of trouble. As I am not absolutely sure that I well understood the changes who propose I send a new schematics. Is it correct ? In that case what must be the X value for the 2 W resistor ?

 

[yungman]

No, I'll draw up something in a little bit.

 

[yungman]

This is what I mean,

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The transorb is P6KEXX where XX is the voltage. I use MBD301 schottky diode. Do it on the other amp.

 

[Ravaner]

Ok, now it's perfectly clear. Thanks a lot.

 

[yungman]

Try this first, it is very hard to pin point where the arc come in, you can just try. This is only based on my experience.