Watch out!

First of all you should be clearly aware that most gear and circuits using tubes run on lethal voltages. If you are new to high tension circuits, learn the according safety rules first - and then act according to them - always. Never get lazy!

It would be wise to do your first steps under the supervision of someone who knows well about the immanent dangers of tube circuits, e.g., a television repair man or a radio amateur who builds his own gear. The older this guy is, the better: First, he obviously survived by knowing the rules and acting accordingly, and second, he actually may know lots of tips concearning construction and layout of tubed gear.

Always remember that the risk is not over even after having unplugged the mains cord, since there may be high voltage capacitors in the circuit which can store lethal voltages for days.

PSU capacitor discharge circuit

The following circuit is fetched from Claudio Bonavolta´s Homepage with his permission. You can find the formulae for dimensioning parts there.

To put it short, this little circuit uses a relay, powered by the heater voltage, which shorts the PSU capacitors out during some seconds after mains is cut, across a suitably dimensioned resistor.

This circuit is very little and easily could be modified and added to most tube gear. Personally, I always have it included on my breadboards, and many of my finished projects.

Here is a picture: