Sunday, April 27, 2014

HV delay using two DPDT switches

Years ago Glass Audio published an article that described a simple way to delay the high voltage when turning on your tube amp. I've used it many times and it works well.

When you turn on either of the switches, the filament transformer comes on first. After a delay of your choosing, turning on the other switch turns on the high voltage. Turning off either switch turns off the high voltage first.

There are two disadvantages. First, the circuit requires two separate transformers. Second, if you lose power and then it comes back on before you can turn off the switches there is no delay. In practice, the tubes are usually still warm if the power outage is short.

The ordinary DPDT switches from Radio Shack work fine and you are only putting 115 vac through them so this circuit is good for higher voltage amplifiers. When testing your amp for the first time, it can be helpful to check that your filaments are lit and at the correct voltage before applying high voltage.

Finally, most people can't turn on the amplifier unless they are given instructions. 

This diagram may help you visualize how to wire the switches.

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