Where is the dangerous voltage in a tube amp?

"UNPLUG YOUR AMPLIFIER AND DISCHARGE THE FILTER CAPS BEFORE YOU START WORK INSIDE A TUBE AMPLIFIER. THERE ARE LETHAL VOLTAGES INSIDE THAT CAN KILL."

Everyone who's started working with tube amps has heard that a hundred times. And it's absolutely essential advice. But obviously you need to turn the amp back on to test your work. Is it safe to run the amp while it's out of the cabinet as long as you're don't poke around inside?

Not necessarily. Dangerous voltages can appear where you least expect them - either through circuit design or component failure.

Here's an Ampeg V4 / VT22 that's in for a tune up. The reverb pan for these amps is under the top cover and if you're doing extensive work it's much more convenient to unplug it. Here your can see the RCA cables that connect to the pan dangling above the chassis:







Now normally one doesn't associate reverb leads with high voltages, so you might be inclined to let them hang where ever they may. Lets take a look at what happens when the amp is switched on:



The meter here is measuring the DC voltage between the center pin of a reverb lead and the amplifier chassis. It's showing 0.013 volts. That wouldn't shock a flea. Nothing wrong there. What happens when the standby switch is thrown into the operate position?







181 Volts! You'll be glad if that's not dangling out the back of the chassis touching the screwdriver you're about to pick up! This voltage isn't there for all that long. The sequence below shows 1 second intervals.







It drops from 181.2 to 104.7 in just a couple seconds. In 15 seconds the voltage drops to 12 volts and keeps dropping from there. Because of the grounding scheme in the standby circuit of the V series Ampegs high voltages appear all over the amp in places you wouldn't usually expect them - even when the amp is in standby!

The point is, never assume that there is no voltage at some point in a tube amplifier just because there's no reason for it to be there. If you're are unsure, check with your meter. If your meter says it's safe, don't assume it's always safe. Things change. Never let wires dangle unprotected from the chassis. You never know what kind of surprise they might have in store. And always work with one hand behind your back. In case you do make a mistake the current is less likely to go straight through your heart.