Tube amp safety so you don’t get shocked

In my experience, most tube amp owners know that they shouldn’t open up their tube amps and poke around in there. They know it’s dangerous, and you might be killed by stored electricity. But that’s usually about it. So, let’s do a quick safety check, so you understand how and why your amp’s innards might zap you to death.

First of all, your tube amp runs on DC high voltage. The AC voltage that comes out of your wall is converted to a larger amount of DC voltage by the power transformer and rectifier (either a tube or diodes). While you will probably survive a jolt from AC wall voltage, 350+ DC volts can do some real damage/unalive you. The worst case scenario is that your body offers a path to ground, allowing the current to flow through your body (and heart), killing you (see: the Keith Relf story).

But you’ve probably heard that your amp is still dangerous even if it’s turned off and unplugged. How is this possible!?

It’s possible because of components found in every amplifier called Electrolytic Capacitors, AKA filter caps. These capacitors are vital to the performance of the amplifier because they filter (hence the name) the dirty, pulsing pseudo-DC power coming from the rectifier.

But they do something else: they also act as a battery of sorts. This is why these capacitors are noted as B+ in schematics and wiring diagrams. The B stands for battery.

In addition to filtering the DC, the caps also store power for the amp to use when it’s being driven hard. See, the power consumption of your amp is directly based on how hard you play it. Because of this, sometimes there isn’t enough power being converted by the rectifier to handle the demands of your playing, so the filter caps step up and provide some of the power they have stored up. The end result of this is that your amp sounds completely normal.

But what about all that power stored inside those caps, you ask? Well, therein lies the danger. Because your amp is likely running at 300+VDC, the filter caps are capable of storing 300+VDC. If you were to touch the + end of the capacitor, you would become the recipient of all that stored power. Technically, anytime you pick up a AA battery, you are getting voltage from it, but it’s only 1.5VDC, so you just don’t notice, and at that level, your skin isn’t a very good conductor. But we all knows what happens if you put both terminals of a 9V battery on your tongue.

Now, most amps (if they are wired and maintained properly) will drain the filter caps upon shutdown. Usually, they drop to around 8-10VDC, which isn’t dangerous. HOWEVER! You can’t ever assume that an amp has properly drained its filter caps. Due to component failure, your filter caps may be quietly storing all of the voltage that goes to the power tube plates (plate voltage). This is why the most vital safety procedure is manually draining the filter caps. You can buy a tool to do this.

Once you have thoroughly drained the filter caps, the amps is safe to work on and you won’t get shocked. But you must make sure that your amp isn’t plugged into the wall when you’re working on it. Even if the amp isn’t turned on, you will have live wall voltage (roughly 120VAC) at several places in the amp that are easy to touch. The power switch and fuse holder will almost certainly have live voltage if the amp is plugged in. That kind of shock hurts a lot (just trust me on this).

So, if your amp is unplugged and the filter caps have been drained, the amp is now safe to touch, but I like to do one additional step just for my own peace of mind. After I drain the caps, I use my multimeter, set to DC volts, to check the filter caps to make sure that the voltage is low enough. Usually, there’s still around a volt left in the caps even after draining, but this is safe. So, what other safety issue might you encounter?

If you come across a tube amp that still has an old two-prong plug, don’t turn it on. If the amp is incorrectly wired or a component has failed, that power cable may allow current to enter the chassis. The first thing you should do with an amp that still has a two-prong cable is remove it and replace with a three-prong cable.

If you are building an amp and are bringing it up to voltage for the first time, use a variac to slowly add voltage. This way, if anything fails, it will fail at a lower voltage, which may help avoid damage to other part of the amp and you. This would be a good time to mention that it’s never a bad idea to wear safety glasses when working on a live amp. If something explodes, you’ll thank me.

As always, if you don’t have experience working on amps, you won’t accomplish anything by poking around in there anyway. Leave it to a professional who knows what to look for. That said, you can replace your own preamp tubes if one fails. Preamp tubes do not need to be matched, but you should replace them with the same type of tube. Power tubes are a little trickier. If your amp has fixed bias (meaning it has a pot that allows you to adjust the bias) you need to re-bias the amp every time you replace the power tubes. If your amp is cathode biased (biased by a resistor in the circuit), you can simply pop in new tubes.

I will update this post with more safety tips as I think of them.