Shocking habits

[tubeswell]

Well it'll teach me!

Last night while experimenting with adjusting the bias and measuring bias current, and plate and screen voltages at different settings (yet again) on my recent 5G9 build, after my last fiddle, just when I had decided I was happy with everything, I went to disconnect the meter after I had switched the standby off, but forgetting to switch the main switch off. Talk about dopey! How effing dumb was that!!!!? I had been switching the amp on and completely off constantly for about an hour with a routine of make sure its off, hitch up the meter clips set the meter to mV, flip the main switch let it heat up flip the standby let the measurement settle down, check the measurement and note it, and flip it all off in reverse order before switching the meter to kV to go through the same rigmarole for the plates, and then again for the screens, before repeating the whole cycle with different bias setting. I guess I got bewildered and forgot to switch off that main at the end - kick myself (KICK!)

So after a 240V jolt and a tingling forearm, I exported myself (cursing my stupidity all the way) to A & E promptly, and thankfully the ECG they did didn't turn up any spooks (touch wood). But man did it made my heart flutter!

Seems that what saved me was I only had one hand in the amp, I was on a wool carpet with a rubber underlay and the other (-ve) end of the meter was still clip-grounded to the chassis, so the current went in my thumb, (which was holding the +ve end on the meter clip) and gone out through my wedding ring which I think I must've, for a fraction of a second, brushed on a contact on the fuse holder with another part of my hand.

I swear I'll never work on an amp at 10:30pm again!

Anyway I though I better let everyone know as a timely reminder of paying attention to safety, not to fall into bad habits. I've been doing this amp hobby for a couple of years now, and that's the first time (and I hope the last) that I get that clumsy. I'm so p*ss*d with myself!

[Paul P]

Well, thanks for sharing the experience with us. Shocking to say the least.
I'm really glad to hear that you're ok. Must be scary. I was going to ask
when I saw a brief mention of something in your other thread because I
wanted to avoid doing whatever you'd done. I then found this post.

Repetitive sequences are dangerous. At some point you fall into a routine
and you sort of stop thinking each step out completely. I've encountered
this in woodwork when I have a pile of similar pieces to cut on a table saw
for instance.

I've been worried I'd do something stupid while working on my amp and am
glad I've got most of the work done. When working on my amp live I wear
rubber boots, stand on a thick rubber mat, wear chemical gloves to my
elbows and have one hand firmly anchored behind my back. The gloves are
not electricians, like the ones rated to 1kv which are expensive, but I figure
they still offer a pretty good insulation and you can manipulate test leads
and write with a pencil while wearing them.

I also use these test leads that are like a syringe with a spring hook on the
end which keep my fingers far away from the hot parts.

Paul P

[tubeswell]

Yep it sure took me out of my comfort zone

[Suburbanite]

welcome to the club of people more likely to be struck by lightning (you are now a better conductor of electricity)

[Steve Conner]

I was put off working with tubes for years by a similar incident. I'm glad I got back into them, though.

Some useful things to remember before getting your hands into high voltage equipment:

SIDE - Switch off, Isolate, Discharge capacitors, Earth. The "Earth" refers to clipping shorting jumpers across the high voltage circuit to make absolutely certain it can't come alive again.

The latter two are really only applicable for big iron, like the trashcan-sized 100kV test transformer in my workplace's high voltage lab, which has an earthing rod that has to be fitted before touching anything. In a tube amplifier, bleeder resistors should do the Discharging and Earthing. You should check that the amp has them before working on it, and that they're functional, and consider adding them if it doesn't have any. It doesn't hurt to familiarize yourself with how long they take to discharge everything.

Plug in your pocket. Or at least remove it from the wall outlet and have sight of it on the bench in front of you before touching anything. Get into the habit of making eye contact with the plug before you delve into something with both hands, since it guarantees that you Switched off and Isolated. (Assuming of course that you pulled the right plug, and not the one for the coffee percolator.)

If you really have to work live:

Don't do it alone. In practice many hobbyists do, but be aware that it ups the risk factor considerably. Lone working on high voltage equipment was forbidden everywhere I've ever worked.

One hand in pocket. What this really means is "Don't let your heart be part of a possible current path". So it also means making sure that your feet are insulated from the ground, and you're not leaning your bare leg against a metal file cabinet, etc. Holding your guitar strings in one hand while prodding live amp guts with something held in the other hand is NOT a good idea.

When prodding around live gear with a meter, use good quality test leads and probes that are rated for the voltages you're using and kept in good condition.

Of course all of this advice is given without warranty, you are responsible for your own safety.

[R.G.]

Working on tube amps is very much like handling live rattlesnakes. You only have to be negligent once.

Worse actually. The amps don't rattle.