High Voltage Safety Manual.

[nosaj]

I liked Gerald Webers tool one end of the discharge tool had a chopstick jammed into the alligator clip .

It's alway fun when you leave the discharge tool connected on power up.
nosaj

[SoulFetish]

I'm just getting started in repairing amps and came across this post. In looking to build a cap discharge tool, I've seen that some use lightbulbs in the circuit. Perhaps that would help avoid the above case where a resistor failed open, as you'd see right away that the bulb wasn't lit? thoughts?

Well,.. I suppose if you found a filament bulb rated for 400-600V; which would limit the current to a reasonable level. Or, a 300V bulb with a series resistor chosen to limit the current over the rated voltage.
The problem is the cold resistance of the filament. Bulbs usually burn out right at turn on due to the inrush current. This is counter productive for the purposes of easing the stress of discharging a fully charged cap. LEDs could work, but you'd probably have to create a parallel circuit to limit the current for the LED to under 10mA (for most LEDs). But then this wouldn't tell you if the main discharging resistor went open. Unless, of course, you didn't mind using a large resistance and just having to wait longer for the voltage to drop to a safe level.
I'm using a 50W/2k resistor. This is admittedly a bit low, and if the voltage is high enough, will cause a small spark. (shhhh)
But it drops the voltage faster than the shake of a lambs tail.

I don't want to discourage you from trying to incorporate some kind of visual indicator. But, the best visual aid is to confirm that the high voltage is bled off using a multimeter to read the voltage. You dig?

[jy999]

The light bulb limiter would only fail to light if there was an open circuit in the AC power supply inside the amplifier chassis. I've never seen this happen, but I'm not a repair tech. It's main purpose is to keep things from popping as they might under full current AND as a diagnostic tool because you can read some symptoms based on the brightness of the bulb (I won't cover it here though. Some searching here will reveal best practice and use.)

True what you say, but I was talking about a light bulb in-line with a capacitor discharge resistor, not a current limiter to the main device. Higher up in the thread a poster had a discharge tool that had a resistor fail open, and then they didn't know that it wasn't discharging. A bulb in line would have avoided that failure case (as in, not knowing)

[jy999]

I don't want to discourage you from trying to incorporate some kind of visual indicator. But, the best visual aid is to confirm that the high voltage is bled off using a multimeter to read the voltage. You dig?

Absolutely! any indicator would only be help, not a replacement for proper checking. I've always been terrified enough of electricity to double and triple check everything with a meter.

I haven't done the math (too late and too many beers in), but I figured with a big resistor and the lamp in-line, the lamp wouldn't require that high of a voltage rating.. but I'm not that experienced!!

Some googling led me to one circuit with two LEDs back to back (to avoid directionality), and a 100k resistor. Another circuit used a vibration motor, which I thought was super clever

But again, as you say, I'll use the meter!

[Enzo]

Actually, we'd need to calculate the current. Your cap may have 300v charge on it, but when you put that 10v car bulb across it, that 300v can't stand.

[SoulFetish]

Actually, we'd need to calculate the current. Your cap may have 300v charge on it, but when you put that 10v car bulb across it, that 300v can't stand.

Exactly right.

I haven't done the math (too late and too many beers in), but I figured with a big resistor and the lamp in-line, the lamp wouldn't require that high of a voltage rating.. but I'm not that experienced!!

sure. so, as an example, lets say that you'll be working on your typical Fender/Marshall type amp
You'll be dealing with reservoir caps charged up to between 420-520VDC or so. Lets use the +520V for our example:
Lets say you have a 15W/120V incandescent bulb with a T7 base you want to use inline with a bleeder resistor. In order to keep the lamp within it's specs and not burn it out, you've got to create a voltage divider. In order to do that we'll use the 125mA current of the filament bulb at 120V, and get our resistor value for the remaining 400V we need it to handle.
400/0.125 = 3.2k
But, the resistor needs to be able to handle an momentary power surge of more that 50 watts, especially if we account for cold resistance of our bulb filament.
I would try to limit the current even more. My advise would be to see if you can find something like a 5W/220V rated bulb (or two 5W/120V bulbs in series) . That way, in our example, the resistor only has to handle about 300V @ ≈22mA or so.
Keep in mind that the voltage-current ratio will be a curve over the duration of the discharge.

[Mick Bailey]

By habit after after discharging the caps and before working on the amp I check the voltage just to be certain they're discharged.

[Kevina]

Every engineer should print this out A3, and display it right in front of them

Based on that cartoon, I'd say the guy holding him is in just as much danger.. My dad when I was younger had me test the magneto on the lawnmower to see if it was producing spark, well he lit me up. So he says go get your little brother you hold the magneto then hold his hand with your other hand. Well I didn't get the shock that time my brother got it.
nosaj