B+ Voltage Indicator

[lmolter]

I was thinking, as a newbie (noob?!), that I might forget to wait until the caps drain sufficiently and start poking around the chassis and eyelet board. With this in mind, other than putting bleeders across the caps, and since there is a little empty space on the eyelet board, what would be wrong with a small LED mounted in a hole to indicate the presence of lethal voltages still lurking about? If the LED was biased correctly for about 4ma of forward current, that would put a 100K resistor across the B+ at the preamp stage. That in itself shouldn't pose a problem, should it?

My thought here is that a visual indication of still-lethal voltages might save someone who's excited to find that hum or other problem and forgets to wait until the caps drain (or worse, forgets to turn off the amp altogether).

But... if it was that simple, howcum no one else ever mentioned it (or have I been living under a rock)?

Gee, do I dare post this?

Larry

[stokes]

Just get in the habit of draining the caps as soon as you open a chassis,and always double check with a DMM.A surefire way to remember to drain and check volts is to get zapped once,did it once back in '75 or so and haven forgotten to drain a cap since,as I'll never forget the sensation of getting slammed in the chest with a sledge hammer.

[tubeswell]

A surefire way to remember to drain and check volts is to get zapped once,did it once back in '75 or so and haven forgotten to drain a cap since,as I'll never forget the sensation of getting slammed in the chest with a sledge hammer.

Yep its not a nice feeling - ironically it stays with you if you're lucky

[mr.jetski]

not so ironically it'll stay with your family if your not

[Enzo]

After over 50 years of playing with electricity, I have taken my share of pokes. DOn't like to get them though.

It would be simple enough to wire an LED in there, but is it REALLY a good idea to learn to rely on an indicator instead of your voltmeter and good safe practice?

And what is this?

other than putting bleeders across the caps, and since there is a little empty space on the eyelet board

If we have the time and energy to wire in an LED, don't we have the time and energy to wire on a couple resistors? The resistors can be wired right across the caps if space is an issue. It is never a good idea to think of ways to avoid safety measures.

[Chuck H]

I agree. Bleeder resistors drain the voltage quite fast actually. Where any kind of indicator circuit may fail and give a false sense of safety. Besides, the only people who SHOULD be poking around in amps like these know well enough how to drain the caps or tell if there are bleeders in the circuit. That isn't to say that I want anyone to get shocked. But the label on most commercial amps that says "CAUTION-LETHAL VOLTAGES INSIDE" is enough warning. After that were just cleaning up the genepool

I've been shocked once. I was young and impatient. Ever notice how many times a child will stick his hand on a lit 100 watt light bulb

I got hit so hard that my vision narrowed from the margin down to a single central dot. Like shutting down on old TV. I was launched up and over the back of my chair and landed my back on the ground. I still had hold of my guitar. I had squeezed the neck so tight that I broke two strings and crushed three frets down flush with the fingerboard I know it seems imposible, but my fingers weren't even cut. As far as I'm concearned I'm lucky to be alive. I always unplug and measure voltage now.

Chuck

[lmolter]

Ok. I just showed the whole community just how much I _don't_ know about safe practices and such, and I feel like such an idiot for even posting my idea in the first place. Can the moderator remove this thread?

Anyway, of course there's no substitute for being careful and aware. Indicators can indeed give a false sense of safety if not combined with other safeguards, and the results of not carefully double-checking voltages can lead to trouble, at the least.

So, I think I'll just visit here and learn and be quiet. I don't have the experience yet to be a contributor (other than to show everyone my latest build, which, despite my newbie-ness, is turning out quite well thanks to these forums). For the meantime, though, I'll just lurk in the shadows, tail between my legs.

L

[Enzo]

No, please stay and air your thoughts. How does one learn if nothing is asked. I make mistakes all the time, and I use them as learning opportunities. Nothing like an oversight or a goof to put something in a new perspective.

We're not picking on you, this is just an important issue. We want to read about you in the show biz news, not the obituaries.

Having a B+ indicator is not a bad idea in itself, the "status" green LED on the PV 5150 for example is exactly that. If that LED stays dark, then I know the B+ is missing from the circuit. That doesn't mean the caps are not charged, but it is useful troubleshooting information.

But having it as a "go ahead and touch things" light is just scary to me. Never look for reasons not to be careful.

I mean you were looking for a way to check if the amp was safe or not, so your heart was in the right place... and still beating.

[Bruce / Mission Amps]

No, please stay and air your thoughts. How does one learn if nothing is asked. I make mistakes all the time, and I use them as learning opportunities. Nothing like an oversight or a goof to put something in a new perspective.....

I agree with Enzo... my father taught me many many years ago that the sign of a good craftsman is how well you hide/fix your mistakes. It just some time, patience and willingness to learn in order to get there...

[tubeswell]

Hi Larry

FWIW since signing onto this board about 18 months ago I've made one mistake after another, each progressively more complex than the previous.

Ya just have to keep on keepin' on.

(Can't wait to see what mistakes are on yonder wall - maybe the Hadron Collider is one collossal mistake one way or another LoL :-), - it will either work and after a few weeks we will wonder what all the money was spent on, or it won't and we will wonder the same thing. - (Can I say?) Kind of like building a DIY tube amp I guess)

[Tremolux55]

Hi Larry,

Good question. It takes courage to ask the question that everyone else is thinking!

What your question has highlighted is the need for good safety habits. Dare I say that they aren't rocket science but just a sensible approach to opening up an amp and preparing to work goes a long way towards safeguarding your wellbeing.

Maybe we could start a thread with a simple list - I'll throw a few ideas out there and please others with more knowledge / experience add to it.

1. Unplug the amplifier before doing anything

2. If the filtercaps are immediately accessible (ie Fender tweed style set up) attach a resistor jumper to the main filter cap to ensure any voltage in the system is drained. Check capacitors for voltage using multi-meter before you begin work - don't assume they have been drained.

3. If you have to remove the chassis to access the circuit do so carefully and refer to the step above.

This should put you in a good place to start working on the amp.
Consider things like a thick rubber mat to stand on (also very comfortable). Using only one hand when probing the circuit etc.

Remember you have to do this everytime the circuit is powered up.

Main thing is enjoy safely!

[jrfrond]

The LED thing is superfluous for anything but B+ indication. Just use your meter and don't think about it.

I personally let the amp drain by pulling the plug, however, if someone has done the Torres diode mod, whereas a series forward diode is inserted after screen grid supply, which traditional is where the preamp gets it's B+ feed. This is supposed to prevent preamp sag. Anyway, if that mod IS in there, the preamp filter caps can still hold a healthy charge, because that diode would be reversed when the amp was shut down, with no where for the current to bleed off. Those caps would need to be drained manually.

Yes, I am rambling a bit on this, but I am attempting to point out that ANYTHING can happen inside an amp.

[tubeswell]

the bits I would add to what tremolux55 said are:

a) Don't do any electrical/electronic work on an amp late into the night. The slightest lapse in concentration due to fatigue can have lethal consequences.

b) Don't work on an amp if you have been drinking alcohol (or imbibing any other intoxicating substances). Same thing as a) above.

c) maintain alertness and concentrate during repetitive sequences like bias setting and measuring. Don't put yourself in a situation where you will be distracted by children, spouses, pet animals or other distractions

d) never ever put your hand inside an amp when it is switched on (even better to have it unplugged from the wall like tremolux55 said).

e) when discharging filter caps, hook the bleed resistor upto the chassis ground first, then to the +ve side of the filter caps. Good discipline to remember to disconnect these resistors in the reverse order. This minimises the chances of you accidentally coming into circuit contact with the +ve side of the filter caps.

f) don't do electrical work on a concrete floor - concrete is a good conductor

[Enzo]

And it may sound cornball, but in our shop, we have had a rule for many years: if you are taking cold medication, you can't work on tube amps.

it may seem cruel, but when you are ill, you wind up fixing tape decks and effects pedals.

[Steve Conner]

It helps to keep your workshop tidier than this
http://www.youtube.com/watch?v=wua8W_Ry644


Also, yeah, drink and exposed live conductors don't mix.

I don't get the discharging filter caps thing: I permanently install bleed resistors on every amp that I build or work on, and check that they're functional.