Question for the amp/electronics gurus

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famous beagle

famous beagle

Well-known member
Hey y'all,

I've done a small amount of electronics work---built several pedals, lots of mods on guitars, am building second amp right now, etc.---but there are still several things I don't understand. I've never taken a class in electronics; everything I've learned has been from reading books on the subject. Here's one thing I've wondered for a while:

I know how to properly discharge a tube amp to make it safe for working on (or, I should say I know one of the many ways). The method I use is to run a wire from pin 1 of the first input tube to the chassis (or any ground connection) with the power and standby switches on. This needs to be done because the filter caps store electricity that can still be lethal or fatal even with the amp unplugged (for a while).

My question is this. When you make this connection, what happens to this electricity? Where does it go? If it's being discharged into the chassis, where does it go from there (since the amp is not plugged into the wall).

I know this may be a stupid question, but I haven't been able to find an answer for it anywhere.

Thanks
 
It's better to use a fairly high resistance (1 megohm or so) resistor from the filter caps to ground. I have a few that I made from jumpers, just inserting the resistor in the middle and heat shrinking over it, and clip one side to ground, the other to the filter cap, leave it for a few minutes, then move on to the next one. It just bleeds off the stored charge to ground.
 
LDS said:
It just bleeds off the stored charge to ground.
Click to expand...

See, this is the statement that I don't really understand. How does it bleed it to ground? When the amp is unplugged, the chassis is not making a connection to ground. Right? Or ... am I totally missing something here?

The concept of "ground" has always eluded me a bit, actually.

Thanks
 
I'm not a guru, just an acolyte. I'll never fully understand it either, but Think about it this way, if you just unplugged the amp, it'll still sound for a short while. So the capacitor doesn't care that the amp is plugged in, and eventually they will discharge anyway, you just use the resistor method to do it a little quicker so you don't get bit by the capacitor.

The circuit ground (chassis or otherwise) doesn't care about the third prong. The house ground is really a safety ground. I have an old amp that only has a two prong AC plug. Or, look at it another way, your guitar plug has a tip (signal) and a sleeve (ground) it doesn't care either about that third prong.
 
I'm not a guru, just an acolyte. I'll never fully understand it either, but Think about it this way, if you just unplugged the amp, it'll still sound for a short while. So the capacitor doesn't care that the amp is plugged in, and eventually they will discharge anyway, you just use the resistor method to do it a little quicker so you don't get bit by the capacitor.

The circuit ground (chassis or otherwise) doesn't care about the third prong. The house ground is really a safety ground. I have an old amp that only has a two prong AC plug. Or, look at it another way, your guitar plug has a tip (signal) and a sleeve (ground) it doesn't care either about that third prong.
 
Blue Jinn said:
I'm not a guru, just an acolyte. I'll never fully understand it either, but Think about it this way, if you just unplugged the amp, it'll still sound for a short while. So the capacitor doesn't care that the amp is plugged in, and eventually they will discharge anyway, you just use the resistor method to do it a little quicker so you don't get bit by the capacitor.

The circuit ground (chassis or otherwise) doesn't care about the third prong. The house ground is really a safety ground. I have an old amp that only has a two prong AC plug. Or, look at it another way, your guitar plug has a tip (signal) and a sleeve (ground) it doesn't care either about that third prong.
Click to expand...

Thanks for the response. I wish I could say it helps me understand, but I'd be lying if I did. I still don't understand what happens to the electricity that's stored in the caps. I mean ... I understand "it bleeds off to ground," but I don't know what that means, really.
 
Grab a 9V battery and go into outer space. Ignore that you can't breathe, your skin is boiling, etc. Put the 9V battery on your tongue and you will notice that it stings just like it does on Earth, even though you aren't touching "ground". Why? Because there is current flowing across your tongue. Where do those electrons go? Into your body? Yes, but then they keep going back where they started--into the battery--except now they are at a lower energy state ("0V") and they no longer feel like going anywhere (they have no more potential energy).

A capacitor is just like a battery, except it stores a charge electrostaticly rather than chemically.

If that doesn't help, read this:

Voltage and current : BASIC CONCEPTS OF ELECTRICITY

This is pretty cool too:

http://www.wisc-online.com/objects/ViewObject.aspx?ID=ACE11805
 
mshilarious said:
Grab a 9V battery and go into outer space. Ignore that you can't breathe, your skin is boiling, etc. Put the 9V battery on your tongue and you will notice that it stings just like it does on Earth, even though you aren't touching "ground". Why? Because there is current flowing across your tongue. Where do those electrons go? Into your body? Yes, but then they keep going back where they started--into the battery--except now they are at a lower energy state ("0V") and they no longer feel like going anywhere (they have no more potential energy).

A capacitor is just like a battery, except it stores a charge electrostaticly rather than chemically.

If that doesn't help, read this:

Voltage and current : BASIC CONCEPTS OF ELECTRICITY

This is pretty cool too:

The Electrostatic Charge of a Capacitor
Click to expand...

Ok, so am I confusing the issue because it's a DC part of the circuit instead of AC? And "ground" doesn't really play a role in DC ... does it? Or is "ground" simply another name for the negative terminal in DC?
 
one key thing is that people always talk about electricity 'flowing' like water. It doesn't really ..... what you have is movement of this electron causing movement in the next electron causing movement in the next and just like bumper cars the motion gets transmitted down the line.
Technically the electron jumps to a higher energy position in the atom I believe but for our purposes we can jusy say it's motion of the electrons ....

So it's not like water flowing where the specific water that goes into one end of the hose comes out the other.
With electrical 'flow' the specific electron at the start of a circuit does not come out of the end of the circuit.
What we call current flow is really more of passing along of movement by excited electrons.

So that electricity that goes to ground .... it's not like there's a pint of electricity in there.
The excited electrons pass that movement on into the chassis but without a continued input of energy the electrons lose their movement ...... it's essentially damped in the chassis or whatever else it might be shunted to.
 
Maybe this will help. The PSU for my RoyerMOd, has pretty large capacitors (220uF) which hold a charge a long time. After it was turned off I could still put my DVM on B+ and Ground and measure about 98v, 97.9v, 97.8v.... so I strapped a resistor between B+ and ground. Then measured, down to a few volts, and the resistor just wasn't pulling it anymore. So for the heck of it I stuck a LED B+ to Ground. It lit for a few seconds, dimmed and went out, and now I was down to about 1/2 a volt or so.

With the power supply on and plugged in, I still measure B+ from the B+ terminal to Ground. It stays steady, because there is a continual supply of AC feeding it. Turn it off, I just have what is stored in the caps.
 
famous beagle said:
Ok, so am I confusing the issue because it's a DC part of the circuit instead of AC? And "ground" doesn't really play a role in DC ... does it? Or is "ground" simply another name for the negative terminal in DC?
Click to expand...

Forget about the concept of ground for a moment. Electricity flows from negatively charged regions to positively charged regions. AC just means that the current flow reverses direction. Watch the animation in that capacitor link until it makes sense.

When you have a charged capacitor you have one terminal with a positive charge with respect to the other terminal. Link the two terminals across a conductor and you will get a current. It doesn't matter if the capacitor is connected to "ground" or not.

(Don't) try this: charge up the caps in your amp, cut them out carefully, one terminal at a time, with a plastic handled tool, being careful not to touch anything but the lead you are cutting. Now you have a charged capacitor in your hand. (Don't) touch both terminals to your tongue. Is there current? Oh yes. Is there "ground"? I dunno. Ground is just a convention in circuit notation, it doesn't really exist as a physical law as far as the electrons are concerned. They just want to flow to positively charged regions, which is also what your tongue is worried about.

You can't really call ground a negatively charged terminal because some circuits have negative DC supplies. So "ground" can be positively charged with respect to some elements of the circuit and negatively charged with respect to others.
 
Blue Jinn said:
With the power supply on and plugged in, I still measure B+ from the B+ terminal to Ground. It stays steady, because there is a continual supply of AC feeding it. Turn it off, I just have what is stored in the caps.
Click to expand...

Not quite. If you have a continual supply of *AC* feeding a cap (depending on what's going on in the circuit) the capacitor will happily pass the AC and retain no resulting charge. Caps need to be charged by a *DC* supply. So first your amp has to rectify the incoming AC from the wall to DC in order for the caps to do their job (which, incidentally, is getting rid of ripple, which is the remaining AC on the DC supply)
 
mshilarious said:
Not quite. If you have a continual supply of *AC* feeding a cap (depending on what's going on in the circuit) the capacitor will happily pass the AC and retain no resulting charge. Caps need to be charged by a *DC* supply. So first your amp has to rectify the incoming AC from the wall to DC in order for the caps to do their job (which, incidentally, is getting rid of ripple, which is the remaining AC on the DC supply)
Click to expand...

I should have been more clear on that!:spank: I'm talking of course about the mains supply to the rectifier portion of the power supply.
 
Hey guys, Sorry to threadjack, bu how are ya on Brushless dc motors.
I've got a cassette tape player with speed issues.


Gona open a new DIY thread here if you've any time.
 
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