Do Amps Hold Power, After Switching Off?

[djhollowman]

Hey all,

A friend has asked me if I could fit a power attenuator into his amp head. It's a kit conversion with full instructions and it seems a straightforward enough job, just wondering about electricity though!

Obviously I will have the amp unplugged, but will it STORE any electricity? Will I need to ground it or discharge it or anything first, to avoid shocking myself?

Sorry, I'm a noob when it comes to amp circuits etc

Thanks,

DJ

[mammoth guitars]

Power supply caps usually have a bleed resistor on them so they will discharge within a few seconds of turning off the amp. With it unplugged you will not be at risk for AC voltage shock.

[Myka Guitars]

If you are new to amp circuits then it would be best to read up on them before you start working on them. Every article I read on amp safety states that the power supply caps will hold a charge that needs to be discharged before you can safely work on the amp. It is better to be safe than dead so check the caps with a meter before you do anything. Also be sure to read the manual and have the schematics nearby.

Read this.

~David

[mikhailgtrski]

Power supply caps usually have a bleed resistor on them so they will discharge within a few seconds of turning off the amp. With it unplugged you will not be at risk for AC voltage shock.

Usually, but definitely not always!

Be careful - it's best to check the caps with a voltmeter to make sure they have fully discharged before you start poking around, even if they have bleed resistors installed. I've watched mine drain on a voltmeter, and it takes more than a -few- seconds to get to zero volts.

[Geo]

Caps in tube amps can "re-charge" too. I've watched, with a voltmeter, while the 20-something V on some PS caps crept up by tenths of a volt.

A good way to drain most of it is leave the standby switch on but kill the main power switch and keep strumming until the amp is totally dead. This will drain most of it. You can also just unplug it from the wall while it's on and keep playing until the sound is gone.

Whatever you do, check the power supply with a voltmeter before you touch anything. Drain the voltage off by clipping a resistor between the PS and ground. (I use 220k).

UNPLUG THE AMP WHEN YOU OPEN IT. I lost count of the times I left an amp plugged in and then shocked myself by bumping a contact on the power switch.

[Paul Marossy]

I lost count of the times I left an amp plugged in and then shocked myself by bumping a contact on the power switch.

Been there, done that!

[ToddW]

You can just get an allegator clip lead, connect it to a 10k resistor and attach one end to ground. Then while holding the insulated part, touch the other end to each cap for a few seconds. Make sure the amp is unplugged!

Me, I'd go with Kevin O'Connor's Power Scaling kit before an attenuator, although an attenuator might be easier to add. Built power scaling into my first amp, and it's an amazing addition!

Todd

Edited May 20, 2008 by ToddW

[Tim37]

I lost count of the times I left an amp plugged in and then shocked myself by bumping a contact on the power switch.

Been there, done that!

your not the only one my wanna be champ has a bite too it.

Edited May 20, 2008 by Tim37

[djhollowman]

Caps in tube amps can "re-charge" too. I've watched, with a voltmeter, while the 20-something V on some PS caps crept up by tenths of a volt.

A good way to drain most of it is leave the standby switch on but kill the main power switch and keep strumming until the amp is totally dead. This will drain most of it. You can also just unplug it from the wall while it's on and keep playing until the sound is gone.

Whatever you do, check the power supply with a voltmeter before you touch anything. Drain the voltage off by clipping a resistor between the PS and ground. (I use 220k).

UNPLUG THE AMP WHEN YOU OPEN IT. I lost count of the times I left an amp plugged in and then shocked myself by bumping a contact on the power switch.

You can just get an allegator clip lead, connect it to a 10k resistor and attach one end to ground. Then while holding the insulated part, touch the other end to each cap for a few seconds. Make sure the amp is unplugged!

Me, I'd go with Kevin O'Connor's Power Scaling kit before an attenuator, although an attenuator might be easier to add. Built power scaling into my first amp, and it's an amazing addition!

Todd

Thanks everyone!

I have some resisitors lying around. They're rated: Power 1/4 W, Value 510 ohm - could I use one of those to discharge?

What does the k-rating on resistors refer to? 10k = 10 kilo ohms, yes? So my resistor I mentioned would be rated at 0.5k?

Is that safe to use?

Gotta learn more about amp circuitry!!! (I did follow and read the link you posted David, thank you! )

I could post up the .pdf which details how to install the attenuator if anyone's interested, and if you can tell me how to post a link to a .pdf! Would I have to host it somewhere first, then post a link to it?

BTW, what's the difference(s) between an attenuator and a power scaling kit, just for my own information?

Thanks

DJ

[Geo]

I have some resisitors lying around. They're rated: Power 1/4 W, Value 510 ohm - could I use one of those to discharge?

What does the k-rating on resistors refer to? 10k = 10 kilo ohms, yes? So my resistor I mentioned would be rated at 0.5k?

Is that safe to use?

BTW, what's the difference(s) between an attenuator and a power scaling kit, just for my own information?

Thanks

DJ

Use good ol' V=IR. Say you have 100v on your filter caps. With a 500ohm resistor, current=100/500=0.2A. So, P=IIR... if I did that right, your resistor would have to dissipate 20w safely! A 1/4W resistor will blow up. If you use something much higher (10k, 100k, 220k, whatever you have) current will be lower (and I think the discharge will take longer). It's safer to keep the current lower.

Of course, this is all different if you only have 20v on your caps. I usually see values in this range, not 100v or more.

That's my understanding anyway.

An attenuator is usually some kind of dummy load that soaks up some of the output power. For example, you could probably use a lightbulb as an attenuator, or say a 10ohm 50w resistor, etc. Power scaling actually lowers the voltage which the power supply gives to the power tubes. Lower V=lower wattage.

Hope that's helpful, someome correct me if I'm wrong.

[Prostheta]

Earthing parts doesn't always guarantee discharge across all parts of the circuit, especially if you have AC to DC conversion or step-up transformers involved (like TVs..egh) and high voltages maintained by power supply smoothing caps can be the other side of rectifier diodes preventing them discharging by earthing points "upstream". Safest method is to LEAVE THE AMP ALONE unless you know what could be in there. If you do have to go inside it, leave it unplugged from the mains but "turned on" for a long long time. At least a week so any residual charges dissipate naturally through component inefficiency or bleed resistors, whatever. Caps can "recharge" through dielectric reabsorption but not general to levels which could be harmful. Again - leaving the amp for a while to settle is your only sure fire way of making it safe to work with if you don't know what to avoid.

Safest method is to make somebody else do it. That means an experienced tech, not making someone else poke around the amp hunting for residual charges. That said, idiots are much more plentiful and cheaper to employ than qualified knowledgable techs. YMMV.

Big screwdrivers make big sparks across big caps. Those big sparks also make a good job of vapourising the metal or welding the screwdriver to the points. Soft wet fleshy sausages do not belong in these areas.

Edited May 20, 2008 by Prostheta

[Tim37]

Safest method is to make somebody else do it. That means an experienced tech, not making someone else poke around the amp hunting for residual charges. That said, idiots are much more plentiful and cheaper to employ than qualified knowledgable techs. YMMV.

True but theres only one way to get experience.

[Mattia]

Safest method is to make somebody else do it. That means an experienced tech, not making someone else poke around the amp hunting for residual charges. That said, idiots are much more plentiful and cheaper to employ than qualified knowledgable techs. YMMV.

True but theres only one way to get experience.

Yes, but you can start by going to see someone with experience, and asking if you can watch them discharge the amp. F'r instance.

[djhollowman]

Thanks for all your input fellas!

I'm going to leave the amp unplugged with switches on to dissipate any remaining electricity, as Prostheta suggested, for at least a week. I'm also actively reading up on amp technology, and electronics in general!

I have no way at all of watching someone work on an amp, which would be ideal, because of where I live etc.

At the moment I'm thinking I will probably return the amp to my friend having NOT done the work.

I don't want to put myself at risk.

I do want to get the experience of doing work like this, though. I feel confident about installing the component, from following the instructions which came with it. It's just the thought of residual charge which is putting me off.

Is there a way to let you guys read the .pdf file I have detailing this?

Appreciate anything constructive.

DJ

[curtisa]

Thanks for all your input fellas!

I'm going to leave the amp unplugged with switches on to dissipate any remaining electricity, as Prostheta suggested, for at least a week.

A week is possibly a little bit extreme. The tube amps I've seen only have a few hundred uFarads of capacitance per voltage supply, which will dissipate fairly quickly after turn-off. Probably all you really need to do is unplug the amp, leave the main power switch turned off, and turn the standby switch on. This will connect the main filter caps to the amp circuitry to speed up the discharging process. I would imagine they would be discharged to a safe level within half an hour, but always check with a multimeter to be sure.

Making up a little "cap discharger" tool is a good idea - just solder some crocodile clips to a 10K 1W resistor and cover the exposed solder joints/wire with some heatshrink or electrical tape. Clip the resistor across the caps with the amp turned off and the amp is safe to work on straight away - $1 insurance.

[ward]

DJ, don't be scared off.

You have a multimeter, right ?

If you measure your caps after the mains is unplugged for some time

you can see the dc voltage with your own eyes.

[djhollowman]

DJ, don't be scared off.

You have a multimeter, right ?

If you measure your caps after the mains is unplugged for some time

you can see the dc voltage with your own eyes.

Thanks! Yeah, I have a multimeter - just don't really understand what I'm doing with it!

I'm not even sure what the caps look like on the amp! I'm such a noob with this stuff.

Argh! Help...

DJ

[Prostheta]

If in doubt, err on the side of caution. It's all well and done knowing what to look for and how to deal with it, but if you don't then it's a recipe for trouble.

Get a tech to install the power attenuator, and if you're nice and friendly then get him (or her?) to give you a 101.

[Tim37]

the caps (the ones that really bite) look like little coke cans with two wires coming out.

you might want to go find some books on electronics and learn some basics like ohms law and a few other things befor you get too deep.

[KeithHowell]

and a few other things befor you get too deep.

Yes like 6 foot under. Power is dangerous and it kills if you don't know what you are doing!!!!

Keith

[Prostheta]

Basic reading:

http://en.wikipedia.org/wiki/Resistor

http://en.wikipedia.org/wiki/Capacitor

http://en.wikipedia.org/wiki/Ohms_law

http://en.wikipedia.org/wiki/Coulomb

http://en.wikipedia.org/wiki/Transformer

http://en.wikipedia.org/wiki/Rectifier#Rec...utput_smoothing

There is a good summary of an amps operation in this article also.

http://en.wikipedia.org/wiki/Instrument_amplifier

Sorry I couldn't provide anything more specific, but the content of these articles (despite being Wikipedia) is basic enough to be truthful and accurate.

[ward]

It's up to you,

measuring your electrolytic caps is very basic but probably the most important thing for your safety.

If you want to do it, we can guide you through it.

we'll need the type of amp,

and pictures of the inside.

If you don't, take it to a tech.

[ToddW]

Saw this thread popped back up and thought maybe DJ had posted a follow up, but nope . . .

So DJ, did you have a go at it?

Todd

[Prostheta]

This is where DJ doesn't post for a week, whereupon we all start panicking and worrying that one of us forgot to tell him about the big-welding-glove and safety glasses bit of "stick a big screwdriver over the big electrolytic smoothing cap's contacts".

[djhollowman]

Ow!

Yeah, I really want to take the job on. Right now the amp is back with its owner, who has a mate that will discharge the caps. However, I'm still going to want to do that myself as soon as I get it back, just to be sure, and so I know how to do it for future reference!

The amp is a Rhino YJM50 50-watt head, here'a link to a Harmony Central review with photos:

http://news.harmony-central.com/Newp/2002/YJM-50.html

and another page with views of the inside:

http://www.yngwie-megafan.org/articles/RHINO.html

rhinoamps.com seems to come back with some kinda adult listing!!??

Once I have it back I'll update this thread and be asking lots of specific questions!

I have a .pdf file from the manufacturer detalling how to install the (what he calls) "drive control", which I would verymuch like you guys to read and evaluate - please tell me how I can make this happen? Can I save it somewhere and post a link to it, like you do with photobucket etc? I really feel it would help!

Thanks!

DJ