general questions re my first adventure in high voltage...

mistermikev · Post by **mistermikev** » 16 Apr 2011, 00:22

hello,
first off, please forgive my newbieness...

just got a valve jr head in trade for next to nuthin. orig owner purchased the 'fenderish' innards from turreboards.com
http://www.turretboards.com/layouts_sch ... sh_mod.pdf
he built it a while back and volume was dropping in and out when tested. he thinks the culprit was bad power tube.

anywho, I'd like to go over it with a fine tooth comb and verify all parts and redo a cpl 'ugly' things.

q1) I have a DT-838 digital multimeter...
http://zzlihao.en.alibaba.com/product/2 ... meter.html
it looks like it's telling me that it is 500v max(you can see on pic at bottom next to neg probe lead) so I should be ok using this?

q2) if I put neg / pos of multi on neg / pos of c8 and see low voltage I should be ok right?

q3) if I see high voltage... my procedure would be: clip one end of 5w 330r resistor to south(neg) side of c8 and other end to grn wire (next to r17) I should be able to re-test filter caps and eventually see low voltage.

thanks in advance for any advice you can provide.

DrNomis · Post by **DrNomis** » 16 Apr 2011, 07:20

mistermikev wrote:hello,
first off, please forgive my newbieness...

just got a valve jr head in trade for next to nuthin. orig owner purchased the 'fenderish' innards from turreboards.com
http://www.turretboards.com/layouts_sch ... sh_mod.pdf
he built it a while back and volume was dropping in and out when tested. he thinks the culprit was bad power tube.

anywho, I'd like to go over it with a fine tooth comb and verify all parts and redo a cpl 'ugly' things.

q1) I have a DT-838 digital multimeter...
http://zzlihao.en.alibaba.com/product/2 ... meter.html
it looks like it's telling me that it is 500v max(you can see on pic at bottom next to neg probe lead) so I should be ok using this?

q2) if I put neg / pos of multi on neg / pos of c8 and see low voltage I should be ok right?

q3) if I see high voltage... my procedure would be: clip one end of 5w 330r resistor to south(neg) side of c8 and other end to grn wire (next to r17) I should be able to re-test filter caps and eventually see low voltage.

thanks in advance for any advice you can provide.

Q1, You should be fine because the 500V max means that you can measure up to 500V DC.

Q2, C8 looks like it is connected as a supply filter cap, so, if you have the black negative test probe of your multimeter connected to the cap's negative terminal, and the red positive test probe connected to the cap's positive terminal, you should read something like 300 to 450V DC.

Q3, if you read a low voltage across C8 it may mean that there's a partial-short inside the cap and it needs replacing,

mysticwhiskey · Post by **mysticwhiskey** » 16 Apr 2011, 08:56

I may be wrong, but I get the feeling the OP is asking about discharging the caps before working on the amp. In which case, zero volts across C8 means the filter caps have fully discharged and the amp should be OK to work on. To discharge, temporarily connect the resistor across the terminals of C8 for several seconds. I've used a 10K resistor to do the job, not sure if 330R is large enough?

DrNomis · Post by **DrNomis** » 16 Apr 2011, 09:38

mysticwhiskey wrote:I may be wrong, but I get the feeling the OP is asking about discharging the caps before working on the amp. In which case, zero volts across C8 means the filter caps have fully discharged and the amp should be OK to work on. To discharge, temporarily connect the resistor across the terminals of C8 for several seconds. I've used a 10K resistor to do the job, not sure if 330R is large enough?

Yeah, the wording of the questions doesn't really make it clear what is intended, so I had to make some assumptions....

mistermikev · Post by **mistermikev** » 16 Apr 2011, 14:40

yup, I am asking how to discharge and keep myself safe.

thanks drnormis, mysticwhiskey for taking the time.

afa 5w 330ohm, yes, I posted this because it seemed to me that I should be using something like a 10-25k resistor -as a greater value would mean a quicker discharge... the orig owner gave me this 5w 330r but I'm guessing it'd take a while.

just to clarify... connecting the resistor to neg side of cap and to ground is incorrect... what I want to do is connect resistor from pos to neg on that cap.

thanks again for the answers!

edit: just measured all the resistors on this thing... and r8 is measuring 88k (not 100k)... any guesses as to the functional impact that would have on this circuit?

mysticwhiskey · Post by **mysticwhiskey** » 16 Apr 2011, 14:48

Yes, connect the resistor from positive to negative on the capacitor. Smaller values of resistor actually mean a quicker discharge, larger means a longer time to discharge. A short circuit is the quickest discharge of all! Not recommended though

mysticwhiskey · Post by **mysticwhiskey** » 16 Apr 2011, 14:56

All joking aside, those voltages can be lethal so take care!

mistermikev · Post by **mistermikev** » 16 Apr 2011, 16:12

right, my electrician friend told me to use the screwdriver method... not doing that!

ahh, less resistance = quicker discharge... that would make sense.

I was aware that anything over 25v was a roll of the dice and 300v is almost sure death.

believe me, once I charge them up... I'm gonna be very nervous if I have to go back in.

so my plan is: I'll get a 5w 25k and solder it to two 10a alligator clips, disconnect power, using a pliers I will connect to neg, then pos. all this w one hand in my pocket. sound about right?

I'm guessing, even though pliers s/b insulated via rubber on aligator clips, it would be wise to buy a pliers w insulation rated for 500v. what do you folks use?

deltafred · Post by **deltafred** » 16 Apr 2011, 17:39

As an apprentice (a very long time ago) I was told to put my left hand in a pocket when poking about in high voltage circuits.

1. It keeps it from wandering and touching anything accidentally.

2, If you do touch something with your right hand then it is safer for the current to flow to your hopefully high resistance shoes through your right side than your left.

3. If you are holding the chassis with one hand and accidentally touch a high voltage with the other then you current path between your hands passes dangerously close to or sometime through your heart.

When working on a live chassis plan what you are going to do, what you are going to measure. Be slow and methodical and think about each operation. And keep your left hand in your pocket. fortunately the chassis is usually grounded so one of your meter leads can be clipped to that for most measurements.

You don't have to be afraid or nervous of it, but treat it with due care and respect.

For discharging capacitors you don't need high wattage resistors, 1/4 or 1/2 watt is fine. they are only passing current for a second or two. A couple of (insulated) croc or alligator leads clipped to a 1k or 10k resistor is what I use.

mistermikev · Post by **mistermikev** » 16 Apr 2011, 18:38

I appreciate your comments.

I'm practicing the one handed stuff while working on things I know have no power right now... and it's easier than I would have thought to accidentally touch something.

I think once I'm at the point were I fire up and have issues... a good pair of electrical gloves might be on my list.

merlinb · Post by **merlinb** » 16 Apr 2011, 19:22

mistermikev wrote: I was aware that anything over 25v was a roll of the dice and 300v is almost sure death.

You are perfectly safe up to 50V. And provided the amp isn't actually plugged into the mains, the filters caps won't kill you either (but they'll bloody well hurt!). Mains is lethal, but the caps in valves amps are generally quite modest, and not as frightening as people make out, unless you have a pacemaker or something. I don't even bother discharging when working on amps less than 350V. After the first shock the thing is pretty well discharged

mistermikev · Post by **mistermikev** » 16 Apr 2011, 21:42

hehe, even chocolate cake is lethal in the right dose right?

don't think I'd like to find out about 300v but I appreciate the knowledge.

so you've taken a high voltage shock then?
when I was a kid we had a garbage disposal that apparently wasn't grounded well... I got one hell of a shock from the wall switch. wasn't something I'd like to repeat.

so... how do you know if your alligator clips are insulated 'enough'... other than just trying them out!

deltafred · Post by **deltafred** » 16 Apr 2011, 23:32

mistermikev wrote:hehe, even chocolate cake is lethal in the right dose right?

don't think I'd like to find out about 300v but I appreciate the knowledge.

so you've taken a high voltage shock then?
when I was a kid we had a garbage disposal that apparently wasn't grounded well... I got one hell of a shock from the wall switch. wasn't something I'd like to repeat.

so... how do you know if your alligator clips are insulated 'enough'... other than just trying them out!

I'll try and answer all your questions.

I would like to do some research into how much chocolate cake you can eat before it becomes lethal, but I don't think my waistline would appreciate it.

I've had more electric shocks than I care to remember and don''t recommend them. Not sure what the highest was but I did get about 450V DC from the top cap of a tube in an old amp once, my arm went numb. I also got full (UK 240v RMS, 315v peak) AC mains through a mic while holding an earthed bass guitar, that was a bit like being kicked in the teeth by a donkey (not that I ever have been, but I can imagine). As far as I remember I was suddenly standing 4 feet from where I had been previously with no recollection of how I got there.

AC is very different to DC, DC tends to tense the muscles up and they don't relax until the voltage is removed, AC gives you more of a kick that can send tools, or you, flying across the room. These are generalisations and in practice anything can happen.

Your question about the alligator clips being insulated enough - The standard plastic insulation will stand a pretty high voltage before it breaks down, several thousand volts I would think. What you need to be careful of is cuts, splits or cracks or gaps that can hold moisture (sweat,condensation,coffee,etc) that will conduct and give you a shock, or strands of wire sticking out.

I use them regularly on high voltages and have never had any trouble - yet.

It is not the voltage that kills you, it is the current (through your heart) and you can minimise the risks by :-

Not touching anything that is earthed (hence the left hand in pocket)
Using your right hand.
Wearing plastic soled shoes or rubber soled boots not leather
Standing on a rubber mat (or putting your seat on one).
Anything that increases your resistance to earth

deltafred · Post by **deltafred** » 16 Apr 2011, 23:53

merlinb wrote:
mistermikev wrote: I was aware that anything over 25v was a roll of the dice and 300v is almost sure death.
You are perfectly safe up to 50V. And provided the amp isn't actually plugged into the mains, the filters caps won't kill you either (but they'll bloody well hurt!). Mains is lethal, but the caps in valves amps are generally quite modest, and not as frightening as people make out, unless you have a pacemaker or something. I don't even bother discharging when working on amps less than 350V. After the first shock the thing is pretty well discharged

The filter caps in an amp can and will kill you under the right circumstances, plugged in or not. You, and I, have just been lucky so far.

The advice that you have given is irresponsible to say the least.

merlinb · Post by **merlinb** » 17 Apr 2011, 08:25

deltafred wrote:
merlinb wrote:
mistermikev wrote: The filter caps in an amp can and will kill you under the right circumstances, plugged in or not.

So can literally anything.
I have never heard of a single instance of a person dying from a guitar amp shock, but I've heard of hundreds who have been shocked, including me (almost every time I work on them). People shouldn't go around deliberately shocking themselves, but they shouldn't be turned into sissies either. Proportional care and attention.
500V frightens me, but the Epi valve junior doesn't.

mistermikev · Post by **mistermikev** » 17 Apr 2011, 14:22

your heart is in the right place deltafred, and I don't want anyone to get hurt either (especially not me!), but I don't think it's fair to call merlinb's comment advice per se. He is just a bit cavalier.

phatt · Post by **phatt** » 17 Apr 2011, 15:29

Attention *mistermikev* re your choise of DMM.

Check the CAT class of the unit you use,, it looks rather cheap as I can't see any CAT specs written on the front.
Here's some of the reasons why not all DMM's are equal.

Cat I, Applies to circuits *Not connected to the mains*

Cat II, Circuits directly connected to mains, Houshold appliances
(You'll need this rating to work on Valve Amps)

Cat III, Circuits connected to mains in a building (Electricians use these)

Cat IV, Power lines, substations, (you won't need this one)

These are Australian ratings. check where you live.
Yes I agree never tempt fate when fiddling around inside live gear no matter what anyone says.
My rule of thumb is; *Don't touch anything you can't outrun*

Some folks like to think they can outwitt these things and they may live to tell but you may not be so luckey.

Industrial welding units have a limit of OCV around 75 volts as the human body can't stand much more than that.
But just the same;; on a wet day working with all steel and body sweat a big welder can really hurt you and in some confined spaces each user has to have a watcher in case anything goes wrong.
I've personally been grabbed from big welding machines so yeah I know how dangerous it can be.
Phil.

tictac · Post by **tictac** » 17 Apr 2011, 17:31

A suggestion if I may...

I've built cap discharge tools with an internal resistor of 1K or so and an aligator clip to attach to ground. Problem is you'll still get occasional sparks a flyin when you make a direct connection with a filter cap, possibly damaging the cap; plus some larger filter caps will actually recharge themselves while your working on the amp! I've been hit with a recharged cap and it hurts!

The best thing I've found is to get one of those "Mini-grabber" test leads, clip one end to a grounded lead and the other end to pin 1 or 6 of the first preamp tube. Now you're bleeding the filter caps thru the power supply resistors and the 100k plate load resistor. It only take a few seconds to discharge the amp and by leaving the mini-grabber connected it prevents the caps from recharging and giving you a nasty little suprise...

Of course you must remember to remove the test lead before you power up

TT

mistermikev · Post by **mistermikev** » 17 Apr 2011, 21:38

phatt... thanks for the heads up. I couldn't find any info on my multimeter so I guess I'll have to snag a decent (catII) one prior to doing any post plug in testing.

tictac... good to know. I was planning on using my 5w 10k, and two alligator clips and just leaving it connected so they don't recharge... but I like the idea of not causing a spark!

deltafred · Post by **deltafred** » 17 Apr 2011, 22:16

merlinb wrote:
deltafred wrote:
merlinb wrote:
mistermikev wrote: The filter caps in an amp can and will kill you under the right circumstances, plugged in or not.
So can literally anything.
I have never heard of a single instance of a person dying from a guitar amp shock, but I've heard of hundreds who have been shocked, including me (almost every time I work on them). People shouldn't go around deliberately shocking themselves, but they shouldn't be turned into sissies either. Proportional care and attention.
500V frightens me, but the Epi valve junior doesn't.

I know virtually everything can kill you, death is one of life's few certainties.

I've never heard of anyone being killed by a filter cap either, but I have pulled my hand away pretty quick and gashed the back of it on something sharp. Not to mention the number of test probes/screwdrivers/etc that I have melted the ends of.

I am not trying to turn everyone into sissies, I am just relaying information that was drilled into me as an apprentice.

Yes I take risks, and have a cavalier attitude to a lot of things, well most things really, but that is my choice and I will let everyone else make their choice. It does not hurt to know the correct way to do something, then you can decide if you want to do it that way or not.