Amp safety question - remove the line filter caps?
« on: March 09, 2010, 10:18:52 AM »
I just bought a '63 Guild 200-S stereo amp that hasn't yet been swapped to a modern power input scheme.  It still has a two pronged plug, with a filter cap to ground that is switchable between the line and neutral legs on the transformer primary/power inlet.  First order is to swap out to a three prong grounded plug, with ground soldered to the chassis.   Should I leave the line filter caps in place or remove them?


volker

Re: Amp safety question - remove the line filter caps?
« Reply #1 on: March 09, 2010, 10:51:30 AM »
Remove it!

Depending on the capacitor it might have gotten leaky over the years or even totally fail
at some point and the chassis will be on some potential or even full VAC! Via the guitar cable
you are directly connected to the chassis, so it's an electrocution waiting to happen.
How unlikely it really may be, I wouldn't risk it for the sake of keeping it original or having
some ancient hum prevention.

Re: Amp safety question - remove the line filter caps?
« Reply #2 on: March 09, 2010, 11:22:32 AM »
This is what I never understood:  Wouldn't having the chassis grounded keep it from ever developing a potential?  If the line filter cap shorted, wouldn't the mains fuse blow before any chance of being shocked?  Even if someone stuck a metal rod in the fuse socket, it seems like the chassis couldn't develop enough voltage drop before a line breaker popped.  I understand how dangerous these caps are on an ungrounded chassis, but the same is true for a grounded chassis?

abbey road d enfer

Re: Amp safety question - remove the line filter caps?
« Reply #3 on: March 09, 2010, 01:18:30 PM »
How "grounded" is a grounded chassis?
When you think about the number of junctions between the chassis and the real earth (chassis to ground conductor, ground conductor to prong, prong to socket, socket to wire, wire to earth, discounting all sorts of distro boxes and secondary junctions) and the possibility for one of them to break loose, I wouldn't want to take any risks.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Re: Amp safety question - remove the line filter caps?
« Reply #4 on: March 09, 2010, 04:55:02 PM »
If you're talking about the ground switch and the .05uF cap that's connected - just remove the cap and switch.  With a 3-prong you only have one designated earth and the hot side should always stay in line with the fuse.



With any significant potential on the chassis -even with the earth pin in place - providing a temporary alternate path(which may appear shorter depending on the mains grounds path) via touching the wrong combination of things could still net you a jolt.  Too risky.


Re: Amp safety question - remove the line filter caps?
« Reply #5 on: March 09, 2010, 08:42:35 PM »
>>>How "grounded" is a grounded chassis?

>>>a temporary alternate path(which may appear shorter depending on the mains grounds path)

This helps.  Thanks for the elaborations!

PRR

Re: Amp safety question - remove the line filter caps?
« Reply #6 on: March 09, 2010, 09:05:38 PM »
> Wouldn't having the chassis grounded keep it from ever developing a potential?

If that is true (it is, sometimes), then the cap is "useless". However it is 47 years old, and was NOT correctly specced when new. (Modern safety specs on to-line caps are much more stringent than what new wax/paper caps were good for.) Lose it.

And when I moved into my last house, it was all 3-pin outlets and none of the green wires connected. It "looked grounded", but was NOT.

They also got the white-black switched on 60% of the outlets. So ignoring safety, if you had the non-polarized 2-prong plug you could at least flip-plug for least buzz; with a 3-pin plug you can't even do that (and probably shouldn't).

When I moved into this house, I found the well-head open to dirt, the oil-line laying on damp concrete across a walkway, and some frightening "splices". So it is very odd that all the electrical outlets that work do have functional ground(*) and correct polarity.

(*) Oh, grounded to the fusebox. There's no bond from house to dirt.

> wouldn't the mains fuse blow

I been testing that here. (Backhoe hit the well wires AGAIN.) Don't depend on breakers. Yes, a fat well-wire shorted by a steel bucket trips-out fast. But a long circuit plus extension cord plus appliance cord to a sick cap may pull not enough to trip a breaker for many-many seconds, yet more than enough to stun your heart.

How it should be, and how it is where you want to play, are different things.

envelope

Re: Amp safety question - remove the line filter caps?
« Reply #7 on: March 10, 2010, 09:12:45 AM »
the cap is question is a line to ground cap, as opposed to across the line cap. If it fails the fuse will not blow with 'line to ground' cap, it will blow when an 'across the line' cap fails.

clintrubber

Re: Amp safety question - remove the line filter caps?
« Reply #8 on: June 14, 2016, 12:45:20 PM »
And then these C24 & C25 caps pre-transformer in a 1977 Peavey schematic.

For some line-filtering OK, but also  risk when plugging in a non grounded outlet, as someone might do.

I looked inside but didn't see them present in the actual amp (export version). Would have removed them otherwise.

PRR

Re: Amp safety question - remove the line filter caps?
« Reply #9 on: June 14, 2016, 09:42:29 PM »
The 1977 Peavey *may* be an attempt to shunt RFI; either external coming in or internal (digital clocks) getting out. This was a big issue when early PCs knocked-out over-the-air radio/TV, and even non-PC makers considered it.

If those caps were rated for Across-The-Line duty, I would leave them (if present). I don't recognize "DVL" as a cap spec. I would be entirely happy without them, at least until RFI caused me a problem.


RuudNL

Re: Amp safety question - remove the line filter caps?
« Reply #10 on: June 15, 2016, 02:48:29 AM »
I seems the two capacitors to 'ground' is also common practise in switched mode computer power supplies.
But if somebody uses a not grounded power socket, the whole cabinet may have half of the mains voltage on it!
There is a solution for every problem!

http://www.vansteenisaudio.nl

CJ

Re: Amp safety question - remove the line filter caps?
« Reply #11 on: June 15, 2016, 03:23:32 AM »
been working on git amps since 1970, never seen an amp with the .047/600 fail,

don't know if it is because they just see 60 hz or because the only see 120 V-ac, whatever the reason, never heard of anybody getting shocked by that cap shorting to the chassis, in fact. that is probably the only part i have not replaced in an amp due to failure,

how many old Fender amps are still out there with that cap? or in these amps>
 Marshall, Ampeg, Peavey, Acoustic, Line 6, Soldano, Trainwreck, Silvertone, Splawn, Kustom, Egnater, Orange, Schaller, Crate, Sunn, Magnatone, Randall, Tradesman, Vox, Hartke, Mesa, Traynor, Musicaman, Carvin, Zenith, Motorola, Ford, Chevy, Chrysler, Hiwatt, Laney, B-52, Bogner, Red Bear, Alamo, GK, Gibson, EMG, Blackstar, Behringer (ugh), Krank(RIP Dimebag), Rolland, SWR, Tech 21, Fishman, Tube King, Epiphone, Phil Jones, Leilani, Nu-Amp, Hughes and Kettner, Kendrick,

take it out if it makes you feel better, but don;t stress too much over it,
would UL pass an amp if this cap was unsafe?

if anybody has a story of that cap causing trouble i would love to hear about it,

that cap can be useful when gigging some places have weird power and using the ground switch can quiet down an amp, three prong plugs are great if you don't clip the 3rd prong with dikes because you forgot the adapter and the house you play is old,

if that cap does short out, it will either self destruct or blow the amps fuse of connected from hot to chassis and the chassis is grounded, or it shorts from hot to chassis on an ungrounded amp you will only die of you touch something that is grounded, what would that be? a toaster? a hot water pipe? not too many hot water popes in the jam ran, unless your playing the Voyeur here in town, you want to see an out of code rats nest check out the tanatula room in the back next time you are in OlyWa, can't say i like the tickle i get when i do get that 56K resistor between me and the mic or the other git player,

not the most conservative answer, but then again, i like free climbing and white water kayaking, Jaywalking, dipping transformers indoors during winter time, having unprotected sex with multiple partners,  ;D
« Last Edit: June 15, 2016, 03:44:38 AM by CJ »
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

moamps

Re: Amp safety question - remove the line filter caps?
« Reply #12 on: June 15, 2016, 03:52:27 AM »
I seems the two capacitors to 'ground' is also common practise in switched mode computer power supplies.
But if somebody uses a not grounded power socket, the whole cabinet may have half of the mains voltage on it!

As I know, all IEC connectors with a RFI filter incorporated have  Y caps, usually 2n2 or 3n3. 
A capacitive divider they form if the ground isn't present has very high impedance, so there is no real danger of  an electric shock.
Of course, caps must be Y types.
IIRC, there is a standard about that.

RuudNL

Re: Amp safety question - remove the line filter caps?
« Reply #13 on: June 15, 2016, 10:01:50 AM »
A capacitive divider they form if the ground isn't present has very high impedance, so there is no real danger of  an electric shock.

Well, I must be very sensitive, because I felt it many times!
(I was already  'electric boogying' before this phenomenon was invented... :P )
There is a solution for every problem!

http://www.vansteenisaudio.nl

clintrubber

Re: Amp safety question - remove the line filter caps?
« Reply #14 on: June 15, 2016, 01:56:23 PM »
Thanks for the reactions.

I scrutinized the terminal strip in the export version and as it seems those C24 & C25 have never been in place for Europe. I also had a look at schematics of various later three pronged guitar amps (Peavey, Fender, Ampeg) and none of these used such caps.

FWIW, adding a line filter before such amps would however re-introduce them again
( see for instance http://eis.apitech.com/power_line.asp ), so if one likes to toy around with three prong amps & ungrounded wall outlets there's still some fun possible.   8)

PRR

Re: Amp safety question - remove the line filter caps?
« Reply #15 on: June 15, 2016, 02:15:43 PM »
> cabinet may have half of the mains voltage on it!

Yes.

I once mediated between a jazz-band, my PA system, and a movie-shoot's sound mixer. The musicians were getting shocked, blamed me. I traced it to the film mixer 2-pin and 2-caps power input, but he would not let me tamper. I finally tied the film-sound mixer chassis to my PA system, properly grounded, and drained his hi-Z 60VA away.

I've been allergic to that connection ever since.

Re: Amp safety question - remove the line filter caps?
« Reply #16 on: June 15, 2016, 05:49:11 PM »
I have over 10 vintage Fenders, Marshalls and Voxes. All have been converted to 3 prong AC cords with chassis grounded and cap removed. On my tweed Deluxe, converted the polarity switch to a standby switch, very handy. Also carry a little test light I clip on my strings and touch the mic to see what I get. Been shocked too many times on the lips and teeth. You know, that might explain some things....

JohnRoberts

Re: Amp safety question - remove the line filter caps?
« Reply #17 on: June 15, 2016, 06:04:29 PM »
With a proper 3 wire line cord and grounded chassis you can remove the "stinger" caps and switch.

A cap between ground and neutral seems like a wasted part, and not like the Peavey I remember, but if the line cord ground pin was cut or floating, the extra cap to neutral will at least divide down the leakage into the floating chassis from line. 

A cap between line and ground needs to be Y safety rated. You can buy caps that are both X and Y rated, but not cheaply (X and Y refer to how the caps fail such that they don't create human hazards.).

JR
Cancel the "cancel culture", do not participate in mob hatred.

CJ

Re: Amp safety question - remove the line filter caps?
« Reply #18 on: June 16, 2016, 03:38:54 AM »
i try to stay neutral on these topics, but sometimes i get so hot that i need to get grounded,  ;D

just a ref drawing for my own selfish needs because we do wire old amps for 3 prong whenever possible,
in that case, by all means do we remove the cap because it will be a moot point if the chassis actually gets tied to ground via a good cord, properly wired wall socket, and a good transformer on the pole, plus, in that situation, the cap does see a real 120 VAC when plugged in,

what happens with a two prong cord chassis that has that cap?
that cap takes the floating chassis (line cord goes directly to pwr xfmr primary) and connects it to either the hot or neutral of the wall socket, but from an AC standpoint only.
Reactance of the cap is 1/377 * .047uf = 56,436 ohms,
what is the potential between the floating chassis and either/or  the power line neutral or hot?
 hopefully zero. no wonder the caps don't fail.
so why bother with the cap?
i would hazzard to guess that sometimes the chassis picks up EMF and that can be shunted to ground via the cap/neutral wire of the cord,
but if the cord is plugged in so that the cap goes from the chassis to Hot, then more hum would be experienced, thus the switch for picking the lesser of the two,

what about a vibro champ with no gnd switch? Leo says flip the cord for least hum and/or shock from the SM58 grill,

once the chassis is grounded via 3 prong, all EMF should go to wall socket ground.

however, sometimes you might find that even with the 3 prong plug you still get a bad hum, maybe due to funky wiring at the club, in that case, and old school player with the tweed bassman might have the jump on you with his ability to choose which side of the line that chassis cap is connected to,

why do two players get a mild shock when using vintage amps?
one might have his cord plugged in 180 degrees different from the other guy, this one guy has a chassis tied to Hot via the cap and the other guy now  has chassis tied to neutral(gnd) via the cap, so they have 120 V-ac between them, but are current limited by 56 k twice = 112K = about 1 ma between them, which is probably not enuff to cause refib/defib,  unless drugs are involved, which is a safe bet for working musicians, hell half the people who come into the git shop are on something or boozed up or both,  :D
personally, being tied to electrical ground via the git cord/chassis and line cord gives me the willys, i can handle the .047 tingle with the floating chassis, but getting across another ground that is 180 out while holding the git and being connected to that green wire scares the crap out of me,

120 VAC wall socket plus mutant cj at the Rhythm and Rye last Thursday on the J bass, $15 jar, $90 gate, /3>
« Last Edit: June 16, 2016, 04:36:15 AM by CJ »
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

CJ

Re: Amp safety question - remove the line filter caps?
« Reply #19 on: June 16, 2016, 04:27:37 AM »
file this under "don't believe everything you see on the net"

they got the line and neutral labels swapped,


was gonna email this site to warn them  but it was just a photo drop,

http://www.pic2fly.com/120V+Outlet+Wiring+Diagram.html

Warning-this pic is not correct>
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html


 

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