knufsaid
Rookie Solder Flinger
Posts: 6
Likes: 0
|
Post by knufsaid on Oct 25, 2006 8:24:24 GMT -5
hi everyone haven't been here for a while - i've copper shielded my strats and was pleased with the results, i'm now thinking of doing the star grounding/wiring mod.
Capacitors!! i'm in the UK where the mains voltage is 250v ac and i've found some .33uf (250v ac working voltage) ones - is this what i want?
i can't find .33uf 400v ones but i can find .22uf 400v and .47uf 400v - should i get these? if so which ones??
Or should i look for higher than the recommended 400v rated capacitors as our mains is higher? i've found .22uf 630v dc - would these do?
thanks for any advice you can give
mark
|
|
Channelman
Meter Reader 1st Class
Posts: 62
Likes: 0
|
Post by Channelman on Oct 27, 2006 3:26:52 GMT -5
In a guitar (not an amp) the working voltage of any component does not need to be more than a few volts. Even with active electronics they only need to be rated at a slightly higher voltage than the battery. (eg 9v battery....12 v caps) I think the only reason 250 v or 400 v caps are used is because they are physically larger and easier to connect. Some of the small ceramic cap can be a bit fragile and are easily damaged if the leads are bent. So, the mains voltage has nothing to do with it, just pick capacitors with the correct capacity. If you want 0.47 uF, get them in any voltage rating.
|
|
knufsaid
Rookie Solder Flinger
Posts: 6
Likes: 0
|
Post by knufsaid on Oct 27, 2006 4:31:19 GMT -5
thanks - i understand it now!
mark
|
|
|
Post by UnklMickey on Oct 27, 2006 11:10:04 GMT -5
WHOA, HOLD THE PHONE!Channelman, what you said is usually correct............ but, in this case, we are talking about a cap that is intended to isolate all the shielding and touch points from the signal ground. the intent here is, if there is a certain type of failure in a tube amp, that would cause a high DC voltage to be present at the guitar cable, the cap will block the DC. although it will take a RARE type of failure for this to occur, it is possible. since we get these kinds of questions from time to time you should read this: and the last portion of this, under the heading, "Why a 400V Capacitor?" Knufsaid, any of the 3 caps you mentioned should suffice. cheers, unk
|
|
Channelman
Meter Reader 1st Class
Posts: 62
Likes: 0
|
Post by Channelman on Oct 27, 2006 13:07:42 GMT -5
OK unk, sorry, I didn't think of that aspect. I hadn't realised that DC voltages might get onto the 'jack ground' and I'm still not clear how that could happen. If it was AC mains that had got on the amplifier ground, which I suspect would be more common, then I still wouldn't fancy my chances with a 0.33uF 'isolation' cap which has an impedance of 10k ohm at mains frequency and our British 240 volts between the lips (microphone) and a sweaty hand (on the guitar strings/bridge) Ouch! CM PS I did think that 0.47uF sounded large for a 'tone cap'..... I should have realised it wasn't
|
|
|
Post by UnklMickey on Oct 27, 2006 14:27:24 GMT -5
don't 'beat yourself up' about it. .:lol:.
it was new to me the first time i read John Atchley's take on it.
even with the smaller (0.1uF) cap that he recommends with the 'strings only' approach, that would only increase the impedance by a factor of 3 for AC problems. still one heck of a rude awakening in the states, twice as bad on the other side of the pond.
in general, for AC problems, a proper grounding scheme IS the only safe solution.
we previously discussed several possible failures in tube amps that could cause a DC voltage to be present at the amp chassis, and likewise at the guitar.
proper grounding, instead of the capacitive coupling of the chassis to neutral (the nasty Fender 'polarity' switch), will protect against all the DC problems, except one i recently became aware of.
on a few tube amps, the input jack is isolated from the chassis.
it makes it's ground connection via a common at the preamp.
if that trace (or wire) is broken somewhere between the preamp, and the power supply (rare, but possible), then it's possible to get a sizable DC voltage on the shield of the input cable.
the other noteworthy point, is the the jackplate will still be a hazard in any of these failures.
unk
|
|
|
Post by ChrisK on Oct 27, 2006 15:40:19 GMT -5
RF anyone? ?
|
|
knufsaid
Rookie Solder Flinger
Posts: 6
Likes: 0
|
Post by knufsaid on Oct 30, 2006 18:03:21 GMT -5
thanks for the info from both of you!
i am a bit paranoid about wiring as i got electrocuted a few years ago by a fault at a rehearsal place - i was fine playing guitar but when i grabbed my mic as well......... i still have the scars across my fingers where the strings were!!!
luckily the bass player turned off my amp with his headstock and i'm still here but it wasn't something i'd want to repeat or have happen to anyone else.
to anyone reading - there is nothing you can do if you're being electrocuted, you can't let go, you can't switch anything off - you can only feel your heart racing and i knew it was only a matter of time before it gave out. if i'd been on my own i would be dead.
i always use an rcd breaker now and it's never tripped but i don't care - i hope it never trips and i hope every place any of us ever play is safe! i don't know if a breaker will prevent it from hapenning again but it gives me enough peace of mind to carry on playing!
i also used to check by touching the strings with the back of one hand and tapping the mic with the back of the other hand - when you get electrocuted the muscles contract and thats why you can't let go, this way you'd get knocked away by the shock rather than gripping onto it.
anyway sorry to get so heavy but it still gives me a cold feeling when i think about it
thanks for the help - i've got the .22uf 400v so i'll put that in
keep on rockin - i have!!
|
|
|
Post by UnklMickey on Oct 30, 2006 18:13:29 GMT -5
knufsaid,
don't get a false sense of security!
the cap will lessen, but not eliminate an AC shock. (that's almost certainly what you experienced.0
there in NO substitute for proper grounding on the AC outlets, and amps.
get a GFCI if you can, but at least get a 'wiring tester'. they only cost a couple of bucks, and you just plug into the outlet you want to test, and check the lights.
or keep a bass player nearby. .:lol:.
unk
|
|
knufsaid
Rookie Solder Flinger
Posts: 6
Likes: 0
|
Post by knufsaid on Oct 31, 2006 2:53:22 GMT -5
hi unk
i try not to get any sense of security!!
an rcd over here is what you call a gfci and i hope it would work if it ever happened again ( i have 3 of them in my gig box and i use them on the PA circuit, the effects circuit and the amp circuit - paranoid, me??)
my other failsafe is that the bass player and the drummer are both doctors!!
thanks again for all your advice - take care
mark
|
|
|
Post by UnklMickey on Oct 31, 2006 10:54:50 GMT -5
2 members of the band are doctors?
good stuff that!
aside from the safety issue, though, i'd prolly rather have a couple of groupies that are nurses. rofl
|
|