mauveman
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Post by mauveman on Mar 17, 2008 7:11:52 GMT -5
Hi all you guitar nuts.
Boy am I relieved to find this forum!
Wading through so much info online is beginning to fry my poor little brain. I have an urgent question that I hope someone can help me with.
I’m following the tried and trusted method to quieten my beast, (An 84 Squire MIJ strat) but have an issue with the recommended Cap value used in grounding. The recommended cap is a 0.33uf 400v metal film, however this is designed to afford some protection for a US style 120V, 60hz supply.
I live in the UK where our power supply is 240V Can anyone recommend the best value cap to use to afford a similar protection?
Huge thanks
The MauveMan
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Post by sumgai on Mar 17, 2008 11:17:37 GMT -5
mm, Hi, and welcome to the NutzHouse! ;D Actually, the cap in question is not designed with that in mind at all...... in fact, it doesn't care about the Mains power supply in the least. What it does do is help to alleviate a potential fault in the high-voltage section of a tube amplifier. That fault could put as much as the full DC voltage on the ground (earth) of an amp. Yes, the circuit breaker/fuse will eventually catch the fault, and break the circuit. But in the meantime, if you as the guitarist happen to be touching your guitar's ground, and if you happen to be playing outdoors in the rain, standing in a puddle, then you could, conceivably, get a taste of that high-voltage that's been shorted to the chassis (ground/earth). All capacitors block DC, so the cap in question should let you have no more than a strong tingle, before the fuse/circuit breaker finally blows. That's the theory anyway. No one here has volunteered to test it, so it's all rather something like insurance - you can't predict the future, so you try to cover as many bases as possible. HTH sumgai
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mauveman
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Post by mauveman on Mar 17, 2008 13:19:19 GMT -5
Cool, that's good news. Huge thanks for pointing me in the right direction Sumgai.
Now all i need to do is track down one of the elusive 0.33uf 400v caps. Elusive in that i don't want to buy 50 or pay the outrageous prices some people are asking for vintage caps.
£21 (42 dollars!) for five Mullard Mustard Caps on Ebay...
mauveman
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Post by sumgai on Mar 17, 2008 14:48:58 GMT -5
mm, Several of our members are from the UK, so one of them might pop in with a suggestion for ya....... In the meantime though, have you checked our Links, Sort Of page? The original post has several dead links, so go immediately to the second page, and scroll down the Post #18, by newey. That's the updated list, and it has some UK suppliers like Maplin and Rapid, and at least one parts-house that has the capacitor you need (although they might not ship outside the USA, I don't know). If all else fails, you can PM me, and I'll send you one via the Postal system. Might set you back a whole whopping pound (sterling or otherwise). But mine are older, and larger, so look around first. HTH sumgai
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nattimus
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Post by nattimus on Apr 13, 2008 0:37:31 GMT -5
mm, All capacitors block DC, so the cap in question should let you have no more than a strong tingle, before the fuse/circuit breaker finally blows. Pardon me, I'm new at this. So if all caps block DC, is a cap that's listed as 400VDC the same as one listed as 400V? Some of the supplier web sites will have listings of caps, and some will be listed as V, some as VDC. Or is it DCV? Anyway...maybe I can just ask if this cap will work...it's a film capacitor, voltage rating is 400vdc, dielectric material is polyester. Thanks, Tom
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Post by newey on Apr 13, 2008 12:23:25 GMT -5
SG-
You said (and this has come up in several places):
However, in rereading JA's original site, he states that the recommendation to use a .33 µf cap was a change in 1998 from the previously-recommended 1µf. According to JA, the original recommendation was to protect the DC amp ground fault problem, but he made the change in order to provide some limited protection against the mike/PA cross-polarization problem once he realized how few musicians checked outlet polarities.
Am I missing something, or do you disagree with JA on this point? The tube amp protection issue doesn't mean much to me, but polarity protection would be useful.
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Post by sumgai on Apr 13, 2008 12:55:37 GMT -5
Alright, alright, don't shove.... everyone stay in line, I'll answer all of you. ;D Tom, Hi, and welcome to the NutzHouse! ;D Yes, V and VDC are the same. Capacitors are made in such a way that they can withstand the ravages of only so many volts of direct current. If you exceed the voltage rating, you risk failure of your cap (sometimes in spectacular fashion ). You can always use a cap of a higher voltage value, but you should never use one of a lower voltage value than you expect to see in that circuit. In fact, you should over-rate your caps by at least 50%, just to be safe. (But do note, most manufacturers don't exceed that rating by more than 10-20%..... something about costs, and all that jive.) John Atchley calls for 400vDC cap, but many amplifiers have more than 500 volts of DC..... how does that square with what I just said? Simple - the cap is gonna self-destruct, but in the meantime, you've been protected! (Or at least, you should've been protected.) At that point, your guitar may be less than usable, but you are still walking around, complaining bitterly about how poorly built the amp was, etc. I'm sure this is more preferable to the alternative, eh? ;D HTH sumgai
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Post by sumgai on Apr 13, 2008 13:05:23 GMT -5
Next! newey, No, you aren't missing anything here, except a few years worth of an engineering education. But don't worry, you're in good company - John Atchley is in the same boat. That's OK, such is not necessary for success in one's life, unless health and safety are paramount in one's chosen profession. Correct procedures can be learned without all the Greek background, unless one likes imitating Animal House. It happens that in order for this capacitor to work as a protector, one needs to remember the original property ascribed to a cap (which was called a condenser*, way back when): Capacitors block DC, and pass AC. Nice and neat, eh? So one has to ask, how can a cap protect a player against a cross-polarity current flow? Answer: it can't. Even when you use a very large cap, where it takes a finite amount of time for the current to raise the charge to a level that you (the player) can feel the tingling, it's still damn quick, and doubly-damned dangerous. Let me put it another way, by analogy: If caps were so capable of this kind of protection, as intimated by JA and others, then don't you think that manufacturers of Ground Fault Interrupt devices would be using them, instead of the more expensive components therein? Doesn't happen, Bunky. No matter how big the cap, the potential for harm is still there, ergo...... no caps for the protection circuitry.** John, Chris and I (and UnklMickey) discussed this some time ago, I'm sure the thread is still extant, if we wanted to search for it, but it got kind of technical, and in the end, we all agreed on what I just said above anyway. HTH sumgai * Extra brownie points to the first person who posts the original name of these devices, before they were called condensers. ** Yes, there is in fact a capacitor in a GFI device, but it's not in the direct path between the source voltage and the user, so let's not quibble here - you understood my meaning, right?
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Post by newey on Apr 13, 2008 16:40:58 GMT -5
* a Leyden Jar. Wikipedia helped.
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