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Post subject: Safety first
Posted: Mon Jun 11, 2012 6:12 pm
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Being new to tube amps I know that I have absolutely no business messing around with anything that's not on the front panel so I want to assure you all that I have no such intention or desire. In fact some of the very informative posts I've read here have put the fear into me and I'm a little apprehensive about turning my amp ON. :lol:

    :?: Is changing tubes something which can be done by a novice or does one need much more experience and knowledge?

    :?: One method of safely discharging capacitors says to "...attach a jumper lead from the plate of a tube to ground." I'm assuming this is a fast and efficient method. However if the amp is turned off and unplugged, will the capacitors discharge on their own, over time? If so, how long would this take?

Thanks and Cheers!
BM

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Post subject: Re: Safety first
Posted: Mon Jun 11, 2012 8:00 pm
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The amp should be switched off before exchanging any tubes, and unplugged from electricity before discharging caps. When exchanging tubes, there is the possibility of contacting the tube pins with your fingers. Avoid contacting the pins, and/or check that the caps are discharged to a safe level first. I routinely change tubes without discharging the caps, never touching the pins, with no problems.

Changing preamp tubes in most any amp is easy enough, just remove the old and plug in the new, being careful to align the tube pins to the socket before inserting, no adjustment required. Changing the output tubes typically requires a bias check/adjustment, and this is where you need to deal with the high voltage measurements.

The caps can discharge by themselves within a few minutes after turning the amp off, depending on the amp, or even when/how it is turned off, but they don't always discharge on their own. Always check to be sure that they are discharged, when necessary. You only need discharge the caps if the work requires it, such as working on the circuit, soldering, etc.

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Post subject: Re: Safety first
Posted: Thu Jun 14, 2012 7:15 pm
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As mentioned preamp tubes are pretty easy to swap out. Common sense is obviously still in order, but you can change those easily enough. Where it can't be stressed enough is swapping of the power tubes.

Read enough forum posts or watch experts on YouTube explain it and you'll be told over and over how dangerous it can be for an inexperienced person to mess with those high-voltage tubes. Their charge can last for days after they've been both shut off and unplugged. In fact, even after the caps have been properly drained they will slowly begin to re-charge themselves. You'll definitely want the necessary tools at hand when doing such work. Safety is the first order of business.


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Post subject: Re: Safety first
Posted: Thu Jun 14, 2012 9:24 pm
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iabssplyr wrote:
In fact, even after the caps have been properly drained they will slowly begin to re-charge themselves.


Sure would be interested in learning how a cap that has been completely drained and has no power applied to it will spontaneously recharge.

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Post subject: Re: Safety first
Posted: Thu Jun 14, 2012 11:21 pm
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Not trying to start an argument, Bill, just trying to save the life of a fellow musician who doesn't sound very experienced in this field. Google "cap filters recharging" and see what you get. Here's an example: http://acapella.harmony-central.com/arc ... 04133.html

I've even watched a YouTube video of an expert with a voltage meter displaying Tge increase as he discussed the increase. I believe there is a threshold of current where if it's below 10 mV, I believe, it's safe to work on the unit, but once it rises above that you either have to discharge the caps again or you maintain the discharge as you go. Most likely the second choice.

There's a bundle to learn and that's part of the enjoyment of the hobby. But it's a fool who doesn't take precautions and learn what he's getting into, especially high voltage. For me, I have only changed preamp tubes so far. If you don't believe me then check it out yourself from what you read, or better yet, find an expert and pick their brain. Let me know what you find out.


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Post subject: Re: Safety first
Posted: Thu Jun 14, 2012 11:50 pm
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bluesky636 wrote:
iabssplyr wrote:
In fact, even after the caps have been properly drained they will slowly begin to re-charge themselves.


Sure would be interested in learning how a cap that has been completely drained and has no power applied to it will spontaneously recharge.


I think one of Dan Ackroyd's "PSI Factor" episodes covered that, Bill.

You must've missed it, check Youtube.

:mrgreen:

Arjay

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Post subject: Re: Safety first
Posted: Fri Jun 15, 2012 7:25 am
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Um, the chemistry of voltage recovery in electrolytics is well understood -- it's not magic or myth. Here's Rubicon's explantion: http://www.rubycon.co.jp/cn/products/alumi/pdf/RecoveryVoltage.pdf

iabssplyr is exaggerating the danger quite a bit -- the recovery voltage can never be more than a tiny fraction of a cap's full charge, so it's never enough to really hurt someone (unless you're working with much bigger caps than a guitar amp's power supply). But it can be enough to startle someone and that could potentially cause you to knock the amp chassis off the workbench, or burn yourself with your soldering iron, or similar mishaps.


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Post subject: Re: Safety first
Posted: Fri Jun 15, 2012 10:41 am
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iabssplyr wrote:
Not trying to start an argument, Bill, just trying to save the life of a fellow musician who doesn't sound very experienced in this field. Google "cap filters recharging" and see what you get. Here's an example: http://acapella.harmony-central.com/arc ... 04133.html

I've even watched a YouTube video of an expert with a voltage meter displaying Tge increase as he discussed the increase. I believe there is a threshold of current where if it's below 10 mV, I believe, it's safe to work on the unit, but once it rises above that you either have to discharge the caps again or you maintain the discharge as you go. Most likely the second choice.

There's a bundle to learn and that's part of the enjoyment of the hobby. But it's a fool who doesn't take precautions and learn what he's getting into, especially high voltage. For me, I have only changed preamp tubes so far. If you don't believe me then check it out yourself from what you read, or better yet, find an expert and pick their brain. Let me know what you find out.


Ah yes. The experts from Harmony Central. Read through the whole thing without finding a single reference to anything to back it up.

I sure don't remember it being taught when I earned my BACHELOR'S DEGREE in ELECTRICAL ENGINEERING. Maybe I slept through that class. :lol:

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Post subject: Re: Safety first
Posted: Fri Jun 15, 2012 10:42 am
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Retroverbial wrote:
bluesky636 wrote:
iabssplyr wrote:
In fact, even after the caps have been properly drained they will slowly begin to re-charge themselves.


Sure would be interested in learning how a cap that has been completely drained and has no power applied to it will spontaneously recharge.


I think one of Dan Ackroyd's "PSI Factor" episodes covered that, Bill.

You must've missed it, check Youtube.

:mrgreen:

Arjay


Must have. :D

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Post subject: Re: Safety first
Posted: Fri Jun 15, 2012 10:46 am
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strayedstrater wrote:
Um, the chemistry of voltage recovery in electrolytics is well understood -- it's not magic or myth. Here's Rubicon's explantion: http://www.rubycon.co.jp/cn/products/alumi/pdf/RecoveryVoltage.pdf

iabssplyr is exaggerating the danger quite a bit -- the recovery voltage can never be more than a tiny fraction of a cap's full charge, so it's never enough to really hurt someone (unless you're working with much bigger caps than a guitar amp's power supply). But it can be enough to startle someone and that could potentially cause you to knock the amp chassis off the workbench, or burn yourself with your soldering iron, or similar mishaps.


Ok. Here is a better explanation:

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

Can't say as I have ever seen this effect when working on any of my tube amps. Of course, I always check the charge on the power supply caps before sticking my fingers in there. As long as it is below 20 VDC I don't worry about it.

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Post subject: Re: Safety first
Posted: Fri Jun 15, 2012 11:28 am
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"Can't say as I have ever seen this effect when working on any of my tube amps. Of course, I always check the charge on the power supply caps before sticking my fingers in there. As long as it is below 20 VDC I don't worry about it."

If you discharge the caps and then start working on the amp pretty much immediately, you'd never see it because it takes time for the chemical/physical properties of the electrolytic to settle down. And 20 volts or so is about the most you'd every see from caps that had been charged to 450~500 volts -- they only "recharge" to 5% or so. (It's more precise to say that they didn't completely discharge -- they still had residual chemical energy stored.)

It's not something they'd spend much time teaching in EE classes because it's not particularly important. But a novice who thinks an amp is drained because he discharged the caps three days ago when he started working on it could easily be surprised and jump when he gets an 18V tingle.


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Post subject: Re: Safety first
Posted: Fri Jun 15, 2012 12:12 pm
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strayedstrater wrote:
"Can't say as I have ever seen this effect when working on any of my tube amps. Of course, I always check the charge on the power supply caps before sticking my fingers in there. As long as it is below 20 VDC I don't worry about it."

If you discharge the caps and then start working on the amp pretty much immediately, you'd never see it because it takes time for the chemical/physical properties of the electrolytic to settle down. And 20 volts or so is about the most you'd every see from caps that had been charged to 450~500 volts -- they only "recharge" to 5% or so. (It's more precise to say that they didn't completely discharge -- they still had residual chemical energy stored.)

It's not something they'd spend much time teaching in EE classes because it's not particularly important. But a novice who thinks an amp is drained because he discharged the caps three days ago when he started working on it could easily be surprised and jump when he gets an 18V tingle.


Like I said, I check the cap voltage EVERYTIME I open up the back of an amp, whether its to change a tube or anything else. That is just common sense regardless if this effect were dangerous or not. Besides, everyone of us here that provides advice on working on amps warns the questioner about the danger of working on a tube amp. We don't ignore it. It would be irresponsible on our part to do so.

I've taken the full brunt of a Blackface Twin Reverb B+ before. I doubt 20 volts would even make me flinch. :lol:

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Post subject: Re: Safety first
Posted: Fri Jun 15, 2012 6:35 pm
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Well said by all those who know a lot more than I do, for sure. I walked into this just trying to be helpful to someone I don't know, but that's what makes forums great; there's always someone more knowledgeable writing next.


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Post subject: Re: Safety first
Posted: Fri Jun 15, 2012 8:11 pm
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iabssplyr wrote:
Well said by all those who know a lot more than I do, for sure. I walked into this just trying to be helpful to someone I don't know, but that's what makes forums great; there's always someone more knowledgeable writing next.


There are a number ofpeople here who are EXTREMELY knowledgable about amp design and repair. Hang around long enough and it will become pretty obvious who they are.

BTW, I only consider myself to be knowledgable. :lol:

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Post subject: Re: Safety first
Posted: Fri Jun 15, 2012 8:59 pm
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I have an old electrical engineering book from the 50s around here somewhere (can't find it now) that actually says, in order to be safe when checking for voltage, use the back of your fingers. :shock: The reason for using the back of the fingers is that if voltage is present, it will cause the muscles in the hand to retract away from the voltage, and if you use the front of your fingers, the hand will clamp onto the voltage and you won't be able to let go (been there, 460 VAC). It also talks about the difference in taste of AC vs DC voltage. :o

I prefer to use a meter!

A former boss used to tell me that a good technician should be able to choke down 460. :lol:

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