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Post subject: Is this as safe as it appears on the video?
Posted: Tue May 21, 2013 4:19 pm
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Aspiring Musician
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While searching the Internets for information on the Blues Jr., I came across the Billm audio site and found this video entitled Discharging the Capacitors

I was surprised and maybe even a little :shock: shocked. Given my very limited knowledge of how things work inside a tube amp, I'm certainly in no position to mount an argument in any direction but what I saw on the video seems contrary to what I've come to know with regard to safely discharging capacitors.

At 0:22 the person in the video says that he's "using a Fender Blues Junior ... as an example but it applies to any other tube amplifier" and that the "volt meter [is] hooked up to the main power supply filter capacitor." Later, at 0:55 there is a disclaimer on screen "Does not apply to amps with tube rectifiers".

Could it really be as simple as turning on the amp, letting the tubes warm up, then turning it off? Is there something I've missed or perhaps misunderstood?

Cheers!
BM

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Post subject: Re: Is this as safe as it appears on the video?
Posted: Tue May 21, 2013 9:04 pm
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That video has been discussed ad nauseum on the Gear Page. I prefer to be safe than sorry and always check the voltage on the power supply caps before sticking my hands in an amp.

As Dirty Harry once said: "You've got to ask yourself one question, do I feel lucky?" :wink:

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Post subject: Re: Is this as safe as it appears on the video?
Posted: Tue May 21, 2013 9:15 pm
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Yes, it works. I agree that it's always best to check with a voltmeter after discharging caps, no matter which method used.

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Post subject: Re: Is this as safe as it appears on the video?
Posted: Wed May 22, 2013 4:18 am
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Aspiring Musician
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    Thank you both for your replies and for confirming that the Warm-Up/Turn-Off method works. A couple more questions which come to mind:

    • Why is this method not applicable to amps with tube rectifiers?
    • Given that the VM was hooked up to the "main power supply capacitor" which read 4V after the amp was turned off, does discharging one capacitor automatically discharge all capacitors?

bluesky636 wrote:
... As Dirty Harry once said: "You've got to ask yourself one question, do I feel lucky?" :wink:
    Absolutely! Always check with a VM to make sure regardless of the method used to discharge the caps.

    The quote is a classic, but you forgot the first part, Bill. Here it is in it's original form. :wink:

      "I know what you're thinking. 'Did the caps discharge or are they still holding 400 volts?' Well, to tell you the truth, in all this excitement I kind of lost track myself. But being as this is a 44µF cap, the most powerful capacitor in the world, and would blow your hand clean off, you've got to ask yourself one question: Do I feel lucky?"


    Cheers!
    BM


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Post subject: Re: Is this as safe as it appears on the video?
Posted: Wed May 22, 2013 6:45 am
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As far as discharging the caps, it is the same whether an amp has a tube rectifier or SS rectifier, no difference. As Billm pointed out, the only difference is that a tube rectifier has to warm up before producing high voltage. With a SS rectifier, if you toggle the power on and back off before the tubes warm up, the caps can hold the charge of high voltage, whereas with a tube rectifier, the rectifier won't warm up enough to produce the high voltage to begin with. Never rely on the caps being discharged without verifying with a meter.

Some amps have capacitors before the standby switch, and if the switch is open (in standby) when the amp is turned off, those caps before the switch might not be discharged. If the standby switch is closed (in play) then all caps will be discharged at the same time.

Can't emphasize too much, ALWAYS check the caps to be sure that they are discharged. Check the schematic to see if any particular amp has disconnected caps such as those with caps before the standby switch.

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Post subject: Re: Is this as safe as it appears on the video?
Posted: Wed May 22, 2013 2:33 pm
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shimmilou wrote:
... whereas with a tube rectifier, the rectifier won't warm up enough to produce the high voltage to begin with.


I have to disagree with that statement.

When checking the bias on my 5F6A clone (with a 5AR4/GZ34 rectifier), I turn the main power switch on and a few seconds later throw the standby switch. Within seconds, the tube plate voltage, as measured by my bias probe, is over 200 VDC and heading quickly to 400+. :shock:

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Post subject: Re: Is this as safe as it appears on the video?
Posted: Wed May 22, 2013 3:12 pm
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I'm not sure what you mean. A cold tube rectifier takes longer to produce voltage than a SS rectifier after the AC is applied, you know that. Compare how quickly the Plate voltage comes up on your BDRI and you'll see what I mean. Really, check the DC output from the cold tube rectifier before the standby switch and compare to the SS rectifier before the standby switch. Off the top of my head, maybe 5 seconds or more before a cold tube rectifier will produce voltage, compared to instantly with the SS rectifier. So, with a tube rectifier in a cold amp, you can switch the power on and then right back off and the rectifier will not have time enough to produce voltage, but the instant that you apply AC to a SS rectifier it will produce full output DC voltage immediately.

Here is the full quote from the Billm site:

"catfishjones40 says:
July 9, 2012 at 9:20 am

Hi Bill. I watched the video and saw the notice about amps with tube rectifiers. In that case would you just use the resistor method to discharge?

bill says:
July 9, 2012 at 5:28 pm

An amp with a tube rectifier will self-discharge just as quickly as one with diodes. The difference is that with a tube rectifier, there’s no high voltage until the rectifier warms up and begins to conduct.

So “bumping” the power (quick on-off) on an amp with solid-state rectifiers will charge the capacitors instantly. Because the tubes are cold, the capacitors will hold the charge. Bumping the power on a tube rectifier amp does not charge the caps, as long as the heater in the rectifier tube is cold. If you suspect that there’s a charge, let the amp warm up fully, then shut it off. It will self-discharge.

But it’s always a good idea to test before you touch!
"

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Post subject: Re: Is this as safe as it appears on the video?
Posted: Wed May 22, 2013 3:32 pm
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shimmilou wrote:
I'm not sure what you mean. A cold tube rectifier takes longer to produce voltage than a SS rectifier after the AC is applied, you know that. Compare how quickly the Plate voltage comes up on your BDRI and you'll see what I mean. Really, check the DC output from the cold tube rectifier before the standby switch and compare to the SS rectifier before the standby switch. Off the top of my head, maybe 5 seconds or more before a cold tube rectifier will produce voltage, compared to instantly with the SS rectifier. So, with a tube rectifier in a cold amp, you can switch the power on and then right back off and the rectifier will not have time enough to produce voltage, but the instant that you apply AC to a SS rectifier it will produce full output DC voltage immediately.

Here is the full quote from the Billm site:

"catfishjones40 says:
July 9, 2012 at 9:20 am

Hi Bill. I watched the video and saw the notice about amps with tube rectifiers. In that case would you just use the resistor method to discharge?

bill says:
July 9, 2012 at 5:28 pm

An amp with a tube rectifier will self-discharge just as quickly as one with diodes. The difference is that with a tube rectifier, there’s no high voltage until the rectifier warms up and begins to conduct.

So “bumping” the power (quick on-off) on an amp with solid-state rectifiers will charge the capacitors instantly. Because the tubes are cold, the capacitors will hold the charge. Bumping the power on a tube rectifier amp does not charge the caps, as long as the heater in the rectifier tube is cold. If you suspect that there’s a charge, let the amp warm up fully, then shut it off. It will self-discharge.

But it’s always a good idea to test before you touch!
"


Well, I don't know anyone who turns the standby on and off immediately (for what purpose?). My point is, within 5 seconds of flipping the switch, there is near full power available at the tubes. May not be instantaneous, but its pretty darn fast.

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Post subject: Re: Is this as safe as it appears on the video?
Posted: Wed May 22, 2013 4:16 pm
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Yes, no reason to do that at all. I think the point that Billm was making was about the caps being discharged in a BJr by simply switching the amp off, and he didn't want someone to think that they could switch the amp on then right back off to discharge the caps. If the BJr amp is warmed up, as in after playing, you can switch it off and the caps will discharge through the warm output tubes. But if the BJr is cold and you turn it on and right back off the caps will instantly charge because of the SS rectifier and not discharge because the output tubes are cold. That's when the difference between a tube rectifier and SS rectifier comes into play. Switching a cold tube rectified amp on and right back off will not charge the caps as it does in an amp with a SS rectifier.

I know what you mean now, "why in the world would someone do that?" (I have to ask that question almost every day :lol: ) But, as you know, people do some weird stuff and I think that Billm was trying to cover himself and be perfectly clear in case of a misunderstanding.

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Post subject: Re: Is this as safe as it appears on the video?
Posted: Wed May 22, 2013 4:23 pm
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Always good to "CYA". :lol: :lol: :lol:

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