discharging caps

[wsaraceni]

building my first amp. want to make sure i am safely discharging the caps. with the amp on, the voltage at the + side of all the caps is about 270v. As soon as the amp powers off you can see the caps drain in 20 seconds to about 21V where it stays until you drain them with a resistor to ground. sound about right?

when i work on the chassis after that i've been clipping a wire from the + side of the resistor to ground

[Liquid Sunshine]

Over at Metro, Larry said this piece of advice:

"Simply clip pin 1 from V1 to ground and wait a minute, then all your caps are drained. But leave the crocodile clip there while working, because caps are 'remembering their previous voltage and ungrounded will be creeping up again to about 70...100V

And don't forget to remove the crocodile before firing up the amp again, otherwise your plate resistor will die" http://forum.metroamp.com/images/smiles/icon_wink.gif

Because I had bought an Xicon 10W 5% High Oh 25Kohms cement resisitor to drain each cap and I am going to put in a new Metro Dagnall OT C1998 and needed to drain the caps. But Larry had suggested that route instead of across each cap.

ps I don't know why the purple unhappy face got up there...lol.

[tremolux]

The purple unhappy face guy FORGOT to drain the caps on his amp before sticking his purple hands into the open chassis. That accounts for the unhappiness. He just show up now for no reason sometimes.

Just a theory....

:messedup Years ago I looked like this for an hour after doing the same thing, I was lucky!

[Richard Guy]

If your tube amp has a 'stand-by' switch, just leave it in the 'up' position for a few minutes. That will drain the charge in the caps. Remember to unplug the power cord before 'going-in' :messedup

[John Phillips]

If your tube amp has a 'stand-by' switch, just leave it in the 'up' position for a few minutes. That will drain the charge in the caps.
NOT IN ALL AMPS!!!

This is absolutely essential to realise. There is NO one correct rule for draining the caps by any combination of switches when powering off. You need to know how the power supply is configured in order to know what is a safe method.

In amps like a 100W Marshall, where the standby switch is upstream of the rectifier and there are divider resistors across at least one of the filter cap stages, that is absolutely true (up being OFF on a Marshall).

In amps like a Fender Twin, the standby switch is downstream of the rectifier AND the first filter stage, but this stage has divider resistors - so turning the power off but leaving the standby ON (which is, in this case, also up) will also drain the caps via these resistors.

BUT in other amps such as the Fender Deluxe, where the standby switch is downstream of the rectifier and the first filter cap stage (which does NOT have a divider chain), you could get a fatal shock from the - still FULLY charged - first filter cap, no matter which position you leave the standby switch in. Once the tubes have cooled, there is no path to ground at all from any part of the B+ chain.


Please, be careful before posting advice like that... no offense intended.

[Boogs]

Once again we are indebted to Mr. Phillips for his exhaustive knowledge of guitar amps - thank you! :)

[wsaraceni]

NOT IN ALL AMPS!!!

This is absolutely essential to realise. There is NO one correct rule for draining the caps by any combination of switches when powering off. You need to know how the power supply is configured in order to know what is a safe method.

In amps like a 100W Marshall, where the standby switch is upstream of the rectifier and there are divider resistors across at least one of the filter cap stages, that is absolutely true (up being OFF on a Marshall).

In amps like a Fender Twin, the standby switch is downstream of the rectifier AND the first filter stage, but this stage has divider resistors - so turning the power off but leaving the standby ON (which is, in this case, also up) will also drain the caps via these resistors.

BUT in other amps such as the Fender Deluxe, where the standby switch is downstream of the rectifier and the first filter cap stage (which does NOT have a divider chain), you could get a fatal shock from the - still FULLY charged - first filter cap, no matter which position you leave the standby switch in. Once the tubes have cooled, there is no path to ground at all from any part of the B+ chain.


Please, be careful before posting advice like that... no offense intended.

wouldnt the safest way be to then ALWAYS drain the caps with a resistor to ground (using a wire and alligator clips) and check with a multi-meter to ensure they are indeed reading 0v? then you can connect all the + side of the caps to ground while you are working.

[John Phillips]

- thank you! :)
This one was learnt by painful personal experience, I can assure you :). I would prefer to help anyone else avoid the same harshly corrective learning process if at all possible...

In my early days poking around in amps I got a very bad shock from a 50W Marshall - one of the ones with single filter cap stages and no divider resistors. I just assumed that if the amp was turned off and unplugged from the wall (and had been for some considerable time, if I remember), it was safe. Wrong.

wouldnt the safest way be to then ALWAYS drain the caps with a resistor to ground (using a wire and alligator clips) and check with a multi-meter to ensure they are indeed reading 0v? then you can connect all the + side of the caps to ground while you are working.
Yes. It's just that in many amps, those with resistor voltage dividers, it's unnecessary because the resistors are built into the amp anyway. You just may have to leave the standby switch on in order to be sure of draining all the caps.

In fact, I wonder really why all amps don't have a bleed resistor somewhere, as a matter of course - if it was something like 1M ohm, it wouldn't affect the amp in any way normally, but it would mean that the caps would always discharge once you turned off.

[BudLite]

In amps like a Fender Twin, the standby switch is downstream of the rectifier AND the first filter stage, but this stage has divider resistors - so turning the power off but leaving the standby ON (which is, in this case, also up) will also drain the caps via these resistors.

BUT in other amps such as the Fender Deluxe, where the standby switch is downstream of the rectifier and the first filter cap stage (which does NOT have a divider chain), you could get a fatal shock from the - still FULLY charged - first filter cap, no matter which position you leave the standby switch in. Once the tubes have cooled, there is no path to ground at all from any part of the B+ chain.

Hi John, When you say fender deluxe, do you mean the deluxe reverb as well? or is the DR like the twin? I've always played the guitar with power on and standby on, just unplug amp and play your guitar....then hook a alligator clip from v1 socket, pin 1 to ground......Does this sound safe to you? Thanks Dave

[John Phillips]

Hi John, When you say fender deluxe, do you mean the deluxe reverb as well? or is the DR like the twin? I've always played the guitar with power on and standby on, just unplug amp and play your guitar....then hook a alligator clip from v1 socket, pin 1 to ground......Does this sound safe to you?Yes. That's using the tubes to get rid of the bulk of the charge, then allowing the rest to trickle out (and prevent recharging) through the B+ chain resistors and the V1A plate resistor. Just remember to unclip it before you power up again!

Yes, the Deluxe Reverb (and the Hotrod Deluxe, and all previous Deluxes AFAIK, if it matters!) is one of those without the 'stacked' first filter stage, which is where the divider resistors are. These amps have low enough plate voltage that a single filter cap (up to 450V rating in the old days, usually 500V now) will handle it.

[brad347]

the "unplug and play your guitar until the sound dies away" method is inexact and not always complete. Not recommended. If you feel the need to take "short-cuts" like this with safety, you probably shouldn't be inside your amp. Sorry for the blunt-ness.

:)

NOW, alligator clip on pin 1 of the first preamp tube going to ground with standby switch in the ON position, at least on most Fender amps i've ever encountered, is an effective method of draining caps, provided you wait about 20-30 seconds. It is using one of the resistors in the preamp circuit as a bleeder resistor to drain off charge, but since the resistor is a relatively low value it takes a little time.

In any case, regardless of your method, CHECK WITH A MULTIMETER BEFORE TOUCHING ANYTHING. I always leave my alligator clip connected to V1 throughout my work too, even after discharge, to prevent the 'charge memory' that some caps seem to have.

[Richard Guy]

NOT IN ALL AMPS!!!

This is absolutely essential to realise. There is NO one correct rule for draining the caps by any combination of switches when powering off. You need to know how the power supply is configured in order to know what is a safe method.

In amps like a 100W Marshall, where the standby switch is upstream of the rectifier and there are divider resistors across at least one of the filter cap stages, that is absolutely true (up being OFF on a Marshall).

In amps like a Fender Twin, the standby switch is downstream of the rectifier AND the first filter stage, but this stage has divider resistors - so turning the power off but leaving the standby ON (which is, in this case, also up) will also drain the caps via these resistors.

BUT in other amps such as the Fender Deluxe, where the standby switch is downstream of the rectifier and the first filter cap stage (which does NOT have a divider chain), you could get a fatal shock from the - still FULLY charged - first filter cap, no matter which position you leave the standby switch in. Once the tubes have cooled, there is no path to ground at all from any part of the B+ chain.


Please, be careful before posting advice like that... no offense intended.

Thanks for the assist :AOK

[wsaraceni]

In any case, regardless of your method, CHECK WITH A MULTIMETER BEFORE TOUCHING ANYTHING. I always leave my alligator clip connected to V1 throughout my work too, even after discharge, to prevent the 'charge memory' that some caps seem to have.


i definately check with a mulitmeter before going back in. ill have to make up a nice stable alligator clip to ground it while working on it.

[brad347]

that's a good idea, though I've never personally seen a cap develop more than 70v on it from 'memory.' And even that much has been relatiely rare in my experience. Not to say that one couldn't do more, so for safety's sake...

But 70v, while enough to "feel it," probably wouldn't hurt you. Right, experts? (I'll try to never find out).

[ChickenLover]

Just my opinion (and maybe this is only true for me and those like me) but if I leave an alligator clip from B+ to ground in an amp...it's only a matter of time before I forget and leave it in and power up an amp. I never seem to forget to discharge the caps though...so I just discharge them and then put the alligator clips away (of course, if it's my amp then the bleeder resistors are already in there or are going in...like, right now). The caps will end up with a little voltage on them but it's so little that I don't worry about it. Plus there is very little current capacity which is the real killer anyway. I've been bitten by 450vdc while the amp was ON..and yes, it hurt (scared me WAY more than it hurt me though) but I would think that a short burst of <70vdc wouldn't be that bad.

Static electricity that zaps you when you touch the doorknob is on the order of what...5K volts? Just no current capacity...so it just hurts a tiny bit.

I think the main concern/hazard about these shocks are if you happen to get the current flowing across/near your heart (like high-voltage at your left hand and ground at your right hand) then it could stop the heart like a defibrillator...only there's nobody/nothing there to restart the heart. Like they say...one hand in your pocket!

[John Phillips]

My granddad just taught me to take an insulated screwdriver and short the two poles. Is this still OK?
Only if you like to damage your tools and possibly your filter caps.

No, it is not OK - although it will 'work'. But the huge current rush as the cap discharges will not only burn spots out of the screwdriver where you can see them, it can also burn the foil inside the filter caps where you can't, but which may cause the cap to fail.

[brad347]

Just my opinion (and maybe this is only true for me and those like me) but if I leave an alligator clip from B+ to ground in an amp...it's only a matter of time before I forget and leave it in and power up an amp.

Good point!

[JoshuaTSP]

I'm assuming the alligator clips need to be heavily insulated when using the V1 #1 pintube socket + ground method?:BOUNCE