Shock Therapy - Newbie Question

[HackRabbit]

Let me start by saying that I am and always be a newbie. (I am fascinated by amp building, but I do not see having the time to indulge. My hats off to all you that make it happen.)

The June 2010 edition of Guitar Player magazine a reader asks the question 'what is the proper way to drain an amp of stored electricity without getting shocked'.

Part of the columnists response is, "On all vintage BF and SF Fenders with 6L6 output tubes, one only needs to unplug the amp from the wall and put the standby switch in the "play" mode. In a minute or two, all the electricity will be drained".

I have a BF Bassman, but I have never heard of this method of draining an amp. I would like someone much smarter than I to confirm that this method is indeed safe.

Thank you.

[Andy Le Blanc]

some amps have bleeder resistors around the filter caps in the power supply, some don't.
That notion will work if the tubes are hot enough to conduct.
Caps can hold a residual charge called soak even after the tubes are cooled
and the unit is unplugged etc.... You can't trust that method.

I rig up a resistor and lead clips to drain or discharge without jumping (sparking) the charge to ground.

In some higher power tube units, communication and radar, there is a discharge procedure,
the soak can be just as bad as if the power supply was still on.

don't take the chance,

[HackRabbit]

Andy,
Thank you for taking the time to respond.
It is good to know that the method in question is not fool proof since I am a fool. )

[Andy Le Blanc]

No worries... I got lifted some months back, bad, and I work to implement safety.

It only takes once.

[Structo]

Yes, one has to be careful when reading articles like that one.
The other way I have seen in magazines is to have the amp on then turn standby off while strumming then while still strumming turn the power off.
This can bleed the residual charge off of the caps but it may not.

As mentioned some amps have bleeder resistors, some don't.
Some have the first large filter before the standby switch, some don't.

So you can't paint the notion with broads strokes, one method for all.

A good method that Gerald Weber mentions in his books is to take a insulated test lead and clip one end to chassis ground and the other to a plate resistor, using the plate resistor to drain the voltage to ground.
But this can be beyond the scope of a beginner so the dedicated cap discharge tool is a better alternative.

The easiest and safest way for a beginner is to use a large resistor (100K or so) in series with an insulated test lead with alligator clips at each end.
Clip one end to chassis ground and the other on the + end of the filter cap(s).
It will take a minute or so.
Then, and this is critical, always measure for voltage across the filters before assuming the caps are indeed drained.
Sort of like a gun, always assume it is loaded and verify with a meter before diving inside.

[IMG:509:466]http://i23.photobucket.com/albums/b392/ ... geTool.jpg[/img]

[Richie]

I agree, never assume. Someone may have taken out the bleeders, or modded the amp to who knows what, the only way for sure is to use a meter and measure.
The resistor /clip lead will drain the amp too. If you have ever been bit by a solid "real" good jolt.... you won't forget it..

[Cliff Schecht]

I don't like the clip method so much because you are still recommending people to get near 400V+ with their inexperienced fingers. I'm sure it works fine if you are careful but some tighter amps have HV in places that could potentially shock someone. If there is room, I've read a lightbulb works well by touching the base to the HV and the "tip" to the chassis. Never tried that approach though, I just put a bleeder resistor in every build .

Something that I would not recommend doing, especially for older caps, is to just short the terminals. This is hard on the caps and will eventually cause them to become leaky and eventually open up completely.

[Structo]

Yeah, I forgot to mention the screwdriver method, for good reason.

Never short caps out with a screwdriver.
Not only can it damage the caps, but it can throw some hot slag right into your eye.
Also put a nick in your screwdriver or chassis.
Dangerous!

[M Fowler]

Instead of the clip on both ends you can have an probe on one end to touch to the caps, the resistor inline and the clip on other end to chassis ground.

I usually just have a jumper wire clips both end and a large resistor clipped to one end that I touch to the caps. Works everytime and yes I use this device everytime even if I have bleeder resistors installed.

[skyboltone]

I don't have the patience for a 100K bleeder. I find a 2.2K 5 watt works just fine and only makes a little spark. Put the leads to your multimeter on the cap and then try the 100K bleeder. You'll be standing there half an hour for a full bleed down. Plus you have to account for spring back. Most caps will drain then spring back with about half dozen volts. Not dangerous but sparky and scary if you don't expect it.

[Structo]

Yeah, I was trying to minimise the sparks......
It really only takes a minute or two with the 100K.
No different than using a preamp plate resistor to bleed them down.
The nice thing about that method is that you can leave the test lead on the amp to prevent residual buildup.

[Cliff Schecht]

When I'm working on an amp, here's what I do. Before ever turning on the amp (assuming no initial charge on the caps, make sure to measure this!) I use a 68k or 33k resistor, 10W or above, and tack this onto the + side of the cap, leaving the negative side floating but near the chassis. When I need to drain the caps, I just use a screwdriver or (if room permits) use the resistor body itself to short the floating end to ground. No sparks, pretty safe and this can be easily removed.

Something else I like to do is always have a DMM measuring the caps when I'm working on a circuit. Since I started doing this I haven't been shocked once. Clippy leads are nice here..

After you drain the initial charge off of a cap and are just dealing with the "memory" of the electrolytic gel, it is safe to just short the two cap leads together. This hysteretic effect doesn't develop enough voltage to shock you (measure measure MEASURE) but it can damage some test equipment.

One more thing that I've heard of but never dealt with is that just using a screwdriver to short caps can be dangerous in that if the screwdriver has any cracks in the plastic, that HV charge won't have a hard time working its way to your hand. Never actually had this happen but it's something else to keep in mind. Safety first guys!

[skyboltone]

Yeah, I was trying to minimise the sparks......
It really only takes a minute or two with the 100K.
No different than using a preamp plate resistor to bleed them down.
The nice thing about that method is that you can leave the test lead on the amp to prevent residual buildup.

I did that with a Broadcast Transmitter 1 KW one time. Only I forgot to remove the bleed lead. Holy Cow man. What a firestorm when I hit the switch.

2250VAC-SS rectifier - 8uf oil filled cap- 50 pound choke of unknown henries-25uf oil filled cap = 3100VDC fireballl. That scared the hell out of me and I'm not eskeert of NOTHIN'!

When I had my airport shop with no power and was using the generator I mistakenly left the bleed lead on little amps and flipped the switch. The generator would just bog down hehhehhehhehheh