Discharging caps?

[OnlyVees]

I've read at least 3 "safe" methods for discharging caps-

Screwdriver to + side, ground on chassis
Wire with resistor
Play guitar while powering down amp

What is the TGP approved method for safely discharging caps so I can tweedify my SF Champ? Specifically, this Champ has a cap can hard-wired to the board...

Thoughts? :dunno

[JDJ]

Wire with resistor... check with volt meter.

[mbratch]

Screwdriver to + side, ground on chassisThat would not be a good way to do it. You could likely blow a cap that way. And I wouldn't regard it as "safe".
Wire with resistorThis is a reasonable way to do it. Hold it there for a few seconds. As JDJ says, check with a volt meter afterwards.
Play guitar while powering down ampI've never used this method, so I can't tell you how effective it is.

Here's the method I use for vintage Fender amps that works well. Basically, it's the resistor method above but it uses a resistor in the circuit. With the amp unplugged (of course), clip a wire from pin 1 of V1 to ground. Then switch on the standby switch, leaving the on/off switch off. This will drain the caps. Again, check with a volt meter before going in. :) I haven't checked whether this works on a Champ which has no standby (i.e., maybe it works ignoring the standby switch step).

[Voodoo Blues]

Use a resistor.

[croto5000]

Here's the method I use for vintage Fender amps that works well. Basically, it's the resistor method above but it uses a resistor in the circuit. With the amp unplugged (of course), clip a wire from pin 1 of V1 to ground. Then switch on the standby switch, leaving the on/off switch off. This will drain the caps. Again, check with a volt meter before going in. :) I haven't checked whether this works on a Champ which has no standby (i.e., maybe it works ignoring the standby switch step).

+1 usually takes less than a minute

[jay42]

Most amps bleed down all by themselves. If I'm going into an amp, I will put a meter across B+ in a couple of places before touching anything.

With my amps, after I turn everything off, I unplug the AC, then turn everything back while packing up cables and whatnot. That takes care of it.

[TweeDLX]

That would not be a good way to do it. You could likely blow a cap that way. And I wouldn't regard it as "safe".
This is a reasonable way to do it. Hold it there for a few seconds. As JDJ says, check with a volt meter afterwards.
I've never used this method, so I can't tell you how effective it is.

Here's the method I use for vintage Fender amps that works well. Basically, it's the resistor method above but it uses a resistor in the circuit. With the amp unplugged (of course), clip a wire from pin 1 of V1 to ground. Then switch on the standby switch, leaving the on/off switch off. This will drain the caps. Again, check with a volt meter before going in. :) I haven't checked whether this works on a Champ which has no standby (i.e., maybe it works ignoring the standby switch step).
Pin 1, V1 to ground is my prefferred method as well. Still breathing, and the amps all work!

Mike

[aleclee]

A few years ago I bought a thingy from Lord Valve that was basically a probe, resistor, and alligator clip. It wasn't expensive.

[Jack DeVille]

Wire with resistor... check with volt meter.

this is my preferred method. controlled discharge of cap with minimal stress on the components and tools. ALWAYS check for residual current before sticking your hands up in the mix. getting bit is weak and unnecessary. plain and simple.

i have a few 5w resistors with alligator clips on each end for this purpose. those tools are my friends.

[StompBoxBlues]

Most amps bleed down all by themselves. If I'm going into an amp, I will put a meter across B+ in a couple of places before touching anything.

With my amps, after I turn everything off, I unplug the AC, then turn everything back while packing up cables and whatnot. That takes care of it.

I don't know how often or realistic it is but I have read in several different sources that SOME caps, in some circuits can actually build up dangerous voltages even with the amp unplugged. Might depend on other factors too, but have read advice that once discharged it is best if you have then an alligator clip from + to ground on there the whole time. Others point out "yeah, but if you FORGET to take it off again?" but...

I'm sure your way has worked for you, and probably with never a problem but this is just something I have read that CAN happen if one is unlucky.

I made an outlet (described in Gerald Webers book) that is in series with a light bulb (current limiter) that would blow the light bulb in case there was any short in the amp (clip or just accidental) on power up.

I also built a little screwdriver tip, connected to wire to resistor to alligator clip for discharging caps.

I'm no pro amp reapair/modder...have done some, but nowhere in the league of many folks here, so take my advice with a grain of salt, but I do know electronics, and researched and this is the way I chose to go.

Also, it is hard to make a habit, but I try to make sure I keep one hand in pocket or behind my back when working on amps...

[mbratch]

Most amps bleed down all by themselves.
My vintage Fenders will hold a strong charge for weeks not being plugged in or powered on.

If I'm going into an amp, I will put a meter across B+ in a couple of places before touching anything.
Always the best advice. :)

[donnyjaguar]

I don't know how often or realistic it is but I have read in several different sources that SOME caps, in some circuits can actually build up dangerous voltages even with the amp unplugged.

I don't know about dangerous voltage, but on a stereo preamp I built for a customer you can short the caps to ground to discharge and they'll creep back up to about 30Vdc. Dielectric absorption is the reason here. In my builds I just strap a resistor across the main filter cap to discharge it when off. 330kΩ ought to do the trick.

[jay42]

My vintage Fenders will hold a strong charge for weeks not being plugged in or powered on.For most of the BF amps, flip the standby switch so that the totem pole 220k's are connected to the rest of the amp. Keep the speakers attached. I mostly build my own amps and regardless of the original circuit, e.g. 5e8-a, they always get a bleed resistor across B+.

[kilaf15]

I followed the great advice in this forum and wired together a pair of bleeder cables using two different resistor values and double checked the results with a meter. One resistor bled the charge quicker which some say might be hard on the cap. Aside from that, they both did the job and I'll probably lean toward the smaller cap; the time to drain is negligible.

As mentioned above after discharging them, most of the caps will re-gain a bit of voltage but no where near their original value. Just touching it again with the probe will bleed that off as well.

[Structo]

You can't depend on a amp to bleed down on it's own.
There are plenty of amps that don't use these balancing or bleeder resistors.

Treat an amp like a firearm, always assume it is loaded.

The plate resistor is a good method if you are familiar enough with an amps operation.

If not it's best to make a discharge lead like this.

http://i23.photobucket.com/albums/b392/Structo/CapDischargeTool.jpg

Be sure to use insulated alligator clips or you will get shocked.

I usually lead a bleeder lead attached while working on the amp.
Just don't forget to take it off when you power up the amp!!!!!

Be sure to read up on safety procedures while working around high voltages.
When measuring voltages or discharging caps, always use just one hand when poking around in the amp.
If you get shocked with both hands the current crosses your chest and heart, sometimes stopping it!

[andrekp]

Use the resistor, but ALWAYS measure with a volt meter before you put your mitts in there.

Don't use the pin 1 trick, unless you know that pin 1 is actually the plate of the tube. It is the plate of the preamp for a huge number of amps, but not all. (What you are doing is using the plate load resistor as the bleed resistor).

And if you put in a jumper to keep it from creeping back up, make sure it is something big and obvious, like with a big pink ribbon on it or something, so that you can't help but see it when you put it all back together. Leave it in and you could have problems...

[rockon1]

I've read at least 3 "safe" methods for discharging caps-

Screwdriver to + side, ground on chassis
Wire with resistor
Play guitar while powering down amp

What is the TGP approved method for safely discharging caps so I can tweedify my SF Champ? Specifically, this Champ has a cap can hard-wired to the board...

houghts? :dunno

Every amp I own (10 of them)will discharge the caps simply by powering down without using the standby. That doesnt mean all amps will though. I ALWAYS check the caps for voltagewith a multi meter before poking around inside anyways Bob

[crazyForce]

Every amp I own (10 of them)will discharge the caps simply by powering down without using the standby. That doesnt mean all amps will though. I ALWAYS check the caps for voltagewith a multi meter before poking around inside anyways Bob

Meaning you unplug it from the wall as you play the powered amp?

When I performed some mods to my Dual Showman Reverb, I used the screwdriver trick, as I was told to do. Didn't like that at all-I'm terrified of electricity!

This as all great info. Thanks!

[rockon1]

Meaning you unplug it from the wall as you play the powered amp?

When I performed some mods to my Dual Showman Reverb, I used the screwdriver trick, as I was told to do. Didn't like that at all-I'm terrified of electricity!

This as all great info. Thanks!

Actually I usually just hit the switch then I unplug it never using the standby. The most important thing I stated though is that I check with a meter before I touch anything. Not every amp will discharge in this manner. FWIW-I got zapped once by my Classic 50/50 and it was quite a jolt! Bob

[StompBoxBlues]

Actually I usually just hit the switch then I unplug it never using the standby. The most important thing I stated though is that I check with a meter before I touch anything. Not every amp will discharge in this manner. FWIW-I got zapped once by my Classic 50/50 and it was quite a jolt! Bob

and that caps CAN recharge even when unplugged.

[rockon1]

and that caps CAN recharge even when unplugged.

So Ive heard. I havent noticed the phenomenom but its a good reason to ground the caps while your in there I imagine. Bob