Safety switch to drain filter caps?

[armstrom]

I'm curious... has anyone experimented with adding a "safety switch to an amp design? With all of the warnings about killing yourself with the +B voltage and how long the filter caps can stay charged I'm surprised I've never seen this. There are a bunch of different ways to implement something like this.

The most simple would be to just add a momentary switch that shorts one of your preamp plates to ground when pressed.

You could get more fancy by changing the main power switch to a DPDT and using the second pole to switch a similar circuit as described above. So when ever the amp is turned off the plate is shorted to ground.

How about a normally-off micro switch pressing up against the lid of the chassis? When you remove the lid the switch shorts the plate to ground... This carries some risk though as you would have to be sure to put a cap over the switch before powering up the amp with the chassis open...

I will probably integrate one of these options into the tube preamp for my hybrid amp. Any thoughts? I know this seems a bit much... But if safety is paramount then why even risk reaching into the chassis to attach a jumper manually? I like the DPDT option best because you don't have to remember to do anything. When the amp is turned off it's "safe" to work on the circuit. Nice and simple and good for beginners (like me!) who are paranoid that they may forget some safety tips sometimes!

[defaced]

I've seen and used resistors parallel to the filter caps to drain them once the amp is turned off. I use 220K, 2W resistors.

[billybillybilly]

... You could get more fancy by changing the main power switch to a DPDT and using the second pole to switch a similar circuit as described above. So when ever the amp is turned off the plate is shorted to ground...

in that instance , would a bleeder resistor be before the switch to prevent arcing in the switch ?

[tubeswell]

What defaced said. Just leave the bleeder resistors permanently connected - you don't need a switch. When the amp is powered up, the filter caps will charge up regardless, the relatively tiny amount that leaks away through the bleeders when the filter caps are in full charge mode does not detract from the effective functioning of the filter caps.

[loudthud]

I think it was Andy Fuchs used a SPDT switch for a standby that connected a 1K 5W resistor across the B+ when the switch was off.

[Enzo]

I think it is overkill. All amps should have bleeders though. The amps all have a warning that there is nothing inside user servicable - basically it says keep out unless you know what you are doing.

I am all for safety, but make things too overprotective of themselves and you lose respect for the system. That ruber band around the safety switch that will inevitably be there for example. And remember when they decided for safety they would make car doors lock automatically while the car was moving? Talk to the angry car owners who found their car at the exit end of the car wash locked out of their cars. That feature didn;t last long.

[jrfrond]

I'm with Enzo on this. Who is the safety switch for? Experienced techs know how to discharge caps, and, theoretically anyway, anyone who doesn't shouldn't be pokin' around inside.

I've designed everything I build with bleeders, but the majority of amps do not have them. If you just pull the plug while it is operational, you will discharge the caps. No reason to add a "better mousetrap".

[armstrom]

Ok, I get it I'll incorporate drain resistors into my power supply...

[billybillybilly]

weber ?

some of the weber kits (if your looking for examples) have the bleeders built in , 220k 2w (in most cases) run parallel to the filter caps as defaced said .

[Enzo]

Wherever they use two caps in series for higher voltage ratings - like the common 100uf/350v stacked caps - they almost have to use bleeder resistors, except then they are really equalizing resistors that happen to also act as bleeders.

But good practice is putting them in all power supplies.

[NorCalTuna]

eq resistors as bleeders only active when stby is closed?

Wherever they use two caps in series for higher voltage ratings - like the common 100uf/350v stacked caps - they almost have to use bleeder resistors, except then they are really equalizing resistors that happen to also act as bleeders.

But good practice is putting them in all power supplies.

Hey great thread, especially about trying to build too smart of a mousetrap... but won't those parallel resistors only work as real safety bleeders if the stby is either on when the amp is powered down or toggled after power down? The rest of the power supply caps are downstream.. Can anyone speak to any ill effects of these powerdown scenarios? I've always used them but seem to remember measuring lethal differences past the stby... I discharge anyway with a resistor and lead.

thanks!

[defaced]

They are down stream, but they still have a path to ground if the screen filter stage has balancing/bleeder resistors. Assuming the preamp filter caps don't have bleeder resistors, even the first preamp filter cap can drain by going through each of the B+ dropping resistors, then the balancing/bleeding resistors of the screen filter caps. It'll take a while, but there is a path to ground. So it doesn't depend on the state of the standby or power switch because this path is hardwired into the power supply.

[Enzo]

The absolute ONLY difference in power down switch sequence is that in SOME amps the cap discharge time is affected. In some it makes no difference. For example many Marshal amps have the standby switch between the PT winding and the rectifiers. Sitting there, that switch will nave no effect on discharge.

On other amps the standby switch breaks the B+ string between plate amd screen nodes. The plate node would not be discharged if the switch were open, however with bleeders it matters not.

Bleeder resistors would be by definition in parallel with their cap. Switch position would then not enter into it.

[drewl]

There's a SS Ampeg (I think) that has a switch to disrupt the ac input when the lid or back plate is removed, just a PIA when trying to work on them.

[cminor9]

I am considering putting bleeder resistors in my amps since I did not when I built them.

So, does anyone think it's necessary to run a resistor parallel to EACH filter cap? Or will just one do (since they are all wired in parallel anyway)?

Also, is 220K 2W pretty standard? Any *other* recommendations?