[sdiy] HV cap discharging re-revisited

[Thomas Hudson]

I have a 470k 25w resistor, one end soldered to some wire terminated  
in an alligator clip. The resistor is taped to the end of a  
chopstick. I can connect the alligator clip to ground and then use  
the chopstick to discharge the cap. The resistor I use is one of  
those in a finned aluminum housing with rigid leads.

But I only have to do this once, since I always install a bleeder  
resistor (470k 3W) in the amp the entire duration of working on it.  
If it is one of my amps, the bleeder resistor stays in permanently.  
Obviously if I am working on someone else's vintage amp, it is  
clipped in and removed after service.

I also have a cheap VOM clipped to the caps the entire time I am  
working on the amp and only work on the amp when the needle is at zero.

May seem overly cautious, but after having been zapped by an old  
Marshall with about 560 on the plates, and sitting there wondering  
how long it would take to die from fibrillation, I decided I would  
rigidly follow the following safety procedure:

- Turn the amp off and disconnect the AC cable, never trust a power  
switch that can short, or a lamp that can burn out.
- Wait for the VOM to register zero.
- Use my DMM to check each stage of B+, since a bad connection  
between filter stages might not drain all the caps.
- Do my changes.
- Turn on the amp and try it out.
- Rinse and repeat.

This is probably more than necessary, but the guys that do less than  
necessary are either lucky or no longer around to talk about it. In  
theory, we are talking about high voltage but low current, so *maybe*  
50 volts wouldn't kill you.

Of course, in theory, brakes only exist to correct driver errors.

Tomy

On Nov 11, 2007, at 4:56 PM, anthony wrote:

>
> I am in the process of designing a new amp in the near-gutted  
> chassis of a Wards Airline radio that uses a 6V6-GT output tube and  
> a 5Y3-GT rectifier. I'm sure I've mentioned it a few times...
>
> Today I had everything disconnected from the power supply except  
> the first filter cap. to see what recified voltage coming off the  
> cathode was (about 330VDC after warm-up). I had planned to  
> discharge the cap using an ordinary lightbulb - I even prepared a  
> special socket with big alligator clips (too big really). So when I  
> used it, nothing happened. And I should have known nothing would  
> happen because I let it sit for 2 hours, turned off connected to my  
> DMM, which showed 0V right before I disconnected it. So, duh! If  
> I'm not in a big hurry, can't I just let my DMM discharge the cap  
> and I can watch it go to 0-ish. What IS a safe DC voltage to let a  
> cap get to? I mean you can melt metal with 9 volts and a 50,000 uF  
> cap. My guess, for the circuits I'm working with, is below 50 VDC.  
> But you can still really feel 50 volts - nothing like getting bit  
> by 400 volts (which I have - fortunately I was not grounded and it  
> was an ever so slight brush to a charged cap in a circuit that was  
> turned off - so it may not have been the full voltage...) and  
> REALLY nothing like getting bit by a mini-bike magneto (that  
> fucking stings!).
>
> What DID we decide was the best way to discharge caps - if one's in  
> a bit of a rush, that is. I remember someone offered their ex-wife?
>
>
> cheers,
> aa
>
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