How to test / discharge capacitors when positive not accessible

Wally, when I saw that BillM video years ago, it seemed contrary to all the dire warnings we see everywhere.

It's a longish vid, but you can scrub to the point where he uses a snubber (about 1:30) and compare to the part where he just switches off the hot amp (about 3:00).

But then rewind back to about 0:50 where he just switches on the amp briefly and lets it drain without hot tubes.

Bill's point was we wouldn't *necessarily* have to drain 'em if we always started with a hot amp, tubes in place, etc. IMHO, we worry about draining 'em due to the three factors in my post above -- all a risk to amp builders and testers. Other scenarios (filters connected, tubes or heaters not working right) must also exist...

I believed Bill's idea, and I tested his idea, but I still drain caps with a snubber, and nowadays put bleeder resistors in amps I build, and even then drain 'em anyway.

 

Since 40 years of tube amp repair and building, I almost never needed to discharge any caps - except for Ampeg SVT, Ashdown BTA-400 and the like.

I just wait, then check with a DMM. Usually in 5-10mn non-dangerous values are reached (less than 10VDC).

But it's me, OK ?

 

Okay, I’ll accept that there is no reason to strum a chord to achieve the discharge. It is merely Sonic proof of what is happening. do caps recharge...yes... to what extent? I don’t know other than to say that a Blues Jr. III on my bench was at zero volts when I left yesterday. Out of curiosity and due to this thread, I checked the voltage before I started any work on it this morning....just over 20 volts present. Why? I don’t know and I don’t really care. I know 8t happens because I have seen it happen. I have seen much higher ‘recovery’ than this 20 volts, too.
long and short of it...drain caps, maintain the drain, be safe.

 

Okay....if you trust that they drain on their own, here is a story to remember. I was working on an old Marshall 100 watt head. It had been off for quite a while, and I forgot to check the voltage and set up a drain maintenance connection. I went to retention tube sockets.....with a small conducive tool. On a power tube, I continued my mindless recklessness. While retensioning pin...the plate....the tool hit the frame of the power transformer. My whole right upper arm ached all day from that kick! Yes, I now use a n9n-conductive tool when retensioning, and I renewed my vow to establish and maintain a drain.

 

One other useful bit in Bill's video is about 2:15 where he shows how the amp, when turned on, charges the filter caps in about a second, while it takes about 15 sec for the tubes to get hot and start to conduct -- and the amp to make sound. It is this interval (which will vary some with different tubes, etc.) that allows the caps to charge and retain charge when the amp is turned on and then turned off before the tubes warm up.

 

Drain resistors have my vote, but that does noting with the present predicament....See those resistors across those 40uf caps right after the fuse. That's what you need....

 

We used to build lasers for the construction industry, when they were glass tubes with helium-neon gas inside. They required a power supply that supplied about 2500VDC @ 2-3 ma. Knock you right off your chair. Those caps didn't drain at all, you'd darn well better ground the laser tube anode before proceeding.

 

Did somebody say 'standby?' Stand back.

Cool. If I'm honest, I wasn't sure of his point for 5 minutes, but then I was pleased to see another lonely voice using empirical evidence about standby instead of myth and misinformation. He shows how 'current inrush' actually isn't better with standby and may be worse. For those who still stand by their standby, the best summary I know is by Merlin, who shows exactly why it isn't needed, how it may be harmful, and how the reasoning behind it was always faulty. For those who need the mute function, that can be easily achieved without standby...

 

I think the cap drain when turning off the amp has something to do with tube rectification.
I could always be wrong.

 

I like your line of thinking.

 

Ah. I know SS may have different a current-inrush behavior, but hadn't thought about whether it changes hot-tube conduction draining the caps.

Hmm, who owns a DMM and at least one amp with each type of rectifier? This is a 1-minute test, at least if you don't have to drop the chassis...

 

If the amp works, when it’s powered down, the heat left in the cathodes will last longer than the charge in the filter capacitor. So they will all have a tendency to empty in short order.

As example, I increased the size of the filter on a hi-fi amp, by a factor of 20 or so. Yes it’s silly, but seemed fun to add an 11th order low pass to get rid of mains hum. Worked great and the tx somehow survives turn on. Anyhow, with such a large capacitance, it takes about 10 seconds for the music to fade when powered down.

but don’t totally trust it. If you can’t prove they’re empty, make sure you stay safe. And when modding or fixing it, find and mark test points for the voltage and perhaps add a bleed cap too.

 

Or fill it with mercury...

 

Exactly, and that is always my point - following a procedure known to work most of the time should not be confused with being able to verify things are safe.

 

Interesting discussion.
I'm not putting my hands in a chassis unless I manually drain them and check dc voltage first.

 

If it all goes wrong and you do get a belt. Don’t count on it being fatal first go. It might be, but there’s a good chance it’ll really hurt and scare the sh1t out of you. Not been bitten by an amp, but can attest that photo flash caps pack a punch. They’re similar in voltage and size. Feels a bit like a bee sting, a wasp sting, a burn and hitting your finger with a hammer all in one. At the same time as causing your arm to spasm and wrap your knuckles against whatever inconvenient sharp thing is near them. And when that’s all over, you find the other hand which was holding something earthed hurts lots too. Finally after figuring out the thing between your arms is a nice wet and conductive heart, it’s time to give up and have a cup of tea.

At least that’s the ending I would have liked. Unfortunately if you do this in the presence of anyone, they’ll shout at you for being stupid. No sympathy...

 

Dang, and here I was counting on it being fatal the first time! So now we know why you call yourself LightningPhil...

As I said earlier, I use bleeder resistors and still discharge or check my caps or both.

OTOH, it's easy enough to leave a full 600-700V stuck in the filter caps while building and testing an amp, and as you say, the heart sits right between your two hands, and also between either of your hands and both feet...

Oh, and try to make sure the shock knocks you off the amp before you die. It isn't rare for the muscles to go into 60-cycle tetanic contracture, and it'd be a shame if you were still live (though not alive) when your spouse rushed up to hug your smoking corpse...

 

The route to ending up deafening people with a musical tesla coil on stage did indeed involve getting a few zaps.

For school stuff involving a Van de Graaff, found out the hard way that thin insulation between your feet and a metal stage is bad. Capicitance goes up proportionally to area and inversely with distance. A couple of sheets of 0.2mm thick Kapton can happily handle the 200kv, but the small distance between feet and floor ends up with about 1nF. This is much, much larger than the 20pF of the metal dome when a person is not connected to it. Thus much more able to give a belt.

To shorten the story - fiancée discharged the generator with me attached and found my suffering funny. Later calculated about 1800A went through forearm for a short while. And that would explain why it hurt.