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FDP Forum / Amp Mods, Repairs, and Projects / Silverface cap can: how safe?

Hilltop201

NM, USA

Jul 22nd, 2019 04:36 PM   Edit   Profile  

Sorry for the silly question, but is it dangerous to touch the can that the filter caps come in? I mean with the chassis closed, just the part that’s over where the tubes are. I always make sure not to touch it when I’m changing tubes, but am I being overly cautious?

Thanks

SonicBlue

Sunbury-on-Thames

Jul 23rd, 2019 03:27 AM   Edit   Profile  

When it comes to the kind of voltages in valve amps, 'over cautious' is good, it keeps you alive.

In theory the cans are grounded to the chassis and are at zero volts, therefore are safe to touch. In practice, an internal fault where the cap blows and the 'hot' plate touches the can (which has happened), combined with corroded terminals or connections where it's supposed to be grounded could result in the can being at some voltage between zero and very unzero.

Your caution will serve you well, Grasshopper.

Hilltop201

NM, USA

Jul 23rd, 2019 10:01 AM   Edit   Profile  

Thanks for the info. Is it safe to check for voltage on the can using a standard digital multimeter? I own one but have only used it to test continuity with guitar electronics stuff.

Peegoo
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If irritation occurs

discontinue use.
Jul 23rd, 2019 10:18 AM   Edit   Profile  

"When it comes to the kind of voltages in valve amps, 'over cautious' is good, it keeps you alive."

+1,000


This can be a problem, especially on older amps with a two-conductor (hot and neutral) AC mains cable, because it's possible that the chassis is live at mains voltage. Since yours is a silverface, chances are it has a three-wire (hot, neutral, ground) mains cable.

How to test with a meter: set your meter to read AC; if it's not an auto-ranging meter, set the dial to the 200vAC position. Plug in the amp and switch it on. Hold one probe against the amp chassis where there's bare metal--not painted--and insert the other probe into an AC socket's ground contact. You should get somewhere between zero volts and a volt or two.

Ideally it's zero volts, but sometimes there's a little stray voltage. Also, meters are sometimes either very sensitive or out of calibration and they'll register trivial amounts of voltage.

Keep your fingers away from the shiny metal pokey bits on the probes :o)

Edited to add:


If your meter has a continuity beep setting, flip the dial to that. Have the amp plugged in and switch it off. Put one probe in the ground contact of an outlet and the other probe on a bare metal contact on the chassis. A proper ground connection on the chassis will make the meter beep. If you're not getting a beep, there's a problem with the ground.

Look

(This message was last edited by Peegoo at 12:23 PM, Jul 23rd, 2019)

willie

Too Near Atlanta GA

Amp Tech Emeritus
Jul 23rd, 2019 11:15 AM   Edit   Profile  

Just as aside and FWIW, on some older (classic) Ampeg amps, the cap can shell is hot as they are wired in series They are supposed to have a cardboard sheath around them, but I've seen some that were missing this protective cover.

Many years ago, I discovered this bit of knowledge about Ampeg quite by accident and have never forgotten it. Over the many years, I've learned many things about electronics from books, in classrooms and various courses, and on the job training...but sometimes, individual experience will stand out indelibly in your memory. :) Very shocking experience...

w

Peegoo
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If irritation occurs

discontinue use.
Jul 23rd, 2019 11:19 AM   Edit   Profile  

"I discovered this bit of knowledge about Ampeg quite by accident and have never forgotten it."

If you live to tell the tale, you'll only do it once! Glad you're still around.

willie

Too Near Atlanta GA

Amp Tech Emeritus
Jul 23rd, 2019 04:59 PM   Edit   Profile  

So am I, and thanks...much like any landing you walk away from being a good one. :) True enough, my early education in radio, and electronics in general was much by trial and error...if it smarts, it sticks with you. :)

w

Hilltop201

NM, USA

Jul 23rd, 2019 09:43 PM   Edit   Profile  

You guys are brave. I mean, I’ve gotten some serious sandpaper burns and string pokes in my day...

Indeed it’s a 3-prong on the SF.


ECS-3
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USA / Virginia

Jul 24th, 2019 07:24 AM   Edit   Profile  

"Just as aside and FWIW, on some older (classic) Ampeg amps, the cap can shell is hot"

And some of the old Hammond organ preamps.

SonicBlue

Sunbury-on-Thames

Jul 24th, 2019 09:57 AM   Edit   Profile  

Long time ago when I was getting my PPL I came across a slightly different version of willie's maxim.

Good landing - one you walk away from.
Great landing- you can reuse the plane.

willie

Too Near Atlanta GA

Amp Tech Emeritus
Jul 24th, 2019 10:01 AM   Edit   Profile  

LOL...I like that. ;)

w

Peegoo
Contributing Member
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If irritation occurs

discontinue use.
Jul 24th, 2019 10:19 AM   Edit   Profile  

SonicBlue, you've probably heard this one too (regarding pilotage):

You start with two bags: an empty one for experience, and one full of luck. The goal is to fill the experience bag before the you use up all your luck.

pdf64

UK

Jul 28th, 2019 04:45 AM   Edit   Profile  

Marshalls, Sound Citys and other UK etc amps with stacked HT can caps have the same issue. Beware, especially if you see 4 cans sticking out of the chassis, 2 of them will have their metal case (hopefully with an insulating plastic film) at 1/2 the HT voltage! Even with the film though, often the metal of the can is exposed at the top.
Obviously, the cans have to be taped over around the mounting clip, to stop them shorting to chassis 0V.

Hence it's very unsafe to operate such an amp with the back panel removed (assuming it's a head).
With the combos where the cans might be touched by the user if they reach around, it's tricky to make them properly safe.

willie

Too Near Atlanta GA

Amp Tech Emeritus
Aug 6th, 2019 04:25 PM   Edit   Profile  

I should have mentioned those as well...good catch there pdf64. You do an excellent job helping folks with tech issues...well done.

....and with that, after a very long time on the FDP, I will say adios amigos and vaya con dios. You're in good hands here. ;-)

willie

Roly
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Whitehorse Canada

I don't get out much
Aug 6th, 2019 04:42 PM   Edit   Profile  

Yikes Willie

Do you mean you will no longer check in and offer advice?

Leftee
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VA

Aug 6th, 2019 04:46 PM   Edit   Profile  

Willie?

tiller2
Contributing Member
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Washington DC

Aug 6th, 2019 06:43 PM   Edit   Profile  

Thank you, Willie, for your companionship and generous and valuable advice! Go well.

pdf64

UK

Aug 12th, 2019 08:27 AM   Edit   Profile  

Yes, that's the end of an era, I hope he's in good health.
Best wishes and farewell!

FDP Forum / Amp Mods, Repairs, and Projects / Silverface cap can: how safe?




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