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Old 10th August 2014, 06:11 PM   #1
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Default Draining a can cap

I have an old Ace Tone Elite that I need to upgrade with a 3 prong cable. I want to safely drain the filter cap. Here's a pic (bottom of can cap in the lower right). I'm assuming the red terminals are + and the black is ground (While I've dabbled in electronics for years, I've never dealt with a can cap before).

I'm assuming the resistor between the two red terminals isn't a bleeder. Or is it? I'd like to draw a schematic for it, but the first step is to safely get the cap drained. Can I just put a 10K resistor from each red terminal to the black to drain it over a few mins (the cap is a 2x 40MFD @350V)? Here's the top side.
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Old 10th August 2014, 06:26 PM   #2
Mooly is offline Mooly  United Kingdom
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Yes, a 10k is fine but make sure its a couple of watts or so rating. And you could just measure the voltage on the cap... there may be nothing there.

The resistor between the two reds isn't a bleeder, more likely part of a filter arrangement such as a C-R-C type.
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Old 10th August 2014, 06:33 PM   #3
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No that isn't a bleed resistor, it's a dropping resistor.

Unplug the amp while still playing guitar through it, usually the filaments stay warm long enough to work for a few seconds and energy gets sucked from the caps as the transformer is no longer charging them. After you do this check the caps with a volt meter to make sure they are drained down, after doing this i usually find that there is still like 5-10 volts left so I just short the caps to ground with a screw driver.

If this doesn't work your method of a 10k resistor to ground will drain the cap over a period of a few minutes.....double check with a meter and usually like I said before once I see the caps under 10 volts I just use a screwdriver.
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Old 10th August 2014, 07:58 PM   #4
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Thank you guys. I had a feeling it was draining because after you unplug it, you can still hear it for a bit. Of course my multi-meter goes to 1000V, but I've never gone up higher than 110/120 with it and I'm just a bit nervous around vacuum tube voltage...

I believe the caps in this thing are stock (I had never heard of Rubycon before, but apparently they're still in business). I can't find a date anywhere, but I believe the amp is early 60s. I'm assuming it would be a good idea to replace them? There's the 2x40MFD @350V can and then there are two 10MFD @300V radial caps (and a bunch of smaller ones). Would I just replace the can, the can and the 2 big radials, or should I replace all of them?

I'm going to bust it apart this evening, drain the cap and start drawing out the schematic. It's got some unusual tubes (a 6GW8 and a 6AV6, along with a standard 12AX7) and I'm curious how the circuit is laid out. Also the reverb and tremolo aren't working, so that'll be my next thing after getting the power cord replaced.

I have a replacement 6GW8 and 6AV6 on the way. I already replaced the 12AX7 (I was kind of hoping it was driving the reverb and tremolo and that replacing it would fix them, but that was a bust).
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Old 11th August 2014, 01:47 AM   #5
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The other big safety rule, is work around voltage over 25, with one hand only. After you've measured every metal under 25 v (2 is better) and the thing is unplugged and batteries out, then you can use two hands to remove and install stuff.
Another rule, is no jewelry on hands wrists or neck. Even under 25 v, energy in caps can burn your finger to charcoal through a ring.
Rule 1 saved my *** last night. Forgot to discharge, had no clip leads, shocked myself through one hand. Whoops, a little pain. If current crosses your heart, you might die.
Here is a thread about using $2 radial lead caps, rated up to 10000 hours service life, to replace can caps that cost up to $45 and have no life rating. Post # 25.
I've replaced can caps 4 times in my ST70 amp, I'm a bit sick of that, I hope the 3000 to 10000 hour caps last long Cant get the underline to go off.
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Last edited by indianajo; 11th August 2014 at 01:52 AM.
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Old 11th August 2014, 08:17 AM   #6
godfrey is offline godfrey  South Africa
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Quote:
Originally Posted by indianajo View Post
...with one hand only.
Ideally with your other hand firmly wedged into a pocket to make sure it stays out of trouble. You'd be surprised how often it jerks as it tries to go over and help. They really seem to have minds of their own sometimes!
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Old 15th August 2014, 09:10 PM   #7
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Listen, here's THE way to drain filter caps (keeping in mind to be careful cause they can still hold a charge after the unit has been off - unplugged).

1. turn the amp on while it is plugged in.
2. pull the power cable from the socket while the amp is still on.
3. this aids in draining the caps
4. purchase a Square D "Wiggy."
5. touch both probes of the wiggy to both pins of the cap(s) one a at a time
6. if there is a charge, you will fill the "wiggy" vibrate and it may give you a quick voltage
reading. do this until each cap has been discharged.
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