Newbie question about discharging capacitors and testing voltage

[BadAssBluesDude]

I'm brand new to this. I'm studying everything I can before I buy and amp kit. I know the dangers and I always want to be extra, extra, extra careful. I've studied about how to discharge a capacitor, but once I do, I want to check to make sure it's discharged. Can I just hook up a voltmeter to both ends of the capacitor to check that it is properly discharged?

Also, if I'm changing a capacitor do I need to only discharge it, or should I discharge everything around it?

Thanks,
TC

[tubesornothing]

Yes you can use a voltmeter to check the voltage of a capacitor,  make sure you set you voltmeter to the largest scale first.

Best to always unplug and discharge all capacitors - that way if you accidentally slip or touch something with a screwdriver you wont get zapped.

[Rosser]

I like to do what many people recommend: attach a jumper from pin 1 of V1 to ground (make sure the amp is unplugged, and make sure to take the jumper off before you turn the amp on again!). Pin 1 is the plate, which goes to the B+ or power supply -- it goes directly to the power supply caps, so once you jumper it, and turn on the "on" and standby switches, the caps will drain in about 30 seconds. It's better than the screwdriver-to-chassis method, though not as exciting. But it does keep the caps from recharging on their own, which they can do. Again, this is something you do with the amp unplugged.

If you jumper it, you can watch the voltage go down with your meter. If you amp has a standby switch, make sure to turn that on too -- it isolates a couple of big caps -- forgetting to flip the standby was a memorable lesson, which I won't forget (ouch).

[rafe]

Is pin one the plate on All (V1)pre-amp tubes? You had better check the particular tube data sheet ....

I do believe some of the older or oddball amps are different.
It's probably a good idea to put a resistor on your jumper or it's initially pretty much the same as a screwdriver.
It's always better to err on the side of caution

[Dynaflow]

 Gerald Weber Demonstrates:

Regards,

Dyna

Ps: He is clowning around, I believe he touches the input jack in the video....

[BadAssBluesDude]

I'm feeling particularly dumb at this point, but I'm really wanting to be safe. Just out of curiosity I wanted to see what voltage my capacitors had in them. I check with my meter, nothing on any capacitor. The amp had been sitting for a while so I turned it on, played a little bit, and even after immediately turning it off, nothing. I checked my voltmeter into an AC plug and a 9v battery and get a perfect reading from both. What am I doing wrong that I'm not getting any voltage off my capacitors?

Thanks,
TC

[Rosser]

Some amps have bleeder resistors that drain the voltage to avoid this problem of charged caps. Also, I've never used a resistor with the jumper method -- there are generally resistors on the B+ rail that I believe help drop the voltage relatively slowly (<1 minute). I've watched the voltage drop with a meter, and it doesn't discharge all at once -- it cycles down. YMMV

[BadAssBluesDude]

It's an Epiphone Valve Jr. that has a turretboard.com Vox board and a Heyboer OT. Like I've said, I'm brand new to this and I've done A LOT of studying, and as far as I know the Valve Jr. does not have a bleeder resistor.

I'm still stumped.

TC

[RicharD]

If the circuit is intact, then there is a load on the power supply and the caps should drain.  If you fire the circuit w/o output tubes installed, then your B+ caps will probably hold a charge.

[BadAssBluesDude]

I hope this isn't a really stupid question, but can I check them with the amp turned on? I know a lot of you experienced amp builders cringe any time a newbie talks about diving head first into his amp without taking the proper precautions. I just want to be 100% sure that when I turn the amp off and test the capacitors and they read zero, that it actually is zero. I just can't believe that I'm not getting any reading of the capacitors even immediately after the amp has been shut off.

TC

[RicharD]

The only stupid question is the one that doesn't get asked.

Of course you can measure a live circuit, just be careful.  only stick 1 hand at a time into a live box.  Always wear insulated shoues and make sure no part of your body is grounded.  PRR once said to someone else on another forum in another time, "Clip leads are your friend."  I had a great epiphany over this statement.  Clip lead your neg to the chassis in a safe, out of the way location and probe with just the red lead.

-Richard
 

[Katie 77]

I just can't believe that I'm not getting any reading of the capacitors even immediately after the amp has been shut off.

Something doesn't sound quite right here.....even w/ a bleeder.....it takes at least 30sec to discharge caps to zero (depending on bleeder value and total charge)....the discharge starts out fast, but slows down w/ diminishing current...so you should be able to read SOMETHING right after you shut it off.

find a 9V battery and make sure you meter is working, or just power up the amp on the bench w/ meter attached and set to VDC (highest range if you don't have autorange)