Bleeding caps safely

[bnwitt]

I know certain amps that will still have enough juice to shock you with this method.

Depending on how well you're grounded, 20 volts is enough to shock you too.
I've been nailed pretty good off of a 24 volt power supply before.

[Enzo]

And as someone mentioned, the tubes continue to conduct. Try the experiment with the tubes removed and see if it drops as fast.

[Diablo]

If I get energized, I may just charge up a new Illinois cap with my amp and some alligator leads, then disconnect the leads and wait 5 minutes to measure the voltage stored in the cap.

[Diablo]

If I get energized, I may just charge up a new Illinois cap with my amp and some alligator leads, then disconnect the leads and wait 5 minutes to measure the voltage stored in the cap.

OK, did the experiment and this is what I measured.
22 mfd/500V Illinois Capacitor charged to 417 volts.
Removed voltmeter, then removed alligator clips connecting cap to power supply. Waited 5 minutes and measured 280 volts on cap.

Then I repeated the experiment with a 40 mfd/500V cap (twist lock triple), 36 years old removed from my Ampeg V4. I only charged the cap up to a little over 400 volts because it took quite a bit longer to charge and the charge rate slowed down a lot once it reached 400 V. Removed voltmeter, then alligator clips, waited 5 minutes and measured 164 volts.

I conclude that new electrolytic caps store charge at least as good as old electrolytic caps. Both can have dangerous amounts of voltage stored after 5 minutes. Also, the voltmeter drains current and voltage much faster than the internal leakage of the cap.

[Chuck H]

Also, the voltmeter drains current and voltage much faster than the internal leakage of the cap.

This, along with the tubes current draw continuing when the amp is shut down must be responsible for my test results. I left my meter connected for the entire test and I only shut the power switch, not the standby.

Chuck

[EFK]

So the meter itself can act as a power drain? That's extremely interesting and would explain to some extent why I got the readings I did; I too left it hooked up the entire time.

[defaced]

A volt meter should have a very large internal resistance, on the order of megaohms. This is to not load down a circuit while taking readings. However for this kind of study, it may be enough to effect the results.

[Enzo]

10 Meg is a very high resistance, but it is a lot lower than an open. SO it will discharge a cap. Also, when you try to measure something like signal level in a circuit where the circuit elements are 1 meg pots and resistors and 470k resistors and the like, then it becomes a significant circuit load and skews readings. From time to time I have to suggest to someone to conect their meter in parallel with some high value resistor to measure the voltage across it instead of readign to ground from each end and subtracting. Meter impedance is why.

[Robmosis]

Could someone please explain what the B+ point is? I have heard this a lot but am not sure what it is.
Also what does a bleed resister on an amps schematic usually look like? I have a schematic for my amp but am in the precess of learning how to read and understand it. I will post it for you guys/gals.

[Diablo]

B+ refers to the high DC voltage supplied to the power tube plates. Typically 430-570 volts, and it's positive.

Bleed resistors are R133, R134, R135, 136, 138, 139, 141, 142.
They are in parallel with the big value filter caps.

That sure is one complicated schematic, not what I would want to tackle for a first try at building/modding/repairing.

[Robmosis]

Thanks

Thanks very much Diablo. Yea my noob head spun when I found the schematic for my amp. But I am learning off of other older schematics, most from Dave Hunters book, not the one for my amp. Just like to try understand different areas of schematics.
Thanks again Rob

[Steve A.]

OK, did the experiment and this is what I measured.
22 mfd/500V Illinois Capacitor charged to 417 volts.
Removed voltmeter, then removed alligator clips connecting cap to power supply. Waited 5 minutes and measured 280 volts on cap.

Then I repeated the experiment with a 40 mfd/500V cap (twist lock triple), 36 years old removed from my Ampeg V4. I only charged the cap up to a little over 400 volts because it took quite a bit longer to charge and the charge rate slowed down a lot once it reached 400 V. Removed voltmeter, then alligator clips, waited 5 minutes and measured 164 volts.

I conclude that new electrolytic caps store charge at least as good as old electrolytic caps. Both can have dangerous amounts of voltage stored after 5 minutes. Also, the voltmeter drains current and voltage much faster than the internal leakage of the cap.

When using a DMM to measure voltages like that you might also measure the currents involved- or measure the voltage drop across a resistive load.

When using a DMM in HVAC repair work I often got what I called "phantom voltages" when measured without a load, but would disappear once a load was hooked up. I never got phantom voltage readings with an analog meter since the meter itself was a more significant load. BTW the phantom voltages were often a parasitic effect like with a deenergized wire that was run in a conduit with an energized wire. Or a power transformer with an open internal thermal fuse on the secondary.

However, I am a firm believer in the power of electrolytics! My question was just on the measurements you posted (you would always measure the voltage of a battery under a significant load).

Another test that you can make on new and old caps- first charge them up as you did and then drain the charge with a bleeder jumper. Remove the jumper and then check for voltage after 5 minutes. (When I've done this I've gotten small sparks indicating the presence of some voltage.)

FWIW I've trashed a few digital meters measuring electrolytics- these were not Flukes but they never did give me accurate dc voltage readings after that...

Steve Ahola

P.S. So nobody here suggested the tried'n'true method of old school electronics techs- BAS: Big Assed Screwdriver... Place the base of the screwdriver shaft firmly against the chassis and just aim the tip at the ungrounded lead on the cap. Keee-r-r-r-r-ackkkkk!