How can I tell if my filter caps are still holding (the correct) charge?

[Dolmetscher007]

I am new to repairing and building amps. I am very aware of the dangers surrounding tube amps. I know all about the one-hand-in-your-pocket rule. I know to always drain the filter caps before working inside an amp. I know about using a current limiter (high-wattage light bulb). I have a descent amount of theoretical "knowledge," but I am super aware (and nervous) that I don't have much hands-on practical experience. Internet forums, books, and YouTube are my only resources.

So my question is... is it okay to turn my Fender tube amp on with the back taken off... and use my multi-meter to... touch the black lead to the chassis, and carefully touch my red lead to the positive leg of the filter caps so I can see their voltage and then check their capacitance?

The reason I ask if it is okay to do while the amp is plugged in, on, and running... is because I tried to just check how much voltage the caps had after 2 days of the amp not being on, and they were are 1 volt. This tells me that the caps already drained. So there must be some kind of bleeder resister in the circuit. OR... it means the caps are dead/broken/garbage. I don't know how to test these caps to see if they need replacing.

I could just drain caps (unplugged) and go ahead and replace the caps. But I'm trying to learn more about how to test and measure things. So please be gentle with the "stupid newbie" comments. Becasue... trust me... I know.

[AlNewman]

The best way is to find the proper schematic for your particular amp, then with the amp off, go through it and check all resistances and continuities, while comparing with the schematic.

Then, after confirming everything should be good, you fire up the amp with no tubes through the LBL, and see if it flashes.  No flash, then you can remove the LBL and check voltages without tubes.  If all is good, you can go through the same steps while adding tubes.

It's unlikely that most amps will hold a charge on the caps after a couple days, (or even more than a few minutes), with the power off...  But, this isn't a rule, and that's why it's best practice to always make sure the amp is discharged before working on it.

[HotBluePlates]

... I tried to just check how much voltage the caps had after 2 days of the amp not being on, and they were are 1 volt. This tells me that the caps already drained. So there must be some kind of bleeder resister in the circuit. OR... it means the caps are dead/broken/garbage. I don't know how to test these caps to see if they need replacing. ...

Let's take a deep breath & think for a sec.

If the amp works when you turn it on, and "sounds like a Fender amp," then the caps are fine.

That means your question reveals the newness & excitement of "amp stuff" is removing some perspective.  Know that "good amp tone" mostly is a function of "nothing is grossly-wrong" and is not a function of "gnat's-ass part-value precision."  When you wonder whether filter caps are doing their job, you might make a measurement... or you might just turn on the amp & see that it functions (which tells you those caps are doing their job).

Other than that, I mostly don't like to poke meter-probes at circuits with 400+ volts present, while the circuit is "live."  I prefer to clip-on the meter probe with alligator-clips or grabber-hooks, and power up the amp with my hands clear.  But I've also been dumb enough to grab 450v DC in the past, so maybe I have a warped view of how to do stuff.   

[tubeswell]

^What HPB said^ Safest practice is to use insulated gator or test clips when metering tube gear - and connect and disconnect the clips when the amp is powered off (and preferably with the power cord pulled out of the wall socket - especially if you're a newb). Sure you can use appropriate category insulated test probes but chances are you can slip easily off some surfaces and suddenly you're shorting high-voltage bits of the circuit where you don't want...

[Lectroid]

@Dolmetscher007,

Good advice.  What both of them said: use clipping probes.  I don't want to get bitten by HT ever again, so I never make freehand voltage measurements and I don't move probes on an energized amp.

Beyond that, this bit from HBP, is really great advice for every over-anxious new amp builder.   (We've all been there)

Know that "good amp tone" mostly is a function of "nothing is grossly-wrong" and is not a function of "gnat's-ass part-value precision."