How to safely discharge tube guitar amp Filter capacitors D-Lab Tech Tip

I have had many requests for this subject video. I delayed since I did not want replicate similar information that already exists. So, I decided to make a cool test accessory, simplify the process. Also I wanted to point out some newer amp design hazards. Many techs/hobbyist have suffered injuries from being shocked while servicing tube amplifier, whether Guitar, Ham or home audio. Best thing to do is check, double check. Dont be in a hurry. I hope this info is of value to you.

[stephendraper]

When I used to repair amps I used a 240 volt UK light bulb with 2 meter leads connected to it . You could see the bulb slowly get dimmer and then go out when discharged.

[PeopleAlreadyDidThis]

Exercise caution with voltage doubler power supplies, or those that output both positive and negative voltages using the same reference (some HV bench supplies, for example). They can have capacitors that are not grounded. Strapping your discharge resistor from B+ to ground won’t catch all of them. Discharge each capacitor individually.

[wadereynoldsgm]

Thanks for the tip on the negatively-switched standby circuit. Never saw that before in an amp but the fact that if you don't know and you think you've discharged the caps is worth being aware of. Learned something new. Keep up the great work!

[BreauxBobby]

Thank you! This could save my beginner’s arse. I want to try an easy amp build, and I love looking inside my hand wired amps. Thank you for this video.

[EJPCRSKW]

The adapter mentioned is a Pomona 1330.

[mikesradiorepair]

Great tip. Learned that from my uncle who learned it from my grandfather. One thing to remember is use a resistor rated at the maximum voltage you will encounter. I once accidentally ordered the wrong series of metal film resistors from Dale. I wanted 750 volt rated resistors and ordered 250 volt resistors by accident. I would say they went poof the instant they had power applied but they didn't do anything but go open. No magic smoke, no snap crackle or pop, just went open.

[dennispeake]

Terry, we need people who take an interest in our capacitors they are not made to last any more. Thanks, mate a good amateur always keeps his electronics working better and safe.73 de Dennis vk4oc.

[kameel]

Just made this little tool. Haven't used it yet, but when I need it I'm prepared. Safety first.

[jcmcclain]

Great tip Terry! Thought it was going to be the same method I always have seen and used, but I learned two valuable lessons here. Good job!

[antoninowoodystella]

Saint D lab thanks. I love your explanation for not technical speaking guys

[WW-jzzd]

Question. When discharging tube amp caps, should the power cable be connected to the mains, while the amp is turned off? Otherwise, where will the charge dissipate via the chassis if the power cable is not connected to the mains ground? Or should the power cable be pulled out from the mains and chassis alone can absorb all that current?

[papawx]

I looked at the schematic for my Vibrolux Reverb{AA270 circuit} and it doesn't look any thing like that, so I guess I'm good to go the old way of discharging caps. An old, long deceased amp tech once told me another way to discharge my caps: The day BEFORE I plan to work on my amp; turn off the power switch and unplug the amp from the outlet. Then flip on the standby switch. By the next day the caps will have discharged and the amp will be safe to service. Now, this was told to me back in the 1970s, and the guy worked on the circuits of those days. Nowadays, it could be a different story with all the different makes we have today. Then it was mostly Fender, Ampeg, Carvin, Marshall and Silvertone/Danelectro in my area.

[rciancia]

Very good Terry. You are correct in that standby switch is weirdly placed. Great idea with the banana jack.. as always. Thanks my friend. Ron

[johngilley]

Thank you for that great tip. I'm always a little on edge when it comes to capacitors.

[gigmandrew]

Great video Terry! I always learn a lot by watching your videos.

[buffit_]

Thank you Terry. This is a very nice tech tip which I will be using on my amp builds.

[jeffm]

Excellent idea using that banana stacker. Maybe some heatshrink tubing over the resistor area of the stacker would be good. Looking at the Vibrolux schematic there is a ground connection at CP1? (can't read it) on the negative rail just after C33. Therefore with your banana stacker in place, the meter negative connected to the chassis and the positive connected to either TP33 or TP34, I believe the main filter capacitors would still discharge. As you rightly point out though, the voltage on all the other filter caps should be checked for discharge as well.

[AndyCinDallas]

Excellent video and tips, thank you. I'm building my first tube-amp kit and I've been looking for exactly this kind of info - the hows and whys.

[stingray]

Thanks so much D-Labs!!!
Great advice and great Channel!
Thank you !
Bob

[TheBladepolisher]

Great video, but you left no link for the banana adapter . . . . . . . D LAB is here for you.