Discharging the Capacitors

People are always worried about discharging the capacitors in their amps. Yes, high voltage is scary and all that, but it’s really very simple:

Turn the amp on, let it warm enough to play through, turn it off. That’s it.

Here’s a video to illustrate:

To recap: Don’t put an amp in standby if it has one. Just turn it off. It doesn’t make any difference if you play through the amp, but it will verify that the charge is gone. And you can always verify with a voltmeter if you want.

30 Comments

  1. TP says:

    Yes, how true. On some amps like i.e. my old Acoustic G60T, the standby switch is after the first two power caps, so if you put the amp in standby mode first and then switch it off, these two caps store hundreds of volts for days.
    regards

  2. dang says:

    hi bill
    great site!

    please forgive my newbie questions, but i just want to make certain i understand:

    r u saying that the caps always discharge themselves within a few minutes after power is turned off if the standby switch is not used?

    what about amps which don’t have standby switches like princeton reverb?

    so the supposedly fatal charge held in the caps is a myth, and it safe to work on amps that have been turned off without discharging the caps?

    please forgive my total lack of tech knowledge.

    thanks again for sharing your expertise!

    • bill says:

      You watched the video, right? There’s your proof. As long as you leave the amp in “play” position and turn it off when it is warm enough to play through, it will self-discharge. As I said in the article, if you quickly toggle the power on a cold amp, you can charge the caps and they will hold a charge. But if you warm it up until you can play through it and then turn it off, it will self-discharge.

      • dang says:

        thanks for your prompt reply. yes indeed i watched the video. i just asked the question because EVERYTHING i’ve read till now warns of near fatal charges stored in caps for extended time periods. so, being a newbie i wanted to double check with you so i don’t wind up going BZZZZZZT!
        thanks!
        dan

  3. catfishjones40 says:

    Hi Bill. I watched the video and saw the notice about amps with tube rectifiers. In that case would you just use the resistor method to discharge?

    • bill says:

      An amp with a tube rectifier will self-discharge just as quickly as one with diodes. The difference is that with a tube rectifier, there’s no high voltage until the rectifier warms up and begins to conduct.

      So “bumping” the power (quick on-off) on an amp with solid-state rectifiers will charge the capacitors instantly. Because the tubes are cold, the capacitors will hold the charge. Bumping the power on a tube rectifier amp does not charge the caps, as long as the heater in the rectifier tube is cold. If you suspect that there’s a charge, let the amp warm up fully, then shut it off. It will self-discharge.

      But it’s always a good idea to test before you touch!

  4. tmlfan says:

    Hello Bill.

    Great vid.

    Quick question, this is all new to me so I need to be clear on this point.

    Are the caps discharging on their own, once tubes have warmed, because the Blues jr. has a bleeder resistor?

    • bill says:

      The caps are discharging because the tubes are conducting large amounts of current. They only do that when they are warm. The Blues Junior doesn’t have a bleeder resistor. If it did, it would not be necessary to warm the tubes. But warm tubes discharge it faster than a bleeder resistor would.

  5. bluesguitar says:

    Hi Bill. I recently purchased a ’76 princeton reverb. I want to keep as many original components as possible. Yet, I want to change all filter caps that need to be changed. How can I test the old filter caps with the chassis intact? If I replaced all filter caps for the safety of the amp, would it harm the vintage tone?

    • bill says:

      You need special gear to test filter capacitors. If the filter can is original, I’d replace it. If the caps are old, it will change the tone, but it will change it back to where Fender originally designed it.

  6. bbutler356 says:

    So do you mean, “don’t use the standby switch when turning the amp ON”, either? Or just don’t use the standby when turning the amp OFF? Or do you mean don’t use it when you go on break at a gig?

    Thanks, Bill. I haven’t been to your site before. I’m very impressed with what you’re doing. I’ve owned a Blues Jr., and while I love the idea of one (lower watt, 1-12″, compact), I’ve always thought that my tweed one with the Jensen sounded small and boxy.
    Isn’t that due to the size of the cabinet, and nothing can be done about it?

    I’d love to think that it was something else, other than a small cab.

    Thanks for your time, Bill.

    Brian Butler
    youtube.com/brianbutlerUS

    • bill says:

      Turn the amp on when you want it on, turn the amp off when you want it off. Use standby when you want it to be ready to go at a moment’s notice or when changing instruments. That’s all.

  7. kdwisdom says:

    Hey there. I did all of the mods I ordered (cream board basic kit, line out, presence, replaced input jack).

    However, I made a mistake and forgot to plug the speaker in. I replaced the tubes because I know one of them blew.

    Now when I check the voltage, it shows the running voltage at 278 on my multimeter while yours was showing around 332.

    Any idea why it would do this? I’m worried I ruined the output transformer. It sounds fine after I put the new tubes in but was concerned about having such a significant amount of less voltage.

    Is this normal or something I should be concerned about?

    • bill says:

      It’s unlikely that you damaged the OT. You’d get smoke and blown fuses if it were shorted. What’s your line voltage from the wall? If it’s low, your plate voltage will be low, too. 278V is pretty darn low, however. What are the measurements on the brown and green power transformer leads?

  8. dezng says:

    Hey Bill! Great site great info! I always wanted to clip the bright cap of my Deluxe Reverb Reissue but I’m worried about danger of high voltage and discharging the caps. Thanks to you my project might really happen. Is the process of discharging my amp the same? What is the best way you suggest to get the job done? Clip or desolder? Thanks and appreciate your advice!

  9. edtoal says:

    If tube amp caps discharge rapidly when the tubes are warm, why do people get shocked? Do the caps only hold a charge when a cold amp is powered on and off quickly? Who does that? Is that the only scenario that creates dangerous voltages in an unplugged amplifier?

    • bill says:

      People get shocked when they reach in while the amp is on. People get shocked when they power up, then power off and amp with no tubes in it. And, of course, if they power up and off quickly and there is no bleed resistor. My point with this page is that you should always verify that an amp is discharged before you begin to work on it, but elaborate rituals are unnecessary. 99 percent of the time, the amp will discharge itself. Just be aware of the 1 percent cases.

  10. mikeganz says:

    Bill – FANTASTIC video, but I have 2 (probably really stupid) questions.

    1) I’m building a mojo JTM45 kit in about a month. To get my feet wet, I’m rebuilding the tube amp in a late 50’s portable record player. It’s a single tube (25ca5) circuit (so a single ended amp) with a selenium rectifier (a 1n4007 and voltage drop resistor in series will replace this, though). The total parts add up to about 15. Using the wire-to-resistor-to-chassis method above, can I just connect directly to the filter cap, or do I need to go to the resistors in the power circuit?

    2) When dealing with a single ended amp with a selenium rectifier, does the method of turning on the player, letting it warm up and then turning it off work the same as the much larger push/pull amp that you’re dealing with?

    thanks in advance!

    • bill says:

      Yes, you can use the resistor and and lead to ground to discharge the amp. Always unplug a no-transformer chassis before doing any work on it. The selenium rectifier makes no difference.

      • mikeganz says:

        that answers my question! it does have a transformer – right now it’s covered in some sort of white residue that makes me just want to replace it. The schematic calls for a 3600ohm primary / 6-8ohm secondary. I found an Edcor 5000ohm primary / 8ohm secondary that has a similar turns ratio which I’m hoping works.

        Thanks for the help!

  11. count_bassie says:

    Hi Bill

    Thanks for the video. I suppose I have a somewhat unique scenario to what you’re demonstrating. The pins in the socket of my rectifier tube are quite loose (Ampeg B-18N amp), and it won’t heat up at all. I’ve tested the tube in another amp, and it’s good, plus I can see how loose the pins are. Considering the tube doesn’t heat up at all, are the caps in my amp charging at all when I’m turning the amp on? I don’t have a tool to safely discharge the amp handy, but I want to fix the sockets (not hard). Just want to make sure that if I start reaching in there, I won’t be zapping myself! I used your method of discharging that you demonstrated, but not sure it actually does anything right now. And if the caps had a charge, approximately how long would they hold it without being discharged?

    Thanks!

  12. KevinO says:

    What about replacing a bad power cord on a silverface Deluxe Reverb? Do I have to drain the voltage as a safety precaution? The plug end is bad. It’s our other guitarist’s amp. One of those ‘wiggle it and it works’ things. It hasn’t been plugged in for probably a week anyway. I was just going to replace the entire cord as it’s all old and crackly. Just unsolder and resolder the new? It’s already a 3 prong originally.
    Great site. Great information. Glad I found it.

  13. vinic says:

    Hi Bill,
    I have your 100µF/400V cap for C25 (cream board) and wanted to try to add one or two 220µF/400V caps that would fit beside C25… Would there be any reason that it hurts some components around C25 to have such a huge cap? (540µF total) I think about initial current sink, when powering it up…
    (A bit off-topic here, but I didn’t know where to post)

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