Discharging Combo Tube Amps

How to safely discharge your combo tube amp capacitors, check the voltage and prep your amp for modding or maintenance.

[@gigthurmond2187]

I'm just beginning to get into working on my own tube amps, and I've watched probably half-a-dozen videos on how to discharge filter caps. Yours the most thorough and the best. Thanks.

[@roycox4415]

thanks so much! Im so glad it was such a help!

[@LeviBulger]

@@roycox4415 Great instructional. I assume you're from the East Coast?

[@kriskjellquist1758]

Some videos should be a bit longer and people should spend more than 90 secs learning something. When talking about lethal voltage it's worth 20 min on YT. Best video I have seen on this subject. Nice job.

[@jlbaxe]

I did the Fromel mods to my ‘96 USA Blues Jr. sounded amazing but I sold it to a friend because I have a Mesa MkIII. Miss the amp but I still get to play it. Nice to see a local Marylander 🤘🏻😎🤘🏻I subscribed for the studio vid. I’m a custom cabinet builder/woodworker and build my own blind dovetail headshells, racks and studio furniture 😎

[@roycox4415]

Right on!

[@silasfatchett5693]

A newcomer to tube amps who wishes to safely work on one would do well to watch and learn from this video.

[@nickraffaele3471]

Thank man, great video. Out of all the 'how to discharge an amp' videos ive seen, this has been the one that has gotten me to actually make me completely feel safe doing it myself.

[@roycox4415]

assuming you are being safe and always respecting the high voltage, its amazing how easy some of the fixes can be that would normally cost a fortune!

[@dealswift]

Very informative and straight to the point. One of the best video I have seen regarding discharging of tube amps. Thank you.

[@modernmusicentertainment2127]

Excellent video, man. You do a great job thoroughly explaining everything. Definitely more comfortable after watching your video, and I’ve watched a bunch. Sometimes these “super pro” repair guys either gloss over things too quickly or spend too much time making a show out of the video. Thanks so much!

[@graveljaw]

Excellent, and very thorough. Probably worth mentioning the one hand rule, since you display the discharge tool with both hands at times (just to show us the tool).

[@thesuccessfulbarber]

BillM also showed that tube amps discharge themselves if allowed to warm up and I thought it was crazy! I feel confident now

[@guydeguido]

A clear, thorough and very useful guide with safety stressed at all times. Thanks!

[@roycox4415]

thanks so much for taking your time to comment!

[@KotaCraig]

Awesome video. Thanks for explaining EVERYTHING and in layman's terms too.

[@hearpalhere]

Thank you for this excellent and thorough explanation!

[@johnandersen9219]

Well done. Better safe than not!

[@2121toso]

Thank you. I am building an amp for the first time. This was helpful.

[@guitarsofvintage4422]

Excellent thorough video! Thanks!

[@klanden]

Thank You. Great info, well explained!!

[@adamkrause6791]

Excellent informative video ! I am glad I watched this one !

[@roycox4415]

Thanks so much for taking the time to check it out and I'm so glad it was useful to you!

[@LewisShieldsUS]

Wonderful! Have a Twin won't light up and needed retube before the problem. Also have an HRD and an 800 in pretty desperate need of retube. Learning proper discharge and bias technique first. Thanks.

[@robbergin9441]

Clear and thorough,thank you!

[@iancox7874]

It’s interesting getting a visual of the excess voltage when the amp is first powered up

[@l3ertuz362]

very detailed and safe oriented, thank you!

[@roycox4415]

I'm glad it helped! Thanks for the comment!

[@freecitizen2760]

On my Silverface Deluxe, I turn the power switch off while playing until the note dies out. I unplug it, and then I measure the voltage across one of the 0.1 caps near the power tube. Usually it’s between 10 and 150 volts still in the cap. I then ground pin 1 of any preamp tube with a screwdriver to the chassis. It’s never enough voltage to cause a spark. Then I measure the cap again and watch the residual voltage drain away due to the grounded preamp tube.

[@PhotographybyCade]

best video so far

[@jmjb67]

Thanks for the lessn Roy. Cheers.

[@markbrooks7257]

Great video, one of the best on KZbin. But, this applies to both heads and combo amps. Hopefully people with amp heads can find it too.

[@roycox4415]

I have a couple of videos specifically for heads!

[@gwbenites]

I don't know if you were referring to Rene Martinez (Stevie Ray Vaughan's tech) or not, but it's funny because his first tech was a guy named Cesar Diaz who was both famous and infamous at the same time. He has since passed, but he did some great things such as making and modding amps, making pedals and such, but he was known to do some things that drew a lot of criticism also. Things such as using WD-40 on pots, which most techs consider a no-no, and being pretty outspoken about doing things that are now considered blasphemous.

[@analogalien]

Great video!

[@Aer01614]

Thanks good safe approach I like

[@jarrydee2799]

great video. Thank you!

[@davidbacheler1399]

Well done. Thank you!

[@backlineguy]

I've watched lots of videos on how to discharge a tube amp, and yours is very comprehensive...I'm preparing to replace the leaky filter caps in my latest Hot Rod Deluxe project amp...definitely need to know how to discharge the caps so as to be able to work on the PCB and caps safely...what's your opinion on using a pomona plug with a 22k 2W resistor connected to discharge while using the multi meter? Great stuff, thanks!

[@clydeg4274]

Rock Solid advice.

[@56WagonWheel]

There is a video of Lee Jackson discharging caps using a screw driver. He obviously has a lot of experience with high power amps.

[@Gratefulman1965]

Good content, Thanks.

[@jonnyfunkson]

Hi Roy can you please tell me does this also apply to a solid state amp ?

[@jasonyu1605]

what kind of resistor to discharge 22000uf 80v capacitor?

[@philf4086]

Will these amps only discharge when plugged in? There must be good continuity with a ground correct?

[@billwilliams6338]

ROY COX, if the B+ voltage is at 450vdc and you increase it to 600vdc, will this increase the AC ripple voltage and output more wattage?

[@beforethelast]

Fantastic... Thanks

[@backlineguy]

Blues Jr discharges automatically...how about the Hot Rod Deluxe?...same deal with the Filter caps? I'm buying one for $100, it needs new caps, tubes and a couple resistors...

[@petershifman9005]

Good job.

[@axedoc2010]

Super video! Really well explained. Thank you very much.

[@mmore]

great learning video.many thanks. just a question here: why did you discharge the capacitors through the positive side and not the negative side of capacitors? thanks again

[@silasfatchett5693]

The negative terminals of the capacitors are connected to the chassis. Connecting one end of the resistor to the chassis, then touching the positive terminal of the capacitor with the other end creates a path for current to flow from the positive terminal, through the resistor, and then through the chassis to the negative terminal.

[@danielpike4164]

Great video - really useful - thank you. Are there any specific things to think about if your amp has a tube rather than a solid state rectifier?

[@silasfatchett7380]

No. The procedure is just the same.

[@bricebowlin8834]

Look at the accessibility of those tubes. ! so much easier than the Crate vintage club

[@philf4086]

Semi-newbie question - I am assuming from the video that you need to have the amp plugged in so that the 3rd prong has access to ground. True? Someone may conclude that the safe thing to do is to unplug it, but you'll lose the ground connection too. Beware that a plugged in amp will have 120V at the power switch, that is a place to avoid while plugged in.

[@silasfatchett7380]

Not true. The amp does not need to be plugged in. In fact, it should NOT be plugged in.

[@acusticsix]

thank you... i've been thinking about replacing the filter caps in my 20 year old twin. after watching many videos on how to discharge amps i"am still scared shit of being electrocuted.. after watching your video i feel i'am up to the task... (proceeding very carefully) anyway i have the one stupid question. you said any pos. terminal on any cap... are they all linked together meaning you drain one and they are all drained ..or do you have to drain everyone one at a time? cheers

[@gabegabe7914]

That was my question as well. My first impression is yes, since he did not discharge all three individually. Also, you cannot discharge any tube that has the voltmeter already hooked up to it. I did not see Roy move the meter around to free up the other two capacitors for draining. Having said all that, I would still get confirmation from Roy!!!!

[@dekrev]

Funny - we used to throw charged capacitors at each other when I was in trade school ☝️🧐

[@roycox4415]

That’s hysterical!

[@westbay5858]

I checked my Caps but they were all showing 0 so they don't always stay charged.

[@gregweber6237]

Hello, I would like to build a discharge tool like yours. Would you mind telling me where you got the wooden tube you used? Thanks in advance, and great video!

[@roycox4415]

I bought mine as a complete discharge tool so it all came together. I bought it from weber I think.

[@gregweber6237]

Thanks for the info, I just built my own last night, so good to go.

[@MarkTillotson]

Wood can conduct at these voltages and shock you, you need a proper insulator.

[@acusticsix]

are all filter caps connected.... wired together? (discharge one and they will all be drained) or you have to drain one cap at a time? I'am thinking about replacing the caps in my twin....

[@silasfatchett5693]

The positive terminals of the filter caps on the B+ line are connected by resistors, which means that draining one cap drains them all.

[@johnvalencia9927]

Thanks. I wonder how it is when you apply the multi meter to big filter cap in a 1969 bassman..you know, an old cap that is still charged and not able to discharge itself like your amp allows for. Am I to simply clip negative to chassis, then positive probe to positive end of cap and I'll see a nice high reading on the meter? Like the Stick of dynamite caps in my Traynor are rated as follows: 80 microFarads at 450 volts!

[@silasfatchett5693]

If you set your meter to its 500V DC range, you can watch the caps slowly discharge through the high resistance of the meter.

[@johnvalencia9927]

Wow. I did it, I've been playing with the multi meter. The caps in my 1975 sounder organ, in the power supply, go from like 50 volts and then drop down immediately after shutoff! Thanks for the video. How many volts are safe to the touch? Like when my cap reads 50 volts, how am I supposed to know the amps though? If it will hurt me? Thanks so much for all the great information.

[@frossbog]

Very good video from a practical standpoint but it should have made mention that it is current (amperage) not voltage that is harmful. Certainly, without voltage there can be no current, but to say that a certain level of voltage is deadly without mentioning current is misleading. A static spark from your finger to a door knob in the winter is thousands of volts, but it's only a few mA.

[@alvin10388]

thank you...

[@roycox4415]

No problem, hope it helped!

[@silasfatchett5693]

According to the schematic of the Blues Junior, it is not fitted with a standby switch. If your amp has a standby switch then draining the caps by switching the amp off when hot will not work if the switch is in the 'standby' position.

[@jeffgnr91890]

can you do this without a discharge stick?

[@silasfatchett5693]

Yes.

[@backlineguy]

CAN YOU RECOMMEND A SITE ONLINE WHERE I CAN BUY NEW CAPS AND RESISTORS FOR MY HOT ROD DELUXE? I HAVE THE CHANCE TO BUY A HRD FOR $100, AND REPAIR IT MYSELF...THATNKS FOR THE GREAT VIDEO!

[@dastave]

Why doesn't the multimeter, that's hooked up to ground, drain the voltage from the filter caps?

[@dastave]

Does it have to do with the fact that the discharge tool has a resistor?

[@edian1895]

Because multimeters are designed to have a very big input impedance. This ensures that the miltimeter only measures a part of the circuit without influencing the circuit in any way. The impedance is too high to produce any significant current flow.

[@silasfatchett7380]

It does, but very slowly.

[@edian1895]

@@silasfatchett7380 ? Not slowly but almost an insignificant quantity. The voltage on the multimeter is calculated by measuring the current flow which is almost insignificant and multiplying this current flow with the fixed impedance of the multimeter.

[@silasfatchett7380]

@@edian1895 If you have a couple of hours to spare, you can watch the caps discharge through the multimeter.

[@cast390]

@11:20 you're talking about Cesar Diaz Ive seen that video.

[@whynotme5926]

I was told to get a 5 or 10 watt resistor , 100 ohm-The guy at the store gave it to me and I got home and it was a 10 watt, 50 ohm resistor. Would that be ok to use? Thanks for any help.

[@silasfatchett5693]

Yes.

[@thesuccessfulbarber]

There is a guy online ( DLab Audio) that shows certain Fender designs where the amp has to be unplugged but the standy has to be on to properly discharge

[@roycox4415]

Cody the Barber if the amp has a standby switch

[@halbertking2683]

Why that's a Fender Bic Jr.

[@T25de]

Never recommend a $10 meter if you want to measure up to 1000 volts My test leads are Fluke brand and very expensive but you can bet your life they’re solid. Buying a preowned Fluke meter is a great way to get quality gear for a budget price

[@MarkTillotson]

2 x 180k resistors is far too high a value, that's taking a minute or more to drain a 40uF cap. A few k ohms is about right.

[@edian1895]

You don't want a resistor that has a low impedance and a low wattage. This can result in a short circuit between the amp and the ground chassis and cause sparks. Its saver to use a higher impedance to be sure that not too much current can flow through. If you have a 500 volt, you want to use a 1 watt resistor with a minimum of 250 kΩ or 2 watt resistor with 125kΩ for example (500^2/wattage). You can increase the wattage by having multiple resistors in series.

[@anthonysilva5312]

10:00 start here

[@danaparsons854]

S found is practically inaudible. Turn up the volume.

[@danaparsons854]

I really would havevvwatcged but I could hear.

[@roosterjones4247]

Not the current, but the voltage.

[@rickardlowstedt3257]

great video. do not think a pencil is used for pointing out. it must be a wooden or plastic stick, the lead in the pencil is a counductor. if you should point with the tip and touch the end of a pencil.

[@roycox4415]

im not using a pencil. its a wooden chopstick my hand. not sure where you are seeing a pencil.

[@rickardlowstedt3257]

@@roycox4415 I know you are not using a pencil. I just want to make sure other viewers do not think you are. I once was electrocuted this way working as an electrician. Best regards

[@jayytee8062]

Your resistor value is way too high for discharging - ridiculously slow. All you need is around 500 Ohms give or take.

[@gabegabe7914]

Audio is way too low. Cant hear you. Too bad. Everyone seems to think its a good video.

[@gabegabe7914]

Ok, I held the video to my ear and listened instead of watching. Slightly inconvenient but it worked, and yes, it is a very good video and the best I have seen as well. Will be keeping this one in my permanent library. Thanks Roy for the effort and well thought out lesson!!!!!

[@stevebarney6735]

I waited 8 minutes for you to provide the information promised in the title, and then quit. Get to the point.

[@roycox4415]

if you don't need the safety info or the tools you need to do the job just skip forward. some people are new to this and can get hurt. This video was for everyone, not just seasoned techs.

[@stevebarney6735]

@@roycox4415 I'm not a seasoned tech. You talked too much. You want to do a long, comprehensive presentation on safety? Fine: put that in the title. You wasted my time.