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How to safely discharge tube guitar amp Filter capacitors D-Lab Tech Tip
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- Опубликовано: 31 июл 2024
- 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.
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.
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.
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!
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!
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.
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.
Great video Terry! I always learn a lot by watching your videos.
Thank you Terry. This is a very nice tech tip which I will be using on my amp builds.
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.
Well, Snoz my Ramus! It's SUCH a no-brainer, I'm shocked (;-D) that no one's done that before! THANK YOU, Terry. Could save a life - or at least a nasty electrical discharge.
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
Thank you for that great tip. I'm always a little on edge when it comes to capacitors.
I’ve been “on the edge” of a few of them.
Thank you Terry. Keep the videos coming my friend!
Very informative, especially about the fender stand by switch. That's crazy. Great to know. Others might be like that.
That's very clever thanks for the excellent meter tip
Thanks for the tip Terry I was using the to ground method .
Exactly what I was looking for, thank you.
Thanks so much D-Labs!!!
Great advice and great Channel!
Thank you !
Bob
Thank you, Terry. Very good advice.
Saint D lab thanks. I love your explanation for not technical speaking guys
Just made this little tool. Haven't used it yet, but when I need it I'm prepared. Safety first.
Such a nice and simple trick to solve this issue ! .............
interesting and informative , thanks Terry !
Brilliant! Thank you very much Terry! I love you channel and when a critical task can be completed with stuff just laying around. This freebie is right up there with the homemade Yamaha Subkick. Pitty the fool who buys something to do it. ;-)
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.
Wait… this guy said to use a 22k 2 watt resistor. If a resistor max voltage is 1.5x the wattage, then a 2 watt resistor can handle a max of 3 volts. How can I tell the max voltage I’ll be dealing with within an amp? I’m planning to work on a fender blues Junior 15 watt amp.
@@chrissturley823 Not sure what math your using but it's not Ohm's law. Power (wattage) = voltage squared over resistance. That would give you the power dissipated in the resistor.
Great tips, man!
Great video, as usual... thanks!
Thank you so much for your very helpful tips
Very useful and instructive! THX!
Subscription earned! Thank you for posting this.
Pretty Slick Terry Thanks !
Thank you for the safety tip Terry. Im very interested in learning about tube based electronics but the voltages are a concern for a neophyte like me.
thx terry every tip is useful to one and all :-)
Very useful, thank you very much!
Thank you for the tip !
Love the banana clip trick. You’re the best, D-Lab. Aloha 🤙
Thanks, Terry!
This is very clever thank you
Thanks for a great tip I did not know that.
cool vid and information, thanks 👍
Great video. Thank you.
The adapter mentioned is a Pomona 1330.
Best comment
Thanks, I was just about to look it up.
Good info.
thanks
Amazing ! thankyou.
Thank you!
Excellent!
I did not know about the neg ativ standby switch. Thank you terry . i myself was using the a 1k 5 to10watt sandblock ceramic 5watt 1 to 10k .what is a gopd ohm value to use?
Hey Terry, first off, thank you for these great videos! They are informative and fun. My question for you is: do you do this procedure with every amp you work on? I would assume so, but havent seen a video yet where youve done this. It appears like you go straight to touching everything so I was curious! Would you do this for every capacitor in the amp? Thank you.
Speaking for myself, it's possible I would turn the power back on and still have that resistor assembly plugged into the meter and connected to the hi V. Is there a resistor value that you would suggest that's high enough to use in that assembly and not damage anything should that happen?
Hi Jerry, So glad I found your youtube channel. Awesome job on your videos that are well done, easy to understand and the visuals are awesome. Thank you for what you do. I taught auto mechanics for 30+ years and really appreciate your professionalism. I do not have a banana plug setup on my Fluke27/FM meter. Any advice on how to build an adapter with the cap to do a power soak on the excess voltage and current? I'm working on a 67 Vibrolux, which is most likely similar to the Showman in your video, but then what do I know. My amp has a sizzle sound after striking a note that decays over a short time. She needs a tune up for sure. Doesn't have that warm Fender tone anymore. New tubes are on the way. I want to go over all the tube sockets to clean them and make sure they are not elongated. Do you have a video on tube socket maintenance? A few of the sockets look elongated. Well, I'll stop rambling here and continue getting educated with your fabulous work on amps.
Thanks again, Dave
When you say to leave the standby switch in the on position, I take it that it's still disconnected from the wall socket/mains power source?
Great video, but you left no link for the banana adapter . . . . . . . D LAB is here for you.
Thank you for the upload good Sir! Can you please tell how many Omh resistor will be the right resistor for all voltage? Some say 120 and other 470omh 5w resistor! Which resistor do all jobs if I may say that?
Thanks for all your quick tips keep up the good work keep the videos coming.
Rectifier tube question and alternative use odd project. Would like to use a rectifier tube to build a low-voltage 5.3V 1.5A to 2A cell phone battery charger. I know it’s not practical, big bulky expensive if you had to buy the parts but if you have already had all the parts for free it should be possible?.
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?
Great videos!!!
My 65 original blackface twin was capped and serviced last year...my question is do i need to discharge the chassis parts to spray out a few scratchy pots??
Thanks in advance
Roger Rainville
My old vox ac30 has no standby switch....whats the best way to discharge the caps?
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.
That's the method I've been using for years when working on tube amps and...I still have all my fingers ;)
Is there any danger in poking around in an amp with the power trans primary still energised by the wall voltage? I've been shocked by picking up a chassis I thought was disconnected from the wall. Tired and late at night.
Hey Terry, I want to get a big, lit magnifying glass like yours. Could you say where you got your one? Thanks! Lee
Thanks for the great video.
I have some hifi amps with virtually all popropyelene non polar caps - even the PSU caps, which are Solen polypropylenes.
If you have non polar caps instead of polar, can you discharge the cap in the same way (one end of the wire to chassis and other to one end of the cap?, - If so, I assume it doesn’t matter which end you touch on the cap?)
Or can you not use the one wire end to ground technique for non polar caps?
I’ve seen other videos where people just use a resistor ends going tk each end of the Individual cap end, - instead connecting one end to the chassis.
That looks more inconvenient though.
Any advice would be appreciated.
is there a sure fire way to check voltage regardless of circuitry, what if completed the circuit between one end of the filter cap to the other with a resistor, or resistor and multimeter. does that eliminate all the charge regardless of where the standby switch is located?
also, could I use a 25w 4ohm resistor to drain the caps?
Can you use 2 watt 20 k or 24 k or just 2watt 22 k...You the best I've learned so much from you...Jeff
After you put a resistor into one of these plugs do you just clip off any extra length of the resistor wire that sticks out the other side? Should I cover the hole with anything?
Is the standby switch 'on' when there is sound or no sound?
That's a great video! Now because you are a HAM what value resistor would you use on a 1kw linear amp for HF Radios? I've been using a 1k ohm 20 watt but think I've been doing it wrong also forgot to leave the stand-by switch in the "ON" WOW I'm glad I didn't get any shock ! Maybe those amps are a little bit different ?
Thanks! In that res with clips method, where do the extra electrons go when touching that metal case and is that case safe to touch after? (If can explain case ground, that would be appreciated)
Back in the 70's I tested the length of time it took a charged capacitor to discharge in open air.
It ranged from 30 minutes up to 3 weeks (!). The 3 weeks was on oil bath military capacitors from surplus equipment. Standard 'lytics took up to a week and a half, depending on their quality. Average on fixed caps was about 1-7 days, depending on capacity.
Some of your viewers may be unaware that air blowing across the terminals of a capacitor can charge it. Many of the Mil Surp caps I bought back then had a shorting wire across the terminals. Your local electric company also has the shorting wires across the HV caps they use in your power system., until they are installed.
Many thanks
What if I am just changing a speaker, would i still need to drain? Amp has not been used in 3 months.
This might be a dumb question but will this work on an AO44 with no standby switch or any tube amp with no standby?
Hello...where can I buy that meter ad-on to discharge while reading voltage? Thanks
Hey Terry! Big fan. Great videos. Where can I get a D-lab shirt?
Thank you!!! 😊
Hi Terry ! Do I need to do the same thing with a solid state amp or not before attempting to work on it ? Many thanks John from England 😀
Thanks 😎 SIR..... Question... I have an old tube cabinet radio... I think philco... where in it would be model number Etc.... I have not disassembled the chasis yet.... it's a large chasis 24"by 10" loaded with large transformer and tubes alike hope to salvage parts but would like to aquire a schematics....
If you’re grounded and the powers off, with the resistor is it ok to touch other components with the wire your using to discharge the Caps?
So do you leave the amp plugged in when you do this?
Hello Terry! What is the schematic that you use for this vibrolux? Is it AA964?
One friend of mine says that is easiest way to discharge caps from amplifier to turn switch on when is amp disconnected from outlet, and just wait one minute...is it really one of correct ways, is it true at all???
I’m thinking of making it a habit to do the discharge dance with the standby switch in each direction
good tip
A question please. At what point on the eyelet board are you discharging the caps from? Since I'm just a beginner when it comes to this stuff, I'd be more inclined to open the dog house and do each one individually, however, my guess is that you're doing it from the B+ rail past the standby switch from which point the internodal resistors and filter caps branch off. I've been zapped but just a few times over the years (luckily), but ever since I was knocked on my arse across the floor from my stereo speakers (electrostatics) I've had somewhat of an awakening...I'm just as cautious, if not more, with this stuff than I am working inside the live panel in my home which I'm used to. Thanks, JC.
is it power switch down and standby up?
What is that jack you plugged in to the speaker for the load?
Looking at the schematic, it seems like the main caps are still grounded even with the standby switch opened. The ground is at the right side of the picture. It's just the bridge rectifier and transformer that becomes ungrounded.
He doesn't know what he's talking about! I saw it too Russell!
Notice the Hoppes No.9 on the work bench
When working on an amp with no standby switch, are there any rules to follow? A lot of the stuff I’ve been reading seems to be instructions for dealing with amps that have a standby switch. I’m assuming that I should just have the amp unplugged and go ahead and discharge the caps using this method. I bought the little banana piece and got a pack of resistors from Amazon. I’m going to set up my multimeter the same way they have it set up in this video. Is there anything else I should be aware of?
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.
Jeff M Exactly so. The only thing the standby switch is shown as disconnecting here is the bottom of the bridge rectifier. The caps are still grounded. But a better reason to leave it on is so that the valves stay heated and so can still conduct away some HT.
If someone turns on the amp and doesn't wait long enough for the power tubes to heat up then flips the amp off. That will leave a very high potential for residual voltage in the caps, assuming amp does not have bleed resistors on filters. That's one advantage to having a tube rectifier in that it requires the same filament supply to be able to produce the DC voltage.
I meant to say flips the stand by switch on without waiting for tubes to heat up. Then turns amp off right away, all while tubes not having time to heat up and conduct. Not saying that will happen very often but it might when you are troubleshooting an amp on the work bench.
Ringo Starr As a matter of fact the whole standby switch thing is based on a fallacy. It is about preventing cathode stripping, but you need kilovolts for that to occur, i.e. transmitter valves. See here: debontamps.com/standby-switches/. They became a kind of fashion statement. I've even seen them in solid-state amplifiers, don't ask me why, and in fact I was once chastised by a band guy whose SS amp I had used during the solo act, for turning it completely off instead of just back to standby. He was convinced it was harmful. Couldn't be told otherwise. And I've seen tube amps whose solution to this non-problem was to put -750V on the grids for 40 seconds: cure somewhat worse than the disease.
Yup you are correct about cathode stripping occurring at high voltages. That SS must have been pretty old to have a standby switch, perhaps an old Vox amp? However the SS Vox amps from way back disconnected the speaker in standby mode which would at least get rid of pops when the amp woke up stupidly.
What about amps without a Standby?
Thanks for your video btw, appreciate it 👍🏻
I know that a resistive discharge is better, removes the risk of damaging the cap and all but the dead short method is so much more fun ... 😉
Back when I was doing domestic field repairs, TVs mainly, I used to carry a ripple cap bypass tester made from a big cap with jump leads.
If a customers little dog just wouldn't go away I used to charge the cap up from the power supply then short the crocs.
Dog runs off in fright from the spark and the noise ... job done.
My amp doesn't have a standby switch. What's the procedure in this case? (besides buying another amp...)
Thanks
So when the standby switch is 'on' its actually an open circuit? Ie opposite of a light switch. As I (mis?) understand it the switch prevents the high voltage (from caps) from hitting a cold tube and creating an arc (hurting the cap and/or tube). So if you want to drain the caps grounding to the chassis, leaving standby ON creates a CLOSED circuit which allows the caps to drain.
So why not just go from the positive to negative on each cap?
I’m needing to work on a newer vibrolux. Kinda scared to mess with it now!
How about hifi tube amps without standby switch?
Does this apply with a standard solid state amp with a transformer? I'm new... lol
I have Nux amp which will sound like a bomb after it switch off.is that because of capacitor.
In circuit, usually the caps will mostly discharge on their own as evidenced by your meter, as the tubes still draw a small amount of current as their filaments cool. That’s why the sound fades out vs abruptly cutting out. Greatest risk comes from when performing tests with the preamp and power amp tubes removed as now there is no current drawn after power is removed. In any event, it is still a good safety practice to measure the B+ rail before performing any service, and discharging the caps if necessary.
I mitigate this in my designs by adding a high resistance (so it doesn’t load the power supply under normal operation) bleeder resistor across the caps so regardless of what state the amp is in, it discharges those caps when power is removed.