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.
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.
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?
I’ve been popped by one of these. Well, a Deluxe Reverb. I wouldn’t recommend it. Had to take a knee and check my vitals 😂. Safety practice is important, kids.
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.
Wouldn't a 450-volt shock be considered lethal? Scary stuff. A tube amp is not something you want to be poking around with unless you absolutely know what you are doing!
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.
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.
I bought this little box called a TKDMR Model: Td-CD01 Capacitor Discharger. It says suitable DC5 - 1000V that has these two little lights and says on it that discharge complete when light OFF. Is this suitable for capacitors in a modern amp? I have a Marshall DSL100HR head and can hardly find anything on it online other than I believe it as 2017 model amp. I'm wanting to discharge mine, so I can clean this damn yellow glue off the board they say could become conductive and seems to be touching certain components in the amp they say could later possibly short the map out and cause damage. The only amp repair guy around me wants to charge nearly $200.00 to do it. Its very little glue in a couple spots, and feel it's a rook for that much money. Or is this what amp tech charge now? or can I safely do this myself? I've always just played and had techs work on my stuff, mostly accept for biasing. Any advice?
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....
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.
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.
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?
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.
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?
Great video and guys like you are teachers to a student like we myself 😉...I'm looking at 2 used amps today..in your opinion which is the best value for my cash?? A used Hughes n ketnar black spirit 200 watt for 550 cash or a used peavey 3120 120 watt for $ 620
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?
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 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!
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?
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?
What about, unplug the amp. Plug your guitar into the amp and play through it. The residual electricity plays through the speaker, but it dissipates. After playing the guitar a few seconds, the volume trails off and is no longer heard through the speaker. Unplugged, try with switches on and off, guitar plugged into channel 1, then plug into channel 2 and repeat. Won’t that drain the juice out of the amp?
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.
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.
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.
I just put an alligator clip and lead on each side of the caps, and just short/spark them out together, I've been doing it like that for years no worries.
The moment you were talking about getting shocked... ive never seen your face so serious in any of your videos... never been zapped my self... musta sucked bad for you to get so serious(face) about it.
I did that to myself two nights ago! I was retubing a little Mesa Boogie combo-rather-I was trying to find the bad tube. Found a V3 shattered preamp tube and accidentally touched the filament with my thumb because only the glass came right off. I tell ya, without a hand in my pocket... I'd probably be fried chicken. That was one hell of a bite.
Hi Terry Did I buy the rite resistor its a 3 watt 100 ohms www.ebay.com/itm/183448625634?ViewItem=&item=183448625634 I have a 64 fender pro reverb / what large filter capacitors do you recomend , under that metal cover/ and why if you dont mind me asking, in the past I had my vintage amps gone thru but they sounded swimmy instead of dialed in is there a way of avoiding that Iam thinking of doing my own large filter caps I am very mechanical and was going to leave the rest to the amp tech thanks in advance/ Mike
What I don't understand is everybody says the same thing - don't put your hands inside the amp until you've discharged the caps. But you've got to put your hands in there to do that.
The reason the standby switch is relevant is that it interrupts the path to ground. Just putting one lead on ground, and then using the other to discharge the capacitor makes it easier. If you don't have a schematic, you can always discharge a capacitor using this method with one lead on one side of the capacitor and the other lead on the other side of the capacitor. You could even do that with the amp shown if the standby switch was "off." You can always discharge a capacitor by connecting a resistor between it's two legs. Always.
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
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
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.
Pomona and Philmore Electronics both make them . Likely available thru Mouser or Digi-Key. Last one I got was about 5 bux, but have found them at ham fairs for a buck or less.
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)
Hi Terry.....Great tip! Would hat to see someone find 'the death cap' the hard way! Please check back to your last video featuring 'shortwave girl.' My DX-160 is on the way!! :-) Thanks!
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. ;-)
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?
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???
