Guest thesloth Posted July 27, 2015 Share Posted July 27, 2015 (edited) Most tube circuits use high voltage. Maybe a sticky should be put on a thread with some warnings about how dangerous it is to work on tube gear. Before modding or servicing your tube gear it is important to bleed the capacitors. Most modern amps will have some bleed resistors already installed but some don't. Not sure how to do this? What I do is use a 2 watt metal oxide 100k resistor. I have leads soldered with insulated alligator clips to each end of the resistor. With one hand in my pocket I attach one lead of the resistor to ground, then I clip the other lead to one of the power supply filter capacitors. This can take some time so check with a volt meter until you reach a safe voltage. **************Some of the old cheaply mass produced amps (lots of console amps back in the day) did NOT use a power transformer, instead it was directly connected to mains. THESE ARE VERY DANGEROUS and should not be used unless retro fitted with an appropriately rated 1:1 isolation transformer.*********************************************************************************************** Edited July 30, 2015 by thesloth 2 Quote Link to comment Share on other sites More sharing options...
minermark Posted July 27, 2015 Share Posted July 27, 2015 Great Advice I think about this everytime i get near a circuit. Generaly the thought comes @ 000.2 seconds after I utilize my fingers 1 Quote Link to comment Share on other sites More sharing options...
mike stehr Posted July 28, 2015 Share Posted July 28, 2015 (edited) Great Advice I think about this everytime i get near a circuit. Generaly the thought comes @ 000.2 seconds after I utilize my fingers As long as you only got one hand in there...the idea is not to get shocked at all. But don't get both hands in there and complete a circuit through your chest. Edited July 28, 2015 by mike stehr 1 Quote Link to comment Share on other sites More sharing options...
Taz Posted July 28, 2015 Share Posted July 28, 2015 (edited) Can't Ya just touch both sides of the capacitor with you fingers to get a discharge? Says the guy that still remembers sticking tweezers into the wall socket at 2 years of age. Edited July 28, 2015 by Taz 2 Quote Link to comment Share on other sites More sharing options...
tube fanatic Posted July 28, 2015 Share Posted July 28, 2015 Most tube circuits use high voltage. Maybe a sticky should be put on a thread with some warnings about how dangerous it is to work on tube gear. Before modding or servicing your tube gear it is important to bleed the capacitors. Most modern amps will have some bleed resistors already installed but some don't. Not sure how to do this? What I do is use a 2 watt metal oxide 100k resistor. I have leads soldered with insulated alligator clips to each end of the resistor. With one hand in my pocket I attach one lead of the resistor to ground, then I clip the other lead to one of the power supply filter capacitors. This can take some time so check with a volt meter until you reach a safe voltage. This is wonderful advice. I'll add to it that even with transformer operated equipment it is always worth using an isolation transformer when doing any kind of servicing. Remember that the power cord is connected to the mains, and it is always possible to touch the hot lead while grounded. Definitely not a good scenario! Also, be sure to have something on your feet (like rubber soled shoes or sneakers), and NEVER work on a metal table. Maynard Quote Link to comment Share on other sites More sharing options...
thebes Posted July 28, 2015 Share Posted July 28, 2015 Most tube circuits use high voltage. Maybe a sticky should be put on a thread with some warnings about how dangerous it is to work on tube gear. Before modding or servicing your tube gear it is important to bleed the capacitors. Most modern amps will have some bleed resistors already installed but some don't. Not sure how to do this? What I do is use a 2 watt metal oxide 100k resistor. I have leads soldered with insulated alligator clips to each end of the resistor. With one hand in my pocket I attach one lead of the resistor to ground, then I clip the other lead to one of the power supply filter capacitors. This can take some time so check with a volt meter until you reach a safe voltage. Our good Forum friend MikeBSA2A3 taught me to make up something similar, but I use it across the plus and minus pins of the power caps. Of course, sometimes I forget... 1 Quote Link to comment Share on other sites More sharing options...
derrickdj1 Posted July 28, 2015 Share Posted July 28, 2015 Can't you just unplug the amp for a few hours? Quote Link to comment Share on other sites More sharing options...
Guest thesloth Posted July 28, 2015 Share Posted July 28, 2015 Most tube circuits use high voltage. Maybe a sticky should be put on a thread with some warnings about how dangerous it is to work on tube gear. Before modding or servicing your tube gear it is important to bleed the capacitors. Most modern amps will have some bleed resistors already installed but some don't. Not sure how to do this? What I do is use a 2 watt metal oxide 100k resistor. I have leads soldered with insulated alligator clips to each end of the resistor. With one hand in my pocket I attach one lead of the resistor to ground, then I clip the other lead to one of the power supply filter capacitors. This can take some time so check with a volt meter until you reach a safe voltage. Our good Forum friend MikeBSA2A3 taught me to make up something similar, but I use it across the plus and minus pins of the power caps. Of course, sometimes I forget... Doing this is usually okay, but I have seen people use two capacitors in series (the voltage rating of the two caps sum) WITHOUT balancing resistors. This is why I like to advice to attach to a known ground. The chassis should be earthed so using the chassis is a good idea. There is also a method to my madness, attaching the resistor to the ground before the cap ensures the resistor is more at earths potential then you are. 1 Quote Link to comment Share on other sites More sharing options...
Guest thesloth Posted July 28, 2015 Share Posted July 28, 2015 Can't you just unplug the amp for a few hours? This doesn't always work. It really depends on the amp. I have seen plenty of amps come to me and there be 500v sitting on the caps waiting for me. Quote Link to comment Share on other sites More sharing options...
minermark Posted July 28, 2015 Share Posted July 28, 2015 Most tube circuits use high voltage. Maybe a sticky should be put on a thread with some warnings about how dangerous it is to work on tube gear. Before modding or servicing your tube gear it is important to bleed the capacitors. Most modern amps will have some bleed resistors already installed but some don't. Not sure how to do this? What I do is use a 2 watt metal oxide 100k resistor. I have leads soldered with insulated alligator clips to each end of the resistor. With one hand in my pocket I attach one lead of the resistor to ground, then I clip the other lead to one of the power supply filter capacitors. This can take some time so check with a volt meter until you reach a safe voltage. Our good Forum friend MikeBSA2A3 taught me to make up something similar, but I use it across the plus and minus pins of the power caps. Of course, sometimes I forget... Doing this is usually okay, but I have seen people use two capacitors in series (the voltage rating of the two caps sum) WITHOUT balancing resistors. This is why I like to advice to attach to a known ground. The chassis should be earthed so using the chassis is a good idea. There is also a method to my madness, attaching the resistor to the ground before the cap ensures the resistor is more at earths potential then you are. There now is the "Key word! "potential" i was always told i had that Quote Link to comment Share on other sites More sharing options...
ZEUS121996 Posted July 28, 2015 Share Posted July 28, 2015 7 more posts to hit the BIG TIME C'mon MM you can do it !!!!! Mark 2 Quote Link to comment Share on other sites More sharing options...
mpeg2 Posted July 28, 2015 Share Posted July 28, 2015 One hand in the pocket is always good advice, as is discharging the power caps via a bleed resistor. Another (which I always follow) is to check that the high voltage is bled off by using a meter to read the actual voltage on the HV side. I don't stick my fingers in there until the meter shows that the voltage is really bled off. I've had enough shocks in the past (in my young & naive days) to pay attention to this. Rich 1 Quote Link to comment Share on other sites More sharing options...
Guest thesloth Posted July 28, 2015 Share Posted July 28, 2015 One hand in the pocket is always good advice, as is discharging the power caps via a bleed resistor. Another (which I always follow) is to check that the high voltage is bled off by using a meter to read the actual voltage on the HV side. I don't stick my fingers in there until the meter shows that the voltage is really bled off. I've had enough shocks in the past (in my young & naive days) to pay attention to this. Rich Yes, always check with a meter before poking around after discharging. I actually just leave my meter connected and watch it drop. If you are impatient you can use a lower value resistor, say 10k. It will just get warmer so watch the wattage rating. 400v supply and a 10k resistor is: 400/10000=.04 .04*400= 16 watts a 2 watt metal oxide power resistor will handle this for short periods. Quote Link to comment Share on other sites More sharing options...
minermark Posted July 30, 2015 Share Posted July 30, 2015 My early learning stage was in the Ham radio hobby, and no one is in that very long before they figure out POWER Rules! Yeah baby we are talkin Eimak tubes here, just about any tube that has an HV wire going to the top of it, built many a Linear Amp growing up, i think iv been zapped by every dam one. Never gave caps much thought, ground a screwdriver with a clip lead and jump around the circuit shorting leads, gets most every charge. but that is just me Quote Link to comment Share on other sites More sharing options...
Don Richard Posted July 30, 2015 Share Posted July 30, 2015 I always solder a 100K resistor from B+ to ground. This only draws a few milliamps, and the unit self-bleeds when turned off. Quote Link to comment Share on other sites More sharing options...
Guest thesloth Posted July 30, 2015 Share Posted July 30, 2015 I added a brief warning in my original post about some old tube gear that didn't use a power transformer. Some of the other forums I belong to ban the discussion of working on these unless they are retro fitted with an isolation transformer. User beware they are very dangerous. Quote Link to comment Share on other sites More sharing options...
Dr Morbius Posted August 2, 2015 Share Posted August 2, 2015 I can understand not having a power transformer is pretty much deadly! Never came across that situation, thankfully. Quote Link to comment Share on other sites More sharing options...
erik2A3 Posted August 2, 2015 Share Posted August 2, 2015 We've discussed this many times in the past. A step further in this thread: Of course it's essential to discharge PSU capacitors in circuits that don't have bleeders across the output of the power supply. HOWEVER: More importantly, if you are unfamiliar with audio electronics or have minimal experience in electronics in general or are uncertain about what you are doing (such as what leads or tube socket (plate) pins may present high voltage storage charges, or how to interpret positive and negative connections on filter capacitors, or understand the nature of the design of the power supply of the particular component in question - do not proceed with the installation of power resistors to bleed dangerously high voltages to ground. I have learned from experience that it is not something about which one should be casual or take lightly. Keeping one hand in one's pocket is the generations-old practice to prevent electrocution between one's hands and through one's heart, but you should have a very clear understanding of what you are doing before that. Since I joined this forum about twelve years ago, there have been times members posted about how they, on other forum member's dare, decided to solder and make connections in a chassis that was plugged into the house mains and fully energized -- and how they subsequently unknowingly touched something that was carrying high AC or DC current and instantly received an extremely hard and painful shock. Trust me when I say that it's not only unpleasant (as some contributors here will know) but potentially truly lethal. I have had amplifiers sent to me that had been unplugged and out of service for several days, yet still had hundreds of volts in reservoir capacitors. Discharging caps and obviously unplugging a component to be serviced is certainly smart. Be comfortable and familiar with working on high voltage electronics first. Quote Link to comment Share on other sites More sharing options...
ricktate Posted August 2, 2015 Share Posted August 2, 2015 Well you know those old tvs they had a great big tube in the back with a wire on top of it looked like a spark plug wire. Well DO NOT TOUCH THAT when tv is plugged in.... that HURTS. lol. That's my lesson in turning off and unplugging I learned at about 12 years old. Rick Quote Link to comment Share on other sites More sharing options...
tube fanatic Posted August 2, 2015 Share Posted August 2, 2015 (edited) I can understand not having a power transformer is pretty much deadly! Never came across that situation, thankfully. The most popular budget radios of the 30s through the 60s connected the tube filaments in a series string which added up to approx. 120V. Since they used a non-polarized power cord the chassis could be "hot" with the full 120V with the radio either on or off, depending on which way the plug was inserted in the socket. This circuit arrangement was also used in less costly amplifiers and console stereo units. Don't ignore this type of audio equipment if it comes your way (you would be amazed at how nice the sound can be with these). By using an isolation transformer, these units can be made as "safe" (a relative term since vintage equipment generally couldn't meet modern electrical safety standards) as other vintage equipment; and, there's a simple modification which can be employed which can eliminate the "hot" chassis. Maynard Edited August 2, 2015 by tube fanatic Quote Link to comment Share on other sites More sharing options...
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