need help bleeding PLEASE!

[Robmosis]

Hello everyone. I am fairly new to tube amp maintenance but I am learning as much as I can before attempting any mods or kits. I bought Dave Hunters book (The Guitar Amp Handbook) and am having a blast learning how everything works and what does what. I am the kind of person that is not satisfied with just having something work right for me, I have to know how and why. BUT I don't understand why everywhere I look for info I get a warning about the dangers(Which I do take seriously) of the voltages but never any explanation on how to drain it correctly. I understand that people don't want to feel responsible for someone who gets hurt or possibly killed, but us noobs who are serious and want to learn the best and correct ways to be safe would appreciate If someone would help us out and tell us how to do it correctly. I own a B-52 AT-100 tube head and when I feel comfortable with what I know I would like to do some mods.
Sorry for being so longwinded but I feel frustrated that I can't find the correct process. If anyone would be able to help I would appreciate it greatly. I will post a schematic on my amp if it helps to be more specific on my amp.
Thanks for the forum Rob
B52 AT100 schematic.pdf

[Brian Evans]

When the amp is turned on, the power supply creates voltages up to 450 odd volts and has some current behind it, so very dangerous. Don't touch anything inside of an amp chassis when it's powered up. When it's plugged in and not turned on, there are open 120 volt sources inside the chassis - so don't touch anything inside the chassis when it's plugged in and turned off, either.

what's sneaky are the capacitors. In addition to smoothing and filtering, they take on a charge and act like a battery once the amp has been turned off. If you look at a power supply filter cap that's filtering 400 volts DC a couple of hours after you turn off the amp - it still has a couple of hundred volts stored inside of it. Since you are a pretty good conductor of electricity, if you touch the lead of a cap like that, you'll get a real good jolt.

To discharge a capacitor you take a resistor - say a 500K - and connect it across the leads of the capacitor. That allows the voltage inside the cap to dissipate through the resistor. The trick is doing that inside the chassis with a well insulated probe that you cunningly created just for that task, so you don't get zapped.

When I was a kid in school, we used to charge up big capacitors from computer power supplies - 10" tall, three inches in diameter - and see how big a piece of wire we could vaporize by shorting out the leads of the cap. When I was a bench tech doing circuit board repair on digital electronic stuff, we used to charge up little tiny tantalum caps and toss them at people...kinda like a decent static electricity jolt... Ah the days of our foolish youth...

I learned how to be cautious doing CRT display repair. Those suckers store 40 thousand volts and levitate you across the room...

The trick for a newbie is knowing ALL of the things that can hurt you inside the amp, and avoiding ALL of them... It's a skill you can learn...


Brian

[tubeswell]

Another way is to use the amps own resistors to discharge the filter caps. Take a lead with an insulated alligator clip at each end and connect one end the chassis first, and then connect the other end to the plate-side of any resistor in the preamp. You will need to wait about 30 seconds for it to discharge. Then check the voltages with your meter. Or you can use your 1000s voltmeter (with secure, insulated alligator clips attached to it for this) and watch the discharge in progress.

[txstrat]

When building an amp you could put a 220k 2W resistor parallel to each of the first two filter caps (I do this ALWAYS). One before and one after the stand by switch. This will discharge the caps "automaticly" as soon as you switch the power off.

[Enzo]

And since we have discussed this many times, do a search on this forum for cap bleeding or cap discharging.

[Robmosis]

Much much appreciated

Thanks to all of your responses to my questions. I really do appreciate it. Before I decided to post this question I tried to search the site for other posts on this subject. I must not have been using the right terms in my search or I used a long winded search. After I posted I continued to try and I did finally find some great advice on how to do it.
I have said this before but I find M.E.F to be the finest electronics forum around( trust me I have looked at a lot). Everyone seems to be so willing to help out. Even noobs that are serious about learning the dying art of tube amp electronics are treated with respect. There is so much bickering and intimidation on many forums. So to all of the members that I see answering all of the questions, Thanks a million volts.
P.S. I still do not understand why such good informative books like Dave Hunters do not put in a chapter on safe procedures of tube amps. There are enough warnings about the dangers and even parts stating that the authors take no responsibility for injurys. This should cover their butts if needed so there is no reason not to make sure that everyone is as safe as can be.
Thanks again, Rob