Members bluesboy Posted June 22, 2007 Members Share Posted June 22, 2007 I recently bought the bitmo trio mod kit for my epi valve jr. I have read/heard that voltages inside the amp can be very dangerous. What is the proper way to discharge the capctors so it's safe to work on? Link to comment Share on other sites More sharing options...
Members Weathered Posted June 22, 2007 Members Share Posted June 22, 2007 From Randall Aiken's website: Electrolytic (and other type) filter capacitors can hold lethal charges - be sure to discharge them before working on an amplifier. This can be done by shorting the "+" side to the "-" side, or the "+" side to the chassis (unless the cap is in a negative voltage supply, like the bias supply, where you must short the "-" side to the chassis), using a wire or a screwdriver. This will make a big spark and a loud noise, which is sure to impress any onlookers. If you'd rather do it more discreetly, make a jumper cable consisting of alligator clips on the ends, with a resistor in series in the middle of the jumper. Use a value of somewhere between 10K and 100K, with a power rating of 5W or so (a high power rating is not really necessary for dissipation reasons, but mainly because the higher power resistors are physically more robust and won't tend to break). Be sure to adequately insulate the resistor with heat-shrink tubing so the wires aren't exposed, or you'll get shocked while trying to discharge the capacitors. The larger the value of the resistor, the longer it will take to discharge the caps, and the less of a spark you will see. If you use a large value resistor, be sure to leave it in place long enough for it to drain all the charge out of the capacitor (if in doubt, measure the voltage across the capacitor to make sure it is close to zero). Note that many amplifiers will incorporate "bleeder" resistors, which will drain the capacitor charge automatically in a few minutes after the amp is turned off. Don't depend on this resistor to do it's job! Link to comment Share on other sites More sharing options...
Members bluesboy Posted June 22, 2007 Author Members Share Posted June 22, 2007 hmmm... i dont really understand that. I mean, theoritcally I get it, but i dont know how to actually do that. maybe i shouoldnt try to put the kit in myself Link to comment Share on other sites More sharing options...
Members bluesboy Posted June 23, 2007 Author Members Share Posted June 23, 2007 any other opinions/advice/suggesstions? Link to comment Share on other sites More sharing options...
Members 496dart Posted June 23, 2007 Members Share Posted June 23, 2007 If you dont know how to discharge a cap, you should not be installing any kits in an amp. Link to comment Share on other sites More sharing options...
Members Boris the Blade Posted June 23, 2007 Members Share Posted June 23, 2007 I think VJ's have a bleeder resistor in the amp. But as stated, don't rely on this resistor because you never know. On the newer VJ's, it's not simple to drain the capacitors across the cap without removing the whole circuit board, and that would be quite dangerous. iirc, You can drain the capacitors by putting one side of the resistor on pin 3 of the power tube socket and the other side on the chassis and holding it there for about 25 seconds. You should also be able to test the voltage with a multimeter this same way. the last resistor I used when opening up an amp was an xicon 25k 10w cement block resistor. And yes, if you don't know what you are doing, do not mess with it Link to comment Share on other sites More sharing options...
Members mikethedigimon Posted June 23, 2007 Members Share Posted June 23, 2007 I know Weber speakers makes a cap discharger which is called a snuffer stick. After seeing a pic of one, I decided to make one of my own. I used 4 68k resistors (.5 watt each), and a bic pen. Link to comment Share on other sites More sharing options...
Members 50wHead&2x12Cab Posted June 23, 2007 Members Share Posted June 23, 2007 While we are on the VJ modding subject, how difficult/dangerous is it to upgrade the transformer in the VJ? Do you have to solder anything, or is it just a few screws and a plug? Is there anything else in the VJ head besides tubes that can be modded by a novice without risk of life? Link to comment Share on other sites More sharing options...
Members Boris the Blade Posted June 23, 2007 Members Share Posted June 23, 2007 The most popular OT I've seen used on VJ's are Hammond's. I think the 125ESE was a recommended one iirc. I also believe that you need to drill a new hole because the Hammond does not line up perfectly with the old OT holes. And yes, soldering is a requirement. Link to comment Share on other sites More sharing options...
Members bluesboy Posted June 23, 2007 Author Members Share Posted June 23, 2007 well, i dont feel comfortable doing it... i dont care if i ruin the amp, but i dont want to hurt myself. anybody know someone i can send it to to do it for me? Link to comment Share on other sites More sharing options...
Members bluesboy Posted June 23, 2007 Author Members Share Posted June 23, 2007 anybody know who puts things like these in? any ideas? how about somebody here? ILL PAY YOU AMERICAN DOLLARS! Link to comment Share on other sites More sharing options...
Members 496dart Posted June 23, 2007 Members Share Posted June 23, 2007 pm me bluesboy Link to comment Share on other sites More sharing options...
Members clay_finley Posted June 23, 2007 Members Share Posted June 23, 2007 On my old Marshalls, I just use a jumper wire. I connect to ground on one side and pin 1 of v1 to the other. It discharges thru the 100k plate resistor on V1. After I've let the discharge for a long enough time, I take a screwdriver and touch the caps to ground, just in case my discharge wire didn't make a good connection or something. Just to be safe... Link to comment Share on other sites More sharing options...
Members bluesboy Posted June 23, 2007 Author Members Share Posted June 23, 2007 On my old Marshalls, I just use a jumper wire. I connect to ground on one side and pin 1 of v1 to the other.It discharges thru the 100k plate resistor on V1.After I've let the discharge for a long enough time, I take a screwdriver and touch the caps to ground, just in case my discharge wire didn't make a good connection or something. Just to be safe... i understand this suggestion more... thanks for the info Link to comment Share on other sites More sharing options...
Members BillM Posted June 23, 2007 Members Share Posted June 23, 2007 ground the plate on any preamp tube to the chassis. pin #1 and #6 are the plate on a 12ax7. Link to comment Share on other sites More sharing options...
Members mamberg Posted June 23, 2007 Members Share Posted June 23, 2007 dude, just do it.......... the caps on the Valve Jr. ARE self bleeding, there is no voltage there. The OT transformer is easy, I did it as my first mod ever. You have to un-solder and solder a few wires, but it was easy. the statements above only apply to the VJr., no other amps..... Link to comment Share on other sites More sharing options...
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