Expert points. Brian Wang over 4 years ago. Please find the enclosure for the Vds data with temp and partial schematic.
Up 0 Down Cancel. Brett Barr1 over 4 years ago in reply to Brian Wang Brian, I am not entirely sure I understand your question but I have forwarded it to our apps team. I will let you know what our apps team's response is. Brett Barr1 over 4 years ago in reply to Brett Barr1. Ok so after talking with the apps guys, it makes more sense that your issue of voltage on the source node pertains to increased leakage through the FET.
Leakage generally increases, sometimes exponentially with temperature. Again though, as long as you stay within the junction temperatures listed on the datasheet, you should be fine, unless you have some specific minimum requirement on what you require this leakage and corresponding source voltage to be. Brian Wang over 4 years ago in reply to Brett Barr1.
Hello Brett, Thanks for reply. Please help on below questions following up to this. This involves using a 10k - 22k ohm power resistor with two crocadile clips at either end. This doesn't mean they are not dangerous.
PaoloJM , Mar 10, Older amps like the 60's and 70's Marshall's, even 80's may not have bleeders on them though. I know the oldest ones do not. They are easy to make if you need to work on them a lot.
American Viking , Mar 10, Joined: Mar 7, Messages: 17 Likes Received: 0. Haven't seen the scematics yet, but I will be shocked if they don't have proper bleeder resistors.
It would be a big oversight if they don't. PaoloJM , Mar 14, Like PaoloJM, I also always check voltage levels at the filter caps but I usually disregard whether or not an amp has a bleeder resistor and never bother looking for one. I'm so paranoid, even if I get zero voltage reading I still try draining the caps with a 20w ceramic power resistor.
The filter capacitors in an amp store enough high voltage DC to kill you, even when the amplifier is unplugged so you must check them for residual DC voltage and drain them if voltage above 30 volts DC is detected. Capacitors can hold their charge for days. Filter capacitors are not always grouped at the power transformer end of the amp chassis.
They can be placed anywhere on the circuit board. This is a 5E3 Deluxe with fixed bias so it has a bias filter cap big blue cap above. If the amp I'm going to work on is in working order I like to power it off with the standby switch in the "play" position. This will usually drain the caps down to a safe level.
I then verify all the caps have less than 30 volts. If they measure above 30 then I will drain them. Many amps like the Fender blackface and silver face amps have their filter caps located in a "doghouse" or metal covered area outside the chassis near the transformers.
Some amps use "cap cans" which are large multi-capacitor cans secured to the chassis by clamps. Many, but not all amps will drain their filter capacitors if you leave the standby switch on amp in playing mode and turn the amp power switch off.
The tube circuits will use up the stored energy in the caps and if you play the amp as you unplug it you'll hear the volume drop off as the caps drain. Unplug the power cord immediately so you won't forget to do it. Some amps have filter cap drain resistors in parallel with the filter caps to automatically drain them when the amp is powered down. But even if the amp you are working with has cap drain resistors you must verify ALL the amp's filter capacitors are discharged before going inside an amp chassis.
Use your multimeter to measure the DC voltage across the caps. You must check all the caps because many amps put some of the caps on the hot side of the standby switch so they can be isolated from the rest of the circuit and hold their charge. Be very careful when powering up an amp without the tubes installed.
Without the tubes to use up the stored capacitor voltage the circuit can hold higher than normal voltage for several days. I got bit by an amp because I expected it to self discharge like it always did before. You can build a capacitor discharge probe using a 2k 5 watt resistor to safely drain the caps' current to ground. The probe uses a resistor to control the current flow from the cap to ground.
Clip the probe's ground wire to the chassis or ground point and touch the probe tip to the filter caps' positive terminal to drain the stored current to ground. It may take up to a minute to drain the current to a safe level of less than 30 volts DC. Some people leave their drain probes connected during their work but too many have accidentally left them in the amp during power up.
If you plan to leave the probe connected put some colorful tape on it to make it more visible. Another cap drain technique is to short the cold tube side of a plate load resistor to ground.
In the 5E3 layout above the K plate load resistors are highlighted at lower right. The "cold side" of the plate load resistor is the tube end of the resistor. Short that end of the resistor to ground and the caps' current will flow through the plate load resistor to ground in a controlled manner.
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