Filter cap on stout tmb

[taylorc315]

This may be a dumb question but i'll adk anyway. The filter cap in my 18 watt stout drain completly down just as soon as i shut the amp off. I'm using the jj 40/20/20/20 filter cap. The cap will get too 310 v and as soon as i shut it off, straight down too 0. Any suggestion would be greatly appreciated.😕

[HotBluePlates]

... The filter cap in my 18 watt stout drain completly down just as soon as i shut the amp off. ... The cap will get too 310 v and as soon as i shut it off, straight down too 0. ...

As long as the amp works well when the power is on, then it's not an issue. In fact, it means you're less likely to shock the crap out of yourself (I've been bit by 450vdc before, and I wouldn't recommend it  ).

You might be asking why that's happening (and I assume you haven't added a bleeder resistor from B+ to ground). When you shut the power off, the filter caps are no longer getting charge pumped into them from the power transformer. However, the tube cathodes are still hot and those tubes still draw current away from the filter caps. If you keep playing through the amp after powering off, you'll notice you still get sound for several seconds (but it gets softer in volume and eventually fizzles out). The "fizzle out" is where either the tube cathodes are too cool to continue emitting electrons, the filter caps are completely drained, or both.


If the amp didn't do this on its own (or maybe even if it does), it would probably be a good idea to have a 330-470kΩ resistor from B+ to ground at one of the filter caps, just to ensure B+ drops to 0v after the power is off. As I alluded earlier, it really sucks when an amp is off & unplugged, you think the filter caps are drained, and your hand slips and touches a high voltage point of the circuit.