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Post subject: Amp Safety Question
Posted: Thu Apr 08, 2010 10:30 pm
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My new JJ's from Eurotubes are due in maybe tomorrow. Since my newly delivered amp had a bum tube in it, I have already been through the taking out of the tubes and putting them back in. In preparation for the new tube change and other things I might have to do, like change the fuses, I was wondering about the question of safety so I did some reading.

Everyone talks about dishcharging the amp, this is obvious, but I wanted to confirm a couple of things.

1. I have read that you can discharge the amp, if the tubes are warmed up, by turning it off, unplugging and playing the guitar until the sound stops in the amp. This is said to "discharge the capacitors through the tubes". True or false or somewhere in between?

2. Is it necesary or recomended to discharge the amp when the only work to be done is changing tubes?

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Posted: Fri Apr 09, 2010 4:23 am
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I'm not expert so I hope someone else jumps in...

Regarding #1, I've heard about that method, I guess it's kinda like doing the off by stand-by trick: turn the power on and stand-by "on" then flip the power off while leaving the stand-by on. This supposedly discharges the caps too but I've tested it and still found high voltage readings in the caps.

Another way to discharge the caps is by shorting them to the chassis. Get and insulated screwdriver, put the tip against the cap lead and then the other end of the screwdriver shaft against the chassis. I've seen videos of Cesear Diaz doing it this way but I'm a little iffy on this one too. There's a pop and the chance of leaving a burn mark on the chassis.

Whenever I drain the caps I use a resistor to drain the load down to zero.

Be sure to check ALL the caps to make sure none of them have any residual charge.

For #2 unless you are taking the chassis off or will be working next to the circuit I don't think it's necessary to drain the caps. For instance on my HRDx when you take the back panel off the circuit and caps are right in front of you so I'd take the time to drain the caps just in case. On my Vibroverb the tubes are under the chassis and you can't get near the caps unless you remove the chassis so there's no danger when changing tubes.

The most important thing is your safety. Amps can be pretty dangerous if you are not carefull. Wear a pair of sneakers and if possible stand on a rubber mat. Keep one hand in your pocket at all times, don't put both of your hands into the chassis. If you're not 100% confident take it to a tech.

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Posted: Fri Apr 09, 2010 6:45 am
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1. Google 'draining filter capacitor' and read up on all of the safe and correct ways to do it. Here http://www.google.com/search?hl=en&sour ... r&gs_rfai= There are many ways to do it (both safe and unsafe) and no matter what you choose to do, how are going to know if the caps are fully discharged? Do you have a multimeter to check them? You only need to discharge caps if you are going to be poking around the circuit board. Like Voodoo Blues said above, I'm iffy too about the touching the caps to the chassis with a screwdriver method!

2. No you do not need to drain the caps when changing tubes or replacing a fuse (turn amp off and unplug from wall when replacing a fuse). Just don't touch anything on the circuit board!

I see in your signature you have a Blues Junior, just plug the new tubes in and that's it. You don't even need to set the bias when replacing the EL84 power tubes in a BJ as long as the tubes are matched duet, which I'm sure Eurotubes took care of.

Here is a popular Blues Junior site that will have more info: http://billmaudio.com/wp/

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