Cap Draining

[FL6]

I'm hoping to start modding amps and was wondering if someone could give a tut on draining the capacitors for safety?
Explain it like I'm 5 kinda thing....where to set your multimeter, what to touch with the probes etc.
It would also be good to know how to measure that so you know it's drained and safe.

Also I was reading the zero loop mod and in the beginning it asks to measure a reading while the amp is running on full, would like to know how to do this safely...what not to touch etc.

Any beginners books on amps or electronics would be helpful too, gotta start somewhere.

Thanks.

[neikeel]

Make a piece of wire - preferably a bright colour (orange and yellow stripes??) of about 18-22 swg, solder a crocodile clip to each end.

When you have the amp open and are about to work on it place it on the bench. In the UK we have wall switches so I leave the amp plugged into the wall but switched off at teh wall, that way I have the chassis securely earthed (I know my house wiring is in excellent shape with 80mA trip protection breakers on each cicuit). In Europe and bits of the US I am not sure that applies.

Take one of the clips and secure it to the chassis (one of the cab mounting holes works well as the croc clip grips the captive nut threads well and does not ping off).

Next locate the turret junction of V1a plate resistor and the first coupling cap (on a marshall blue plate wire to pin 2 of V1) clip the other end of the croc wire to that turret. With the wall mains switch and the amp mains switch off throw the standby to on. This will drain all the caps through the B+ dropping resistors in a controlled manner into the chassis and off to ground. If it is a pcb then use the pin on the tube base.

Before you work on the amp use your voltmeter on DC volts between ground and the +ve pins of your filter caps to check less than 20v left in the caps and you are good to go.

Leave the wire connected when you work on the amp but do not forget to remove it before you fire up again (hence the bright colour).

[rsi]

It also helps to make one of these. There's a 2W resistor under the heat shrink in the middle.

Something like 10K-50K. The higher the value, the slower the caps will drain. Without a resistor it can spark and make a scary noise.

[FL6]

I have one of these.
https://taweber.powweb.com/amptechtools/stick.htm

But don't know what a turrent junction, plate resister, coupling cap looks like.
Is V1=valve 1? First preamp or power tube?

[rsi]

This is assuming you have a Marshall type circuit. For V1 (should be the tube at the input jack side of the amp), the plates are pins 1 and 6 (assuming it's not an EF86 amp or something). Pins are numbered counting clockwise. There should be a gap between pins that is larger than the other gaps. The gap is where you start counting.

Pins 1 and 6 should each be connected to a resistor and a cap soldered to the same point. Those resistors are plate resistors and the caps are coupling caps. The caps should be smaller ones, not the big can-type caps. It would help to know what amp you have in mind because if it's something like a Vox, then V1 may be an EF86 pentode which is completely different.

[FL6]

It's a Metro 12xxx

[danman]

Have you looked on youtube for videos of the cap draining procedures? Last year when I started getting interested in building an amplifier I looked at quite a few videos of biasing procedures and cap draining to get a better understanding of how to proceed. I spent nearly a year reading and researching the basics of electricity and amplifier layout before I was comfortable enough to service a live amplifier. The cap draining procedure will come easy once you see how its done but you must be very careful and know safe working habits before sticking your hand in a live amplifier to take measurements. One slip and you could shock your self pretty bad or burn out valuable components in your amp. Also study up on proper lead dress before building so that your amp will be easier to troubleshoot and modify later if needed. Bad lead dress can also cause a very noisy amp which will drive you crazy trying to fix. Invest the time to learn the basics and you will have a great time building your amp and hopefully enjoy it for a lifetime!

[neikeel]

It also helps to make one of these. There's a 2W resistor under the heat shrink in the middle.

Something like 10K-50K. The higher the value, the slower the caps will drain. Without a resistor it can spark and make a scary noise.

Not on a turret board Marshall - the dropping resistors control it for you

[FL6]

Have you looked on youtube for videos of the cap draining procedures? Last year when I started getting interested in building an amplifier I looked at quite a few videos of biasing procedures and cap draining to get a better understanding of how to proceed. I spent nearly a year reading and researching the basics of electricity and amplifier layout before I was comfortable enough to service a live amplifier. The cap draining procedure will come easy once you see how its done but you must be very careful and know safe working habits before sticking your hand in a live amplifier to take measurements. One slip and you could shock your self pretty bad or burn out valuable components in your amp. Also study up on proper lead dress before building so that your amp will be easier to troubleshoot and modify later if needed. Bad lead dress can also cause a very noisy amp which will drive you crazy trying to fix. Invest the time to learn the basics and you will have a great time building your amp and hopefully enjoy it for a lifetime!

Yes thanks, I ended up watching a few videos. I found this one to be the best. He also shows with the multi meter that letting the amp warm up then turn it off will drain the caps also, as opposed to turning it on and off quickly. The meter shows its draining faster than with a resistor/wire/clip thing. I think he says the warm output tubes help it drain. Not that anyone should blindly trust this method without measuring.
[youtube]http://www.youtube.com/watch?v=sw9KnSFy3Fs[/youtube]

[demonufo]

Yes thanks, I ended up watching a few videos. I found this one to be the best. He also shows with the multi meter that letting the amp warm up then turn it off will drain the caps also, as opposed to turning it on and off quickly. The meter shows its draining faster than with a resistor/wire/clip thing. I think he says the warm output tubes help it drain. Not that anyone should blindly trust this method without measuring.
[youtube]http://www.youtube.com/watch?v=sw9KnSFy3Fs[/youtube]

Absolutely you should not. It will differ with every amp, and is not foolproof. Always measure to see what the residual voltage is before working on anything. Never leave it to chance.