#1
How to drain your filter caps, True or False...

To drain your filter caps, unplug the amp from the power, turn the "on" and "stanby" switch on with the speaker plugged in and just leave it for 10 minutes and then it should be fine.

Best to check with a meter afterward to be safe.
#2
Pretty much but it depends on the amp. If there's no drain resistor then the caps won't discharge below about 100V for quite some time.

How you do it is to get/make a probe with a nice handle with an alligator clip to one end. So butcher an old multi meter probe. Ground the alligator end then touch the probe to the positive terminals of the capacitors. It's not hard, just keep your pinky finger tucked in - that little bastard is always sticking out touching things if you don't watch him.
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band
#4
And of course make sure it's unplugged from the mains.
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band
#5
you'll get sparks if you do that. of course you can for some excitement, put a 100k 5w resistor in series just in case. if an amp operates at 500v and the caps have 500 on them, 100k to ground would see 500v/100k, 5mA current, which is 2.5w.

this is an extreme case, and hopefully your caps dont have full HT on them. but if you turn on the amp and turn off before the tubes are warm, theres a chance a 500v amp will have more than 400v on the caps a few minutes later.
Call me "Shot".

ShotRod Guitar Works

Custom Hand-wired Amplifiers and Effect Pedals.

Est. 2007


Source to everything I say about Guitars, Pedals, and Amplifiers: I make them.


UG's Best DIY PedalBoard
#10
It never hurts to play it safe when dealing with high voltage caps. Even if you can verify that your filter caps are drained via the standby method it still only takes a minute or so to manually drain them, just to be 100 percent certain, with a tool designed specifically to do so. A fully charged or even half charged 400v capacitor can stop your heart faster than you can blink your eyes.

Isn't your life worth a couple of minutes wait and a simple capacitor draining tool?
Last edited by sparkeyjames at Mar 25, 2012,
#11
I like sparks, you know the job is done. With no resistor they discharge faster, pretty much instantaneously. With 100K resistor it's a fairly slow process, I gave up on them. What do sparks hurt?
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band
#12
Quote by Cathbard
I like sparks, you know the job is done. With no resistor they discharge faster, pretty much instantaneously. With 100K resistor it's a fairly slow process, I gave up on them. What do sparks hurt?


The first time you do that and weld your article of choice to the cap wire you'll learn why it's not a good idea to do that.
#13
Usually, there is enough of the cap leads to attach an alligator clip to.
I attach one end of my cap draining probe (with a 20k 5W resistor in line) to ground and the other end to the cap.
Then go outside and have a smoke or something.
And I always measure the cap to make sure its drained.
#14
I measure it from the start and when the cap voltage gets below 50v or so I short the resistor. Little spark, not a big one.
Call me "Shot".

ShotRod Guitar Works

Custom Hand-wired Amplifiers and Effect Pedals.

Est. 2007


Source to everything I say about Guitars, Pedals, and Amplifiers: I make them.


UG's Best DIY PedalBoard
#15
Quote by sparkeyjames
The first time you do that and weld your article of choice to the cap wire you'll learn why it's not a good idea to do that.

Hasn't happened in 30 years of doing it. Get little marks on the capacitor leads at times but never had one actually weld into place. How big do you think these caps are? It's an amp, not a telephone exchange.
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band
#16
Quote by Cathbard
I like sparks, you know the job is done. With no resistor they discharge faster, pretty much instantaneously. With 100K resistor it's a fairly slow process, I gave up on them. What do sparks hurt?

I typically use smaller resistors to discharge caps for this reason, anywhere from 100 - 1k. Just make sure it has a decent wattage rating so it doesn't burn up.