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Posted: Fri Sep 17, 2010 3:29 pm
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This may sound like a silly question....but..what is the safest way to drain the caps?? I hate all the waiting.
ABS :D


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Posted: Fri Sep 17, 2010 5:05 pm
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Yep, glad to hear you are going to scrub off the rest of the residue, because since carbon is a conductor, it gives you an unwanted connection between the two pins. It is a resistive path between the two pins, but still unwanted.
This little blurb is meant more for the non-technical ones among us.

Glad to hear that you may have gotten away without any serious damage.


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Post subject: Cap discharge
Posted: Fri Sep 17, 2010 5:24 pm
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airbornestrat wrote:
This may sound like a silly question....but..what is the safest way to drain the caps?? I hate all the waiting.
ABS :D


If you turn off the amp right after playing, while the tubes are warmed up, many times the caps will discharge on their own. If the caps still have a charge, use a pair of pliers to hold a 5-watt, 300-ohm resistor in the middle so as not to touch the leads (it can get hot). Then contact the resistor leads to the capacitor leads for a few seconds and they will safely discharge through the resistor. You can use different values on the resistor, but a smaller wattage and/or lower resistance will make the resistor get very hot. :)

Edit: Don't assume that the caps will be discharged after using the resistor once, double check with a volt meter and discharge again if necessary. The voltage will never be zero, but anything below 10 volts is OK.

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Posted: Fri Sep 17, 2010 5:30 pm
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stratele52 wrote:
Yes, I agreed with you if you like the sound.

have good time with your amp


I increased the bias to about 38 mA per tube tonight. The amp sounds really sweet. That is about as high as I want to go.

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Post subject: Re: Cap discharge
Posted: Fri Sep 17, 2010 5:30 pm
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shimmilou wrote:
airbornestrat wrote:
This may sound like a silly question....but..what is the safest way to drain the caps?? I hate all the waiting.
ABS :D


If you turn off the amp right after playing, while the tubes are warmed up, many times the caps will discharge on their own. If the caps still have a charge, use a pair of pliers to hold a 5-watt, 300-ohm resistor in the middle so as not to touch the leads (it can get hot). Then contact the resistor leads to the capacitor leads for a few seconds and they will safely discharge through the resistor. You can use different values on the resistor, but a smaller wattage and/or lower resistance will make the resistor get very hot. :)

Edit: Don't assume that the caps will be discharged after using the resistor once, double check with a volt meter and discharge again if necessary. The voltage will never be zero, but anything below 10 volts is OK.


Shim,
Thanks my brother. As always your advice is great!
ABS :D


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Posted: Fri Sep 17, 2010 5:33 pm
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LesK wrote:
Yep, glad to hear you are going to scrub off the rest of the residue, because since carbon is a conductor, it gives you an unwanted connection between the two pins. It is a resistive path between the two pins, but still unwanted.
This little blurb is meant more for the non-technical ones among us.

Glad to hear that you may have gotten away without any serious damage.


Guess you didn't read my followup post on the previous page regarding my cleanup procedure and results. :wink:

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Post subject: Re: Cap discharge
Posted: Fri Sep 17, 2010 5:37 pm
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shimmilou wrote:
airbornestrat wrote:
This may sound like a silly question....but..what is the safest way to drain the caps?? I hate all the waiting.
ABS :D


If you turn off the amp right after playing, while the tubes are warmed up, many times the caps will discharge on their own. If the caps still have a charge, use a pair of pliers to hold a 5-watt, 300-ohm resistor in the middle so as not to touch the leads (it can get hot). Then contact the resistor leads to the capacitor leads for a few seconds and they will safely discharge through the resistor. You can use different values on the resistor, but a smaller wattage and/or lower resistance will make the resistor get very hot. :)

Edit: Don't assume that the caps will be discharged after using the resistor once, double check with a volt meter and discharge again if necessary. The voltage will never be zero, but anything below 10 volts is OK.


I unplug the amp and turn the standby switch to the play position. You can watch the plate voltage drop with the DVM. Then I switch the DVM to read amps and touch the red probe to the cap and the black probe to ground. You can watch the current drain through the DVM.

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Post subject: Re: Cap discharge
Posted: Fri Sep 17, 2010 5:56 pm
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bluesky636 wrote:
I unplug the amp and turn the standby switch to the play position. You can watch the plate voltage drop with the DVM. Then I switch the DVM to read amps and touch the red probe to the cap and the black probe to ground. You can watch the current drain through the DVM.


That's cool, and handy! I am up for any method other than using a screwdriver. :lol:

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Posted: Fri Sep 17, 2010 10:07 pm
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Ah bull, I just use a Buck knife ! Yeah, I go the 16awg jumper cable with insulated alligator clips, a soldered & heatshrinked 470ohm 1 watt resistor in line(about 24") route. I drain the caps at a preamp plate for a minute and the output tube plate for a minute. Then check with the meter. I like the jumper, because it can stay hooked up while you work on the amp and there is no problem of a return charge stored in the amp. Art

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Post subject: Cap discharge
Posted: Sat Sep 18, 2010 9:27 am
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aclempoppi wrote:
Ah bull, I just use a Buck knife !......


:lol: C'mon! The most manly way would be to put both hands on the chassis, then touch tongue to cap and discharge through the fillings in your teeth. :lol: (obviously I'm joking)

Seriously though, I still know many "old electricians" that insist on using a screwdriver (or knife) from cap to ground! :shock: That can't be good for the caps! (or anything else). Believe it or not, I still have some engineering books from the 50s (somewhere?) and the authors actually talk about touching circuits with your hands to check for power, making sure to use the back of your hand to be.....ummm, safe? (anti-clamping maneuver) WTF!?! They also describe the difference in the TASTE of AC vs DC......!?!?!? And, yep you guessed it, they also say to make sure and discharge the capacitors with a screwdriver! :shock:

But, yes the pre-made jumper/resistor is a great idea too for discharging the caps. 8)

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Posted: Sun Sep 19, 2010 2:42 am
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Discharge filter capacitor with screwdriver to groud can damage the cap. This method ask too much current from the cap.

I use 1500 ohms 1/2 watts with jupers and I kept this connected almost all time I work in the amp . It's so small


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Posted: Sun Sep 19, 2010 6:23 am
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Here's another easy way. Connect a jumper from pin #1 from any 12AX7 preamp tube to the chassis and just give it a few minutes and all the voltage will drain from the circuit. As always, check the voltage. :wink:

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Posted: Sun Sep 19, 2010 1:46 pm
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Good trick Supro63, but don't fortget to put the Standby Swich On. Some Fenders amps have caps before this swith.


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Posted: Sun Sep 19, 2010 4:23 pm
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Excellent point, Stratele ! When going at the discharge from the circuit side of the amp, the standby must be in the "play" position. Otherwise, you're relying on the bleed resistors in the first cap resevoir, and some amps don't have these. Thanks for making that point, Stratele. Art

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Posted: Sun Sep 19, 2010 7:52 pm
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Sorry about that. Some things I just do automatically without much thought. I keep forgetting some folks are new at this. :oops: It does work well.

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