Draining an Amp to MOd

wetland10 · March 29th, 2008, 01:09 AM

I just purchased an Epiphone Valve Junior and want to do a ssimple Bitmo mod. However, I am afraid of shocking my brains out! I've wired guitars before, but never an amp. THe mod seems simple enough. But, what exactly do I need to drain first and how do I drain it?

Any help is greatly apreciated.

Thanks,
Wayne

Scott S · March 29th, 2008, 05:03 AM

You'll want to attach one end of the alligator lead to the chassis, then touch the other end to the "+" side of the capacitors. The cool kids install bleeder resistors in amps that don't have them already.

- Scott

wetland10 · March 29th, 2008, 08:44 AM

Sounds like a lot of work, depending on how many caps are in there. I haven't looked yet.

Any specific type of alligator clips?

Wayne

DrewB · March 29th, 2008, 08:47 AM

I put that mod into a couple of those amps, and I didn't bother draining them. Foolhardy? Maybe, but you're working on the ground side of the amp and only have to touch two connections (IIRC) on the board. If you don't stray from touching things only in that area, you should be fine.

I have a couple of alligator clips connected to the legs of a 5W / 470-ohm resistor that I use to bleed amps. I clip it between the positive lead of a filter cap and the chassis and leave it in place while I'm working. You can't do that in that amp because the caps have radial leads, so clip it to the plate lead (pin 1 or 6) of the first preamp tube. Since the sockets are mounted on the board in that amp, figure out which solder connection is pin 6 and clip onto the closest connection to that pin (follow the trace from that solder connection to the resistor tied to it).

wetland10 · March 29th, 2008, 08:50 AM

Quote:

Originally Posted by DrewB

I put that mod into a couple of those amps, and I didn't bother draining them. Foolhardy? Maybe, but you're working on the ground side of the amp and only have to touch two connections (IIRC) on the board. If you don't stray from touching things only in that area, you should be fine.

I have a couple of alligator clips connected to the legs of a 5W / 470-ohm resistor that I use to bleed amps. I clip it between the positive lead of a filter cap and the chassis and leave it in place while I'm working. You can't do that in that amp because the caps have radial leads, so clip it to the plate lead (pin 1 or 6) of the first preamp tube. Since the sockets are mounted on the board in that amp, figure out which solder connection is pin 6 and clip onto the closest connection to that pin (follow the trace from that solder connection to the resistor tied to it).

Interesting. I'm not all that familiar with working on amps, guitars seem much easier. If it is not a problem to do those mods without draining, might make life easier since I don't want to do something wrong a blow it out.

Either that or just bring to someone else to do.

Wayne

jefrs · March 29th, 2008, 09:23 AM

1. Always, but always unplug from the mains. If the mains doesn't kill you, it hurts a lot.

2. After it's switched off, wait for the caps to bleed out their charge a bit, before going in with your fingers.

3. Then I just dump the big caps with an /insulated/ screwdriver. Or a wire with /insulated/ croc clips. Resistor not required.

4. Anything much bigger than the usual 50µF, like 1mF (yes, that's milli-Farad) needs to have a big wire soldered across its connections. You are not dealing with those brutes.

5. Everything in an amplifier obeys Murphy's law.

6. If you don't unplug, you will always touch the back of the mains switch!

Guitarslinger1 · March 29th, 2008, 11:36 AM

"3. Then I just dump the big caps with an /insulated/ screwdriver. Or a wire with /insulated/ croc clips. Resistor not required."

Discharging the caps this way is not good for them, and it's not so great for the screwdriver or other shorting device either.

It is preferable to use an alligator clip power resistor arrangement to slowly bleed off the charge. Discharging the caps slowly is better for everything.

One way it can be safely done without a power resistor, is to use a wire jumper with alligator clips, and clip one end to ground and the other end to pin 1 or pin 6 of one of the preamp tubes. This discharges the caps slowly through the preamp tubes plate resistor, and the series resistors in the power supply. Very safe and easy on the caps.

G-P · March 29th, 2008, 03:24 PM

Wetland, I suggest you just take the amp to a tech. Guitar wiring is passive, no voltage or currents involved. Amplifiers are an entirely different beast and will bite you before you even know what happened. They can bite very hard.

Proceed at your own risk, but given the tone of your questions and statements, I don't get the sense that you are very familiar with the workings of an amplifier. Save yourself some grief and take it to a tech. Or at least study up on some basic amplifier tech. Some excellent books are available on the topic and excellent forums, including this one.

I haven't heard of the bitmo mod. What does it entail?

wetland10 · March 29th, 2008, 06:45 PM

Quote:

Originally Posted by G-P

Wetland, I suggest you just take the amp to a tech. Guitar wiring is passive, no voltage or currents involved. Amplifiers are an entirely different beast and will bite you before you even know what happened. They can bite very hard.

Proceed at your own risk, but given the tone of your questions and statements, I don't get the sense that you are very familiar with the workings of an amplifier. Save yourself some grief and take it to a tech. Or at least study up on some basic amplifier tech. Some excellent books are available on the topic and excellent forums, including this one.

I haven't heard of the bitmo mod. What does it entail?

The Bitmo mod is this:

http://thetubestore.com/bmspanky.html

I talked to my guitar tech today and he suggested a local guy that works specifically on this amp. He agreed that it is not worth messing around since guitars and amps are two very different things. He is supposed to get me the guy's contact information. Seems to be the safest bet.

Thanks for the suggestions,.

Wayne

jefrs · April 2nd, 2008, 09:18 PM

Quote:

Originally Posted by specialty guitars

"3. Then I just dump the big caps with an /insulated/ screwdriver. Or a wire with /insulated/ croc clips. Resistor not required."

Discharging the caps this way is not good for them, and it's not so great for the screwdriver or other shorting device either.

It is preferable to use an alligator clip power resistor arrangement to slowly bleed off the charge. Discharging the caps slowly is better for everything.

One way it can be safely done without a power resistor, is to use a wire jumper with alligator clips, and clip one end to ground and the other end to pin 1 or pin 6 of one of the preamp tubes. This discharges the caps slowly through the preamp tubes plate resistor, and the series resistors in the power supply. Very safe and easy on the caps.

There is always some circuit through the amp which will bleed them, you will notice I said wait. They really do not hold enough charge to damage a screwdriver - they are not car batteries. Dumping then does them no great harm. Dumping through you does. We use much bigger beasties than 50uF, to service we discharge through the system then dump them with a long plastic stick with a metal bar across the end, then have to clamp them because they charge themself up again (residual charge).
If you must bleed them, park your voltmeter across them, you will see when they are down.
I cannot believe someone elsewhere on here recommended plugging it into the mains to provide an earth - argh!

6x47 · April 2nd, 2008, 10:11 PM

Quick cap discharge is plug in turn on and crank amp, beat the hell out of the strings and while doing so pull the plug.

This always drains the caps for me but then I always probe for voltage across each cap as multimeters are cheaper than down time.

T_red7882 · April 2nd, 2008, 10:23 PM

can anyone post pics of an alligator clip draining?

Guitarslinger1 · April 2nd, 2008, 10:23 PM

Quote:

Originally Posted by jefrs

Dumping then does them no great harm.

I have carbon marks and dings on a few of my hand tools from not properly discharging the caps before sticking them in to do something. Sure, it's not going to "ruin" your screwdriver or needle nose pliers, but it sure makes it look like you might be a hack.

All of the amp gurus who I have read and talked to have said that it is not good for the electrolytic caps to be discharged in that way. Keep in mind that we are dealing with capacitors in a musical instrument amplifier, where all of its makeup effects the tone, not some piece industrial equipment.

If you use the discharge resistor alligator clip method, or the alligator clip and preamp plate resistor method, leaving it clipped on while you work on the amp, residual charge is not an issue, and it can work as a safety feature as you can train yourself to not touch a circuit unless you see that it's attached, and to remove it when you're done, before plugging the mains. Sort of a "safety tag" that shows at a glance what the status is.

But, there are many ways to skin a cat, and I prefer to suggest methods that are considered safe and sound by the strictest of safety freaks, to people who may be sticking their hands into an amp for the first time. Down the road, when they are an "old hand" such as yourself, they can develop their own methods and take any short cuts they see fit.

Larry F · April 2nd, 2008, 10:49 PM

Then there is the technique of plugging your guitar into the amp, continually strumming it, and unplug the amp from the wall, while continuing to strum. I've heard of this for years and, in fact, have used it in conjunction with the alligator clip and resistor method (just in case). How do the posters here feel about that method?

Dacious · April 2nd, 2008, 11:38 PM

I always do the unplug/strum method. I always check at the first filter cap and I've never seen more than a few volts present - certainly well below anything that will shock you. Whereas if you just turn off there can be several hundred volts there. The alligator clip will prevent filter caps regrowing charge, but I've found simply leaving the power/standby switches to on does that.

You have to be careful to leave the amp unplugged from the wall while working on it once you have discharged the caps, and put it in standby before replugging it in, but then you also have to remember to remove the alligator clip if you use one.

The alligator clip might be safest for a newbie who may make mistakes and need to discharge the amp while troubleshooting, because if you make a bad connection, power it up and the amp won't play, you obviously can't subsequently discharge it using the strum/unplug method.

I was told never to earth out the positive of charged caps by experienced people here and elsewhere - it can punch holes through the internal foil wrapping which can't be good, and it's unnecessary.

Don't make the mistake of leaving the amp plugged in even with power off on the amp and the wall - depending on the switch there could still be a live phase there and if you brush the switch or AC plug wires it can zap you.

candh · April 2nd, 2008, 11:41 PM

Quote:

Originally Posted by Larry F

Then there is the technique of plugging your guitar into the amp, continually strumming it, and unplug the amp from the wall, while continuing to strum. I've heard of this for years and, in fact, have used it in conjunction with the alligator clip and resistor method (just in case).

...after which you confirm with your DMM. Has always worked for me.

tazzboy · April 3rd, 2008, 01:48 AM

Someone ought due a video on the correct way to drain caps from amp and put on google or youtube so people can watch and learn how to do it safely without getting shock and killed.

Guitarslinger1 · April 3rd, 2008, 02:17 AM

Quote:

Originally Posted by tazzboy

Someone ought due a video on the correct way to drain caps from amp and put on google or youtube so people can watch and learn how to do it safely without getting shock and killed.

I think Gerald Weber of Kendrick Amplifiers covered it in his DVD.

tazzboy · April 3rd, 2008, 02:30 AM

I wonder if it's on Youtube?

sjhusting · April 3rd, 2008, 02:51 AM

I put bleeders in all my amps.

When I have to discharge an amp without a bleeder I use a big power resistor with the alligator clips.

Then I put a bleeder in.

I have a small DVM whose only purpose in life is to measure across the first filter cap. When I work on an amp, before I do anything else I hook that one up, and it's the last thing I remove before I button everything up.

Even without signal, the caps should discharge through the tubes - depending on the position of the standby. You might have to turn the power switch off and the standby on.

But I always measure, and always discharge if necessary, even if I'm 'only working on the ground side of an amp.' Mistakes can happen.

Never assume the caps are discharged

steven

Rob DiStefano · April 3rd, 2008, 07:01 AM

For years I've done the strum/pull-the-plug cap drain method, and afterwards I always check the cap voltage and have never seen more than a few volts remaining.

If not so feasible to do the strum/drain (such as working on the chassis), I use a resistored jumper and check for any remaining voltage.

IMHO, the strum/drain works quite well, at least in all the amps I've done it to, which is a very considerable number and type of tube amps.

Expanding Man · April 3rd, 2008, 05:00 PM

Here are a couple of links for you. BillM's Blues Jr. site discusses draining the caps.

Also, this Hot Rod Deluxe site covers several methods.

March 29th, 2008, 01:09 AM	#1 (permalink)
wetland10 Tele-Meister Join Date: Oct 2006 Location: New Jersey Posts: 403	Draining an Amp to MOd I just purchased an Epiphone Valve Junior and want to do a ssimple Bitmo mod. However, I am afraid of shocking my brains out! I've wired guitars before, but never an amp. THe mod seems simple enough. But, what exactly do I need to drain first and how do I drain it? Any help is greatly apreciated. Thanks, Wayne

March 29th, 2008, 05:03 AM	#2 (permalink)
Scott S Tele-Afflicted Join Date: Dec 2005 Location: SW CR IA US NA PE Age: 28 Posts: 1,922	You'll want to attach one end of the alligator lead to the chassis, then touch the other end to the "+" side of the capacitors. The cool kids install bleeder resistors in amps that don't have them already. - Scott Last edited by Scott S; March 29th, 2008 at 05:27 PM.

March 29th, 2008, 08:47 AM	#4 (permalink)
DrewB Tele-Afflicted Join Date: Feb 2005 Location: Independence, MO Posts: 1,108	I put that mod into a couple of those amps, and I didn't bother draining them. Foolhardy? Maybe, but you're working on the ground side of the amp and only have to touch two connections (IIRC) on the board. If you don't stray from touching things only in that area, you should be fine. I have a couple of alligator clips connected to the legs of a 5W / 470-ohm resistor that I use to bleed amps. I clip it between the positive lead of a filter cap and the chassis and leave it in place while I'm working. You can't do that in that amp because the caps have radial leads, so clip it to the plate lead (pin 1 or 6) of the first preamp tube. Since the sockets are mounted on the board in that amp, figure out which solder connection is pin 6 and clip onto the closest connection to that pin (follow the trace from that solder connection to the resistor tied to it). __________________ There is no substitute for Sound Pressure Level

April 2nd, 2008, 10:49 PM	#14 (permalink)
Larry F Friend of Leo's Join Date: Nov 2006 Location: Iowa City, IA Age: 56 Posts: 2,665	Then there is the technique of plugging your guitar into the amp, continually strumming it, and unplug the amp from the wall, while continuing to strum. I've heard of this for years and, in fact, have used it in conjunction with the alligator clip and resistor method (just in case). How do the posters here feel about that method? __________________ larry

April 2nd, 2008, 11:38 PM	#15 (permalink)
Dacious Friend of Leo's Join Date: Mar 2003 Location: Godzone Posts: 2,623 35 Photos	I always do the unplug/strum method. I always check at the first filter cap and I've never seen more than a few volts present - certainly well below anything that will shock you. Whereas if you just turn off there can be several hundred volts there. The alligator clip will prevent filter caps regrowing charge, but I've found simply leaving the power/standby switches to on does that. You have to be careful to leave the amp unplugged from the wall while working on it once you have discharged the caps, and put it in standby before replugging it in, but then you also have to remember to remove the alligator clip if you use one. The alligator clip might be safest for a newbie who may make mistakes and need to discharge the amp while troubleshooting, because if you make a bad connection, power it up and the amp won't play, you obviously can't subsequently discharge it using the strum/unplug method. I was told never to earth out the positive of charged caps by experienced people here and elsewhere - it can punch holes through the internal foil wrapping which can't be good, and it's unnecessary. Don't make the mistake of leaving the amp plugged in even with power off on the amp and the wall - depending on the switch there could still be a live phase there and if you brush the switch or AC plug wires it can zap you. __________________ My other Telecaster is a Thinline The Tele Bible, Ch 1, v 10 Love thy Telecaster, covet not thy neighbour's Strat!

March 29th, 2008, 08:44 AM	#3 (permalink)
wetland10 Tele-Meister Join Date: Oct 2006 Location: New Jersey Posts: 403	Sounds like a lot of work, depending on how many caps are in there. I haven't looked yet. Any specific type of alligator clips? Wayne

March 29th, 2008, 09:23 AM	#6 (permalink)
jefrs Tele-Holic Join Date: Nov 2007 Location: Newbury, England Age: 54 Posts: 809	1. Always, but always unplug from the mains. If the mains doesn't kill you, it hurts a lot. 2. After it's switched off, wait for the caps to bleed out their charge a bit, before going in with your fingers. 3. Then I just dump the big caps with an /insulated/ screwdriver. Or a wire with /insulated/ croc clips. Resistor not required. 4. Anything much bigger than the usual 50µF, like 1mF (yes, that's milli-Farad) needs to have a big wire soldered across its connections. You are not dealing with those brutes. 5. Everything in an amplifier obeys Murphy's law. 6. If you don't unplug, you will always touch the back of the mains switch!

March 29th, 2008, 11:36 AM	#7 (permalink)
Guitarslinger1 Banned Tele-Holic Join Date: May 2005 Posts: 966	"3. Then I just dump the big caps with an /insulated/ screwdriver. Or a wire with /insulated/ croc clips. Resistor not required." Discharging the caps this way is not good for them, and it's not so great for the screwdriver or other shorting device either. It is preferable to use an alligator clip power resistor arrangement to slowly bleed off the charge. Discharging the caps slowly is better for everything. One way it can be safely done without a power resistor, is to use a wire jumper with alligator clips, and clip one end to ground and the other end to pin 1 or pin 6 of one of the preamp tubes. This discharges the caps slowly through the preamp tubes plate resistor, and the series resistors in the power supply. Very safe and easy on the caps.

March 29th, 2008, 03:24 PM	#8 (permalink)
G-P Tele-Meister Join Date: Jan 2007 Location: PDX Posts: 154	Wetland, I suggest you just take the amp to a tech. Guitar wiring is passive, no voltage or currents involved. Amplifiers are an entirely different beast and will bite you before you even know what happened. They can bite very hard. Proceed at your own risk, but given the tone of your questions and statements, I don't get the sense that you are very familiar with the workings of an amplifier. Save yourself some grief and take it to a tech. Or at least study up on some basic amplifier tech. Some excellent books are available on the topic and excellent forums, including this one. I haven't heard of the bitmo mod. What does it entail?

April 2nd, 2008, 10:11 PM	#11 (permalink)
6x47 Tele-Holic Join Date: Mar 2007 Location: Northern ON - Northern MN Age: 64 Posts: 511	Quick cap discharge is plug in turn on and crank amp, beat the hell out of the strings and while doing so pull the plug. This always drains the caps for me but then I always probe for voltage across each cap as multimeters are cheaper than down time.

April 2nd, 2008, 10:23 PM	#12 (permalink)
T_red7882 Tele-Holic Join Date: Dec 2006 Location: San Francisco Age: 26 Posts: 828	can anyone post pics of an alligator clip draining?

April 3rd, 2008, 01:48 AM	#17 (permalink)
tazzboy Friend of Leo's Join Date: May 2005 Location: Eugene, Oregon Age: 31 Posts: 3,297	Someone ought due a video on the correct way to drain caps from amp and put on google or youtube so people can watch and learn how to do it safely without getting shock and killed. __________________ tazzboy

April 3rd, 2008, 02:30 AM	#19 (permalink)
tazzboy Friend of Leo's Join Date: May 2005 Location: Eugene, Oregon Age: 31 Posts: 3,297	I wonder if it's on Youtube? __________________ tazzboy

April 3rd, 2008, 02:51 AM	#20 (permalink)
sjhusting Tele-Holic Join Date: Jul 2003 Location: Germany Posts: 684 15 Photos	I put bleeders in all my amps. When I have to discharge an amp without a bleeder I use a big power resistor with the alligator clips. Then I put a bleeder in. I have a small DVM whose only purpose in life is to measure across the first filter cap. When I work on an amp, before I do anything else I hook that one up, and it's the last thing I remove before I button everything up. Even without signal, the caps should discharge through the tubes - depending on the position of the standby. You might have to turn the power switch off and the standby on. But I always measure, and always discharge if necessary, even if I'm 'only working on the ground side of an amp.' Mistakes can happen. Never assume the caps are discharged steven

April 3rd, 2008, 07:01 AM	#21 (permalink)
Rob DiStefano Friend of Leo's Join Date: Mar 2003 Location: Park Ridge, NJ Age: 63 Posts: 4,918	For years I've done the strum/pull-the-plug cap drain method, and afterwards I always check the cap voltage and have never seen more than a few volts remaining. If not so feasible to do the strum/drain (such as working on the chassis), I use a resistored jumper and check for any remaining voltage. IMHO, the strum/drain works quite well, at least in all the amps I've done it to, which is a very considerable number and type of tube amps.

April 3rd, 2008, 05:00 PM	#22 (permalink)
Expanding Man TDPRI Member Join Date: May 2005 Posts: 12	Here are a couple of links for you. BillM's Blues Jr. site discusses draining the caps. Also, this Hot Rod Deluxe site covers several methods.