First build - safety question

photoelectric1357

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Howdy,

I'm about to get started on my first build - a Deluxe Reverb clone from StewMac. I have no prior electronics experience, education or building experience so I'm quite aware that I've bitten off a large mouthful in the DR but am going forward slowly and methodically. I'm trying to address every question that comes up in my mind as I learn about electronics and building from various sources and in terms of safety practices for working on amps there is one question I haven't found an answer to.

I am comfortable with what I've learned about the process of draining the filter caps. Are these the only components that can store lethal or dangerous voltage after unplugging? What about the other capacitors on the amp such as the big orange drops etc? Are there other components that need to be drained? I've not seen anyone mention draining anything but the filter caps but didn't want to just assume. I'd like to know for sure.

Thanks!

Mark
 

Rich_S

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Filter caps are the ones to worry about. The others are mostly low voltage. For example, the "Bypass Cap" shown below in the AB763 Normal channel only has about 1.3 volts across it, Even that small voltage will drain away immediately when the amp is turned off, through the 1500 ohm resistor.

The filter caps on the other hand have high voltages on them by design, and not much load to drain them down. Try tracing a DC path to ground from the line marked 280C B+4. There isn't one... every path is blocked by cap in the tone stack. Once the heater is shut off, the tube stops conducting and there's no way for the 180V on the plate to drain away. Eventually, leakage will drain the filter caps, but it might take a long time unless we provide a drain path on purpose.

Screenshot 2021-11-27 152008.jpg
,
 

printer2

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Keep your iron tinned, clean the tip with a damp sponge and flow some solder on it. When there is not a lot of the tip wetting with solder clean it off and re-tin the tip. Clean the socket and pot lugs with a quick wipe of alcohol to remove any oil from manufacturing (might not be any but could help) or oxidization. Soldering flux floats off the dirt/oxidization and helps the solder to flow. A little dab (and I mean little) can help on the surfaces. But only flux meant for electronics and not acid flux for plumbing. Most wire you will be using is tinned with solder and the same with the part leads. Doubt you will have oxidized ones but if you do the alcohol wipe will clean it off.

Yes I am focusing a lot on soldering, if you can follow instructions and put the right part in place the only real downfall is the soldering. When soldering feed a little bit of solder where the tip touches the parts and once there is a little filet of liquid solder bridging the part and the iron (which aids in conducting heat from the iron) you can add solder farther away from the tip, the solder will flow towards the heat. If you have too much solder on the tip you can just flick it off onto a newspaper or somewhere. The joints should look shiny and smooth and not lumpy. If you have trouble getting heat onto the part the tip might be too dry of solder and you can melt a little more on it. The solder is there to insure a good electrical connection and while it does provide a mechanical bond the part wire (or wire) should be fastened to provide a good mechanical connection. In the case of eyelets there is enough solder flowed in to make the mechanical connection (about the only case).

As long as you can adequately bolt things together the construction end of it should go well enough. Just for kicks you could measure the resistors before putting them in place to make sure you have the right part. Check against the schematic or layout diagram and check off on the diagram whenever you have put the part in place. A Deluxe Reverb is a complicated amp for a beginner but if you manage the construction end of it well enough you should have a working amp.
 

sds1

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I have no prior electronics experience, education or building experience so I'm quite aware that I've bitten off a large mouthful in the DR but am going forward slowly and methodically.
Good for you man, my first build was AB763 as well, also no prior experience. You'll be fine, IMO circuit complexity is non-issue if you are good at breaking big systems into small ones -- and, working methodically as you already suggested. For the scary part, nothing puts the mind at ease like having a lightbulb limiter on hand. Have fun!
 

Lowerleftcoast

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Welcome to TDPRI.
Post pictures of your progress. We like pictures.
Follow Rob Robinette's sites for power cord installation and seriously consider following a different ground scheme than what Stewmac shows.
Use an alligator clip or other heat sink on the capacitor leads when soldering. They are sensitive to heat. Consider all of the parts fragile. Handle with care. Do not bend the leads right at the body of the resistor or cap. Use a needle nose pliers to support the lead so the component is not damaged. Don't break those orange drops where the orange meets the lead.;)
What about the other capacitors on the amp such as the big orange drops etc? Are there other components that need to be drained?
@Rich_S above covered the bias bypass caps. Most only *see* low voltage and they all have a resistor in parallel so they are drained when the amp is shut off.

The coupling caps (Orange Drops) are below 1uF so even if charged, they do not have the *capacity* to cause harm. They won't carry enough *current* to bite you.

To ease your mind a *bleeder* resistor can be installed for safety. It can be left in. It will not draw enough current to bother the Deluxe Reverb. It will drain the caps when the amp is powered *off*. Simply install a 220k 2W resistor parallel to the first filter cap. Always check with your DMM to make sure the voltage has been drained before putting your hands in. No jewelry when working in there... even take off the wedding ring!
 

sds1

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Simply install a 220k 2W resistor parallel to the first filter cap.
It's worth mentioning in most AB763's (Deluxe being the exception I guess) you have a 440k bleeder already installed, in the form of 2x220k balancing resistors that are strapped across the series filter caps at first node.

The Deluxe could definitely benefit from your suggestion!

It's also worth mentioning, the bleeder in front of the standby switch won't bleed the filter caps after the standby switch (when standby is open of course).

Therefore, I'd suggest putting another 220k bleeder across the last filter cap (D node in this case) as well.
 

Peegoo

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The only ones that store dangerous voltages are large electolytics.

These are super easy to identify because like a battery, they are polarized: one lead or terminal is positive (+) and the other lead or terminal is negative (-).

"Can" caps (multiple electrolytic caps in a single aluminum can/container) are a bit more difficult to ascertain polarity beause they often have four terminals on one end. You can discharge these by connecting one end of a jumper wire to the common ground buss and connecting the other end of the jumper to each terminal in succession.

If the amp has a metal "dog house" cover, be extremely careful to remove it straight up to prevent is making contact with one of the caps' leads. You could get zapped if you're not careful.

More info here:

 

JamesAM

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The only ones that store dangerous voltages are large electolytics.

These are super easy to identify because like a battery, they are polarized: one lead or terminal is positive (+) and the other lead or terminal is negative (-).

"Can" caps (multiple electrolytic caps in a single aluminum can/container) are a bit more difficult to ascertain polarity beause they often have four terminals on one end. You can discharge these by connecting one end of a jumper wire to the common ground buss and connecting the other end of the jumper to each terminal in succession.

If the amp has a metal "dog house" cover, be extremely careful to remove it straight up to prevent is making contact with one of the caps' leads. You could get zapped if you're not careful.

More info here:


Thank you for this YouTube channel- I’ve feel like I’ve seen every amp repair account on there but have never come across this one! She is awesome! The one with the old tweed gibson is fantastic.
 

wangdaning

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When you drain charge out of the circuit it drains all of the charge. You can target the filter caps, but if they loose their charge you can be sure the others have as well*. I usually do an initial discharge on the filter cap with the resistor to ground probe I made and then directly alligator clip to chassis and wait a few minutes. Next, check for safety that it has worked (multimeter it, it only takes a second and can save you).

I got shocked building my last amp, and I was one hand behind my back, but my needle nose pliers flew across the room. My son was watching me, as we do (have someone in the room with you in case), but those pliers really flew past his face. The shock bit me, but the pliers could have been an issue. I now always double check even if I think I drained them.

*What if your circuit is not closed, maybe a bad connection. That means it is not draining them all. I caught myself being lazy there, tap all electrolytics with the meter and be safe!
 

wangdaning

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I forgot about the furballs. I leave open stuff out, but my cat and son saw me get shocked. My son is almost seven and knows, the cat is about nine and seems to know. I think the cat heard me talking over and over again about the danger.

*This is this cat and this child, I would not trust others around my stuff like this. My work area door needs to be closed if any other people visit, I won't even trust adults (even my wife is suspect).
 

photoelectric1357

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Filter caps are the ones to worry about. The others are mostly low voltage. For example, the "Bypass Cap" shown below in the AB763 Normal channel only has about 1.3 volts across it, Even that small voltage will drain away immediately when the amp is turned off, through the 1500 ohm resistor.

The filter caps on the other hand have high voltages on them by design, and not much load to drain them down. Try tracing a DC path to ground from the line marked 280C B+4. There isn't one... every path is blocked by cap in the tone stack. Once the heater is shut off, the tube stops conducting and there's no way for the 180V on the plate to drain away. Eventually, leakage will drain the filter caps, but it might take a long time unless we provide a drain path on purpose.

View attachment 923618 ,
Thanks Rich, appreciate that!
 

photoelectric1357

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Posts
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Keep your iron tinned, clean the tip with a damp sponge and flow some solder on it. When there is not a lot of the tip wetting with solder clean it off and re-tin the tip. Clean the socket and pot lugs with a quick wipe of alcohol to remove any oil from manufacturing (might not be any but could help) or oxidization. Soldering flux floats off the dirt/oxidization and helps the solder to flow. A little dab (and I mean little) can help on the surfaces. But only flux meant for electronics and not acid flux for plumbing. Most wire you will be using is tinned with solder and the same with the part leads. Doubt you will have oxidized ones but if you do the alcohol wipe will clean it off.

Yes I am focusing a lot on soldering, if you can follow instructions and put the right part in place the only real downfall is the soldering. When soldering feed a little bit of solder where the tip touches the parts and once there is a little filet of liquid solder bridging the part and the iron (which aids in conducting heat from the iron) you can add solder farther away from the tip, the solder will flow towards the heat. If you have too much solder on the tip you can just flick it off onto a newspaper or somewhere. The joints should look shiny and smooth and not lumpy. If you have trouble getting heat onto the part the tip might be too dry of solder and you can melt a little more on it. The solder is there to insure a good electrical connection and while it does provide a mechanical bond the part wire (or wire) should be fastened to provide a good mechanical connection. In the case of eyelets there is enough solder flowed in to make the mechanical connection (about the only case).

As long as you can adequately bolt things together the construction end of it should go well enough. Just for kicks you could measure the resistors before putting them in place to make sure you have the right part. Check against the schematic or layout diagram and check off on the diagram whenever you have put the part in place. A Deluxe Reverb is a complicated amp for a beginner but if you manage the construction end of it well enough you should have a working amp.
Cheers, Printer. Good info and, I agree, knowing enough to just put the thing together solidly and correctly and have it turn on at the end is a good chunk of learning all on its own.
 

schmee

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Yep, just take your time and double check things.
You will find you forgot or misplaced something on start up. Or you are better than the rest of us!
For soldering to the back of pots or to the chassis some flux is very good to have. I like No Ko Rode.
Have fun!
 

photoelectric1357

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Good for you man, my first build was AB763 as well, also no prior experience. You'll be fine, IMO circuit complexity is non-issue if you are good at breaking big systems into small ones -- and, working methodically as you already suggested. For the scary part, nothing puts the mind at ease like having a lightbulb limiter on hand. Have fun!
Hey, thanks sds1. Appreciate the support, especially from someone who's been through it without experience. I am for sure going to do the light bulb limiter. I was alternately thinking of seeing if I could borrow a variac locally for power up. We'll see, I'll report back!
 

photoelectric1357

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Joined
Nov 27, 2021
Posts
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Age
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Location
Canada
Welcome to TDPRI.
Post pictures of your progress. We like pictures.
Follow Rob Robinette's sites for power cord installation and seriously consider following a different ground scheme than what Stewmac shows.
Use an alligator clip or other heat sink on the capacitor leads when soldering. They are sensitive to heat. Consider all of the parts fragile. Handle with care. Do not bend the leads right at the body of the resistor or cap. Use a needle nose pliers to support the lead so the component is not damaged. Don't break those orange drops where the orange meets the lead.;)

@Rich_S above covered the bias bypass caps. Most only *see* low voltage and they all have a resistor in parallel so they are drained when the amp is shut off.

The coupling caps (Orange Drops) are below 1uF so even if charged, they do not have the *capacity* to cause harm. They won't carry enough *current* to bite you.

To ease your mind a *bleeder* resistor can be installed for safety. It can be left in. It will not draw enough current to bother the Deluxe Reverb. It will drain the caps when the amp is powered *off*. Simply install a 220k 2W resistor parallel to the first filter cap. Always check with your DMM to make sure the voltage has been drained before putting your hands in. No jewelry when working in there... even take off the wedding ring!
Thanks LLC. Interesting point about the grounding scheme. I've seen a lot of different opinions and approached to that in my online travels. Are you thinking of the bus bar approach or do you advocate a specific idea? I must admit, being as green as I am, the thought of moving away too far from Stew Mac's step by step directions is a daunting one to me. For better or worse, the kit components matching up to those step by step directions are why I went with the Stew Mac kit. Said another way, I wouldn't trust that I would succeed with just a schematic and a wiring diagram. I just don't know enough about electronics.

Good info on the handing of the resistors and caps and about the non-filter caps. Luckily, the wedding ring is no longer an issue! Hah! Much appreciated!
 

photoelectric1357

TDPRI Member
Joined
Nov 27, 2021
Posts
8
Age
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Location
Canada
The only ones that store dangerous voltages are large electolytics.

These are super easy to identify because like a battery, they are polarized: one lead or terminal is positive (+) and the other lead or terminal is negative (-).

"Can" caps (multiple electrolytic caps in a single aluminum can/container) are a bit more difficult to ascertain polarity beause they often have four terminals on one end. You can discharge these by connecting one end of a jumper wire to the common ground buss and connecting the other end of the jumper to each terminal in succession.

If the amp has a metal "dog house" cover, be extremely careful to remove it straight up to prevent is making contact with one of the caps' leads. You could get zapped if you're not careful.

More info here:


Thanks Peegoo. Appreciate the heads up about lifting the doghouse. I'd run into her youtube channel recently. Great stuff!
 

photoelectric1357

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If you have little kids and pets around, make sure to never leaved an energized (or unplugged/charged) chassis where little fingers or paws can get into it.

Here's Sparky, my CATacitor Discharge Tool in action; when Sparky's eyeballs stop blinking, the caps are drained.

GIF-Catacitor-Discharge-GIF.gif
Man, that's a cat for ya! Sparky's a rebel!
 




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