Resistor Safeguards Leaky Coupling Capacitor?

[tbryanh]

It appears the send line of effects loops have a high value resistor between the send jack and the coupling capacitor to prevent people from being shocked if the coupling capacitor leaks.

Is this correct?


The Marshall Silver Jubilee uses a 100K ohm resistor.
http://www.schematicheaven.com/marsh...5_50w_2550.pdf

The Mesa Boogie Studio .22 uses a 330K ohm resistor. (As a side note it appears the send jack is mislabeled as the return jack, and the return jack is mislabeled as the send jack.)
http://www.schematicheaven.com/boogi...ogie_22cal.pdf

[Enzo]

No, that is not why it is there. 100k would limit current, true, but considering the high resistance through your body, it would not reduce the voltage much, so you still would get a poke.

Look at the 2550, and there is your 100k R34. But look closer and there is also a 10k R33 to ground from there. These two resistors form a classic voltage divider, in this case 10 to 1 more or less. The signal at C24 is WAAAYYY too hot for the line level output, so they need to reduce it before it hits the jack. And that is why V2B is there - to boost the signal level back up to where it was before the loop after the return jack.

R33, 34 make up a series resistance of 110k from C24 to ground. The signal for the loop is taken out at the 10k point. Imagine instead of the resistors that you had a 110k pot there. Turn the pot from zero up to "1" and connect the wiper to the SEND and you have this exact circuit. It would look like any volume control, like the 1M pot at VR1 over by the input stage.

Look on the Mesa. There is your 330k, but look below at the 5.6K. Those two make a voltage divider too, though in this case it is more like a 60 to 1 divider.

And yes, the send and return labels are reversed.

[tbryanh]

http://www.allaboutcircuits.com/vol_1/chpt_3/4.html

This article says body resistance can be anywhere between 1K ohm and 1M ohm.

1M ohm is typical dry, 17K is typical wet, and 1K ohm is only under extreem circumstances where you are standing barefoot on a grounded water pipe and an electrical contact is made to the gold ring you are wearing on your finger.

According to the chart, for the extreem condition, 300v / 1K = 300ma, you would be unable to let go of the wire, experience severe pain, have difficulty breathing, and could possibly die from heart fibrillation.

300v / 1M = 0.3ma would give a slight sensation at most, and 300v / 17K = 18ma might be a little painful, but you would still be able to let go of the wire.

Whether it was the intention of the circuit designer or not, it appears that under extreem conditions, the resistor between the coupling capacitor and the send jack does provide protection against a leaky coupling capacitor.

300v / 100K = 3ma which would give a slight sensation.

[tbryanh]

As a side note, it appears that extreem conditions can easily exist when working on a guitar amp by putting one hand on the metal chassis and one hand on the 300v. It is important to use the single-hand rule when working on a guitar amp. Ground the scope or VTVM to the chassis of the amp before turning on power to the amp. Then use one hand to move the probe around in the circuit keeping the other hand behind your back or under the table, etc.

[tbryanh]

Another side note.

I plan to use a momentary contact footswitch that operates a contactor/relay.

The contactor/relay will provide power to the device under test, DUT.

In the event that I am getting shocked, I will lift my foot from the footswitch to power down the DUT.

[tbryanh]

Here are links to the some of the parts needed for the momentary contact footswitch.

Contactor
http://www.amazon.com/Contactor-90-244/dp/B000H5WMXG

Transformer
http://www.amazon.com/Honeywell-AT14...591889&sr=1-23

[R.G.]

Enzo, judging from the responses, you're shoveling sand against the tide.

[R.G.]

Whether it was the intention of the circuit designer or not, it appears that under extreem conditions, the resistor between the coupling capacitor and the send jack does provide protection against a leaky coupling capacitor.

If you knew the answer, why did you ask?

As a side note, it appears that extreem conditions can easily exist when working on a guitar amp by putting one hand on the metal chassis and one hand on the 300v. It is important to use the single-hand rule when working on a guitar amp. Ground the scope or VTVM to the chassis of the amp before turning on power to the amp. Then use one hand to move the probe around in the circuit keeping the other hand behind your back or under the table, etc.

We have a saying back in the Ozarks where I came from - "Don't try to teach your grandmother to suck eggs."

Another side note. I plan to use a momentary contact footswitch that operates a contactor/relay. The contactor/relay will provide power to the device under test, DUT. In the event that I am getting shocked, I will lift my foot from the footswitch to power down the DUT.

Kewl. It's always good to - when you find you're hurting yourself - quit.

Here are links to the some of the parts needed for the momentary contact footswitch.

Huh?????? Appropos of what????? Is this an informational announcement or a request for an engineering review?

[tbryanh]

Thanks for the eggs and stuff.

I didn't know the answers in advance. I did the research on the website after posting the question.

Morgan Jones in Chapter 5 (Faultfinding to fettling) of his book Building Valves Amplifiers advises people to never work alone on vacuum tube amps. I must work alone, so I will use the momentary contact safetey switch.

Although I am a junior member, I am not new to electronic circuitry. I was a bench technician for 10 years troubleshooting analog and digital circuitry. I am new to vacuum tube equipment, but I am coming up to speed quickly.

Please try to refrain from sucking or throwing eggs. If you can do this, you might benefit from the discussion too.

[R.G.]

Thanks for the eggs and stuff.
I didn't know the answers in advance. I did the research on the website after posting the question.

It would have helped in understanding you if you'd said that.

Enzo is extremely experienced, and didn't need the one-hand pointer, as many of the readers here do not.

Morgan Jones in Chapter 5 (Faultfinding to fettling) of his book Building Valves Amplifiers advises people to never work alone on vacuum tube amps. I must work alone, so I will use the momentary contact safetey switch.

A momentary safety switch is not enough. Electrical shock, especially if you get hold of the AC line, will tend to freeze you in place. You may not be able to get off a footswitch, just as you may not be able to let go of a conductor. We had people who removed themselves from the gene pool by working alone in a number of ways. Fail-safes sometimes don't and only your buddy will be able to size up the situation and save you.

You know what that kind of momentary switch is called, right? It's a dead-man switch in the vernacular. I remember my old buddy Ivan, the world's Russian Roulette champion. His career record was 487-1.

Although I am a junior member, I am not new to electronic circuitry. I was a bench technician for 10 years troubleshooting analog and digital circuitry. I am new to vacuum tube equipment, but I am coming up to speed quickly.

Kewl. We share some background. I've only been doing this for 36 years, and tubes only since 1984.

Please try to refrain from sucking or throwing eggs. If you can do this, you might benefit from the discussion too.

I'll try to understand a few of the pearls, though I'm sure most of the really good nuggets will fly right over my head.

But I do try to catch some of them. I have found all of these points and more in postings in the archives here. It's a good place to start.

[tbryanh]

I plan to wear heavy leather gloves while testing and keep my left hand in my pants pocket.

[tbryanh]

I plan to have a safety throw switch with a large handle in a visible location on the test bench for someone in the room to be able to use.

[Steve Conner]

What are you making, a clone of Frankenstein's Monster?

[Brad1]

I plan to have a safety throw switch with a large handle in a visible location on the test bench for someone in the room to be able to use.

I plan to wear heavy leather gloves while testing and keep my left hand in my pants pocket.

I plan to use a momentary contact footswitch that operates a contactor/relay.

It's good to have a healthy respect for electricity, and even a bit of healthy fear, but it seems like you are going to extremes worrying about it, and trying to devise all these "safety features".

One has to wonder whether you are really ready to poke around inside of a tube amp? If you are THAT afraid of getting buzzed, then it's doubtful that you can really concentrate on the task at hand, because you'll be concentrating on not getting zapped! And having those footswitches and throw switches could even give you a false sense of security. Also, what do plan to do in there with a clumsy pair of thick leather gloves?

You'll be better served by reviewing, remembering, and implementing ALL the safety procedures of working on this stuff. Be alert at all times and don't turn your head away while you are poking around in there. Discharge caps after unplugging and before poking around. Don't randomly poke around with a metal object. The one-hand thing. Don't work on it in the bathtub. Etc.

I'm sure most people here have been zapped at least once. Luckily, I'm sure most of us were following simple rules, and were either jolted away from the thing, or were able to pull away quickly. If you don't make yourself a conduit to ground, then it's less likely to freeze you in place and kill you.

Getting REALLY zapped is when I was 18 and working at a place that repaired all types of electric motors. Here's a tip: Don't go outside for a smoke in the rain, come in with wet sneakers, and hold the lift hook on a huge 440V, 3 phase motor from the local utility company, while someone flips the switch!

I was completely blacked out for a few minutes. They shut it down immediately, and said I was thrown about 4 feet back. THAT'S where gloves may have come in handy. That's also where I abandoned common sense.

I've been zapped in my youth by touching my mouth to a mic while using an old ungrounded Fender amp. I abandoned common sense by not testing that first.

Just keep your wits about you, and use common sense. If you are SO afraid that you can't get past all that, maybe you shouldn't be doing this? Sometimes, we will our own destiny.

Just something to think about.

Just be careful.

Brad1

[Steve Conner]

Don't go outside for a smoke in the rain, come in with wet sneakers, and hold the lift hook on a huge 440V, 3 phase motor from the local utility company, while someone flips the switch!

Holy crap! I bet you felt that all right! However, the 400-500V DC inside a typical 50 watt guitar amp will do pretty much the same thing to you, if you let it.

A GFI won't trip when you are getting shocked by the DC part of an amp, because the power transformer isolates you from the circuit that the GFI is sensing.

Dead men's switches may or may not work, in the way other posters described.

Emergency shutoff switches are only useful if there is someone else around to pull them.

Really the best thing is to understand electricity as best you can, and respect it. In other words, instead of buying 20 different fire extinguishers, just don't smoke in bed in the first place! I have given myself some painful zappings in the past, including one with 400V DC that put me off playing with tube circuits for a long time. Since then, I have been very careful and never got another jolt from an amp.

Probably the scariest thing I worked on was a Tesla coil that ran off 600V DC at 5 amps and stepped it up to about half a million volts. I was more worried about a shock from the 600V DC bus than from the high voltage output, and you can bet I had a buddy standing by with a broom handle and a knowledge of CPR. ;-)

Remember that scene in The Taking Of Pelham 123 where the guy gets fried by the subway third rail? Those are 600V DC too. So is an Ampeg SVT for that matter.