Resistor Safeguards Leaky Coupling Capacitor?

[Enzo]

tb - your original question was about the resistor and was it there as a protection. Regardless of whether it indeed might offer some, that is not why it was there. I had hoped to explore its real purpose for you.

The resistance through the human body might be as low as 1k in some circumstance, but unles my fingers are wet, 1M is a low reading. Turn your meter to ohms and grasp each probe tip in your fingers. Right now my Fluke says 2.5M and if I squeeze really tight I can get it dowb almost to 1M.

I applaud your concern for safety, but electricity is faster than anything you can do, including add fuses. And while the one hand rule is better than nothing, if you touch B+ and your wrist leans against the chassis edge, ZZZT, you will hurt. Caution and respect work a lot better than anything.

And you might check out RG's web site. It is full of very useful tips and techniques yuo can use in servicing.

[tbryanh]

So I am curious at this point if it the gain of the stage was deliberately set high or an extra stage added so that a voltage divider network was required to bring the level down to line level thereby allowing a high value resistor to be used as a safety device between the coupling capacitor and the send jack.

I have not looked at alot of vacuum tube amp schematics, but the two that had effects loops had the high value resistors.

As a comparison, I am thinking that coupling between stages typically does not have this resistor. Of course this is an idea coming from observing a very limitted number of vacuum tube schematic diagrams.

[Enzo]

The signal in most tube amps going into the PI - that is the output of the preamp section - is rarely going to be 1 volt or anything like line level. If you are going to have an effects loop, you cannot spit 30 volts of signal out, you need to drop the stuff down. And of course the returning line level signal will not remotely drive the PI, so it has to be boosted.

In the case of the MArshall, the signal path to the power amp is through the loop. You COULD break a send off from the signal path in a branch and then drop it down, and also have the return gain stage mix back in - the parallel loop - but those functions of level reduction and level restoration are stil required.

Level drops are common in amps, mainly I look for them in higher gain amps, but they are all over. Look at a couple Peavey amps. (You can get any PV drawings you ever need by asking at parts@peavey.com. And Steve has a few popular ones on his site, www.blueguitar.org) The Classic 30 is popular. Look after the first stage, there is a 680k over 250k divider for the clean channel volume. The dirt channel basically branches the signal through a couple stages of V2. Leaving V2A there is a 470k over 470k divider that cuts level in half. And after V2B ther is a 1M over 10k divider at the post volume control. COming out of the tone stack, the signal is low enough to feed the loop. That loop driver is an emitter follower with no gain, so the level is the same there. The return is buffered by V3A and brought back up to tube levels for the PI.

Look at the PV 5150 - a gain monster - the 5150-2 is bery similar and the drawing is easier to follow, but look at either. On the 5150-2, in the dirt channel I see dividers after four of the five triodes in the circuit. And in the "clean" channel, after both triodes. None of these parts connects to the real world.

Look at the VOX AC15. There is a 3M3/470k divider between second and thrid triode, and exiting the tone stack is another 220k/470k drop. MArshall DSL100 R29/r30 betwen triodes of v2. And a several resistor divider between V2 and V3. DSL401 has some.

You can crank as much gain as possible from A tube, but you can also reduce the level after a stage and run it through another to give you a second chance to shape the tone, or a third or a fourth... SO an old tweed or something with barely enough tubes to make sound come out will not likely have a lot of signal reduction within. But something with a ton of balls and going for distortion might have several more stages than it "needs" and reduce the levels between.

[R.G.]

So I am curious at this point if it the gain of the stage was deliberately set high or an extra stage added so that a voltage divider network was required to bring the level down to line level thereby allowing a high value resistor to be used as a safety device between the coupling capacitor and the send jack.

I have not looked at alot of vacuum tube amp schematics, but the two that had effects loops had the high value resistors.

As a comparison, I am thinking that coupling between stages typically does not have this resistor. Of course this is an idea coming from observing a very limitted number of vacuum tube schematic diagrams.

Preconceived notions die hard, don't they?

No, the high resistance network is not there to be a safety feature.
No, the high resistance network is not there to be a safety feature.
No, the high resistance network is not there to be a safety feature.

Let it go. Enzo has told you the clear and present reason for the divider. It's to match signal levels from the inside of the amp to what get plugged into the loop. Trying to reason out a tortuous way it could possibly have been a safety measure will not be all that fruitful.

I have a suggestion for you. It could save you a huge amount of time and frustration. Go buy a copy of Kevin O'Connor's "The Ultimate Tone" - volume one. You'll get a huge amount of your questions answered, including in particular the question about loops.

[tbryanh]

OK, so your usually looking to generate tone with the nonlinear range of the tube, and in most case avoid cutoff or saturation.

Operating one stage in the nonlinear range can be OK, but many times you want to operate a series of stages in their nonlinear ranges. To keep the tubes from saturating or cutting off, you need some attenuation between the stages.

[tbryanh]

OK, so the voltage swing on the plate can be 300v p-p, and the voltage swing on the grid of the next stage can only be about 10v p-p. This mens that attenuation between stages is typically needed.

[Peter Clothier]

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.

I'm not so sure about this RG.???????
In My experience AC throws you off and DC acts like superglue.

It was 1972 the last time I had a really decent belt from a old valve radiogram amp. It was switched off but I managed to get hold of the B+ line (I cant remember how exactly I was a teenager back then) It felt like the incredible hulk had grabbed me by the throat squeezing the life out of me and i couldn't move anything. Eventually, it did let me go just before I felt like it was all coming to an end. Since then I have always had a huge respect for DC >50V.

Peter.

[tbryanh]

SUMMARY

1. Attenuation between stages or between a stage and a send jack is typically needed in vacuum tube circuitry.

2. Because of the small currents and high voltages typically found in vacuum tube preamp circuits, high value resistors are required to drop the required voltages to provide the needed attenuation.

3. This means that a high value resistor will typically be connected to the coupling capacitor.

4. It is clear that under ideal conditions, a leaky coupling capacitor can kill people.

5. Send jacks are typically standard 1/4" phone jacks, and it is easy for a musician to mistakenly plug his or her guitar into a send jack.

6. The high value resistor will protect the musician from a leaky coupling capacitor if the musician mistakenly plugs his or her guitar into the send jack.

7. When it comes to send jacks, the high value resistor has a dual role, to provide needed attenuation and to protect the musician.

8. When it comes to send jacks, it cannot be concluded that because the high value resistor provides needed attenuation that it does not provide protection to the musician. Likewise, it cannot be concluded that because the high value resistor protects the musician, it does not provide needed protection.

[tbryanh]

8. When it comes to send jacks, it cannot be concluded that because the high value resistor provides needed attenuation that it does not provide protection to the musician. Likewise, it cannot be concluded that because the high value resistor protects the musician, it does not provide needed *attenuation*.

[Enzo]

That some level of current limiting would occur does not make that the resistors "purpose." But you are free to spin it as you see fit, and welcome to think what you like. As long as you understand the purpose of the parts in terms of the circuitry itself, then you will be better able to understand the operation of the circuit in question.


COnsider: a sailing ship sinks at sea. The large wooden mast breaks off and floats. Sailors can cling to the mast to avoid drowning. Clearly the purpose of the mast on the ship is as a safety device in case the ship sinks.

[tbryanh]

***some level of current limiting***

300v / 100k = 3ma

3ma is an extremly safe level of current.

[R.G.]

I seem to catch a faint smell of walters and markphaser.

SUMMARY
1. Attenuation between stages or between a stage and a send jack is typically needed in vacuum tube circuitry.

Attenuation between stages, etc. is OFTEN needed for signal level reasons.

2. Because of the small currents and high voltages typically found in vacuum tube preamp circuits, high value resistors are required to drop the required voltages to provide the needed attenuation.

Because of the high impedances of vacuum tube circuits, high value resistors are more appropriate for attenuation than low values.

3. This means that a high value resistor will typically be connected to the coupling capacitor.

This means that a high value may often be connected to coupling capacitors for signal attenuation.

4. It is clear that under ideal conditions, a leaky coupling capacitor can kill people.

It is clear that under worst case conditions, the failure of any one of a large number of parts in a tube amp can kill people.

5. Send jacks are typically standard 1/4" phone jacks, and it is easy for a musician to mistakenly plug his or her guitar into a send jack.

Given the average intelligence of a guitar or bass player, yes.

6. The high value resistor will protect the musician from a leaky coupling capacitor if the musician mistakenly plugs his or her guitar into the send jack.

A high value attenuation network may or may not protect a musician from the several hazards that might exist under worst case conditions.

7. When it comes to send jacks, the high value resistor has a dual role, to provide needed attenuation and to protect the musician.

When it comes to send jacks, a high value resistor in an attenuation network, if it's used there at all, has an intentional role of adapting the signal levels to those appropriate for the send jack. Any other role is accidental.

8. When it comes to send jacks, it cannot be concluded that because the high value resistor provides needed attenuation that it does not provide protection to the musician. Likewise, it cannot be concluded that because the high value resistor protects the musician, it does not provide needed attenuation.

When it comes to send jacks, it cannot be concluded that there is a safety/protection role for the attenuation network, except by accident.

OK, so tell us - are you losing a bet if you can't make people agree the resistors are for safety?

Otherwise, let's get back to the questions:
If you already knew and were not willing to take information you were given, why did you ask?

As I said:

No, the high resistance network is not there to be a safety feature.
No, the high resistance network is not there to be a safety feature.
No, the high resistance network is not there to be a safety feature.

Let it go. Enzo has told you the clear and present reason for the divider. It's to match signal levels from the inside of the amp to what get plugged into the loop. Trying to reason out a tortuous way it could possibly have been a safety measure will not be all that fruitful.

Here's another bit you may have missed. The connection to a send jack is typically from the plate of a 12AX7, with typically a 100K plate resistor. If the coupling cap is replaced with a dead short, the plate resistor all by itself limits current to your "safe" 3ma. While I'm not happy with 3ma being "safe", you are. And the circuit as is, no high value attenuation network already limits to that.

Therefore the high value resistor in the attuenuation network is not even theoretically needed for safety reasons - using your numbers.

The plain fact is, it's not there for safety reasons. Any safety effect is a happy accident.

[Tom Phillips]

Mark Phaser was exactly what I was thinking.

[tbryanh]

Great point.

Here's another bit you may have missed. The connection to a send jack is typically from the plate of a 12AX7, with typically a 100K plate resistor. If the coupling cap is replaced with a dead short, the plate resistor all by itself limits current to your "safe" 3ma. While I'm not happy with 3ma being "safe", you are. And the circuit as is, no high value attenuation network already limits to that.

Great point. The 100K plate resistor made us safe already. I'm convinced now. See how reasonable of a person I am

3ma is safe. Barely perceptable if perceptable at all. Check the table in the link http://www.allaboutcircuits.com/vol_1/chpt_3/4.html

Wow, you senior techs are great.

[Steve Conner]

Good point, but what if I accidentally put a two-prong plug on the other end of my guitar lead and plugged it into a wall socket? (After all, we know guitarists and bass players aren't that smart.) If the shield of the cable ended up connected to the hot, then all the metalwork on the guitar would end up hot too! If I somehow managed not to notice the smoke billowing from my control cavity and pickups, and then touched a grounded mic stand, I would get KFC'd almost instantly

I'm going to campaign for a law that forbids the storing of two-prong plugs near guitars, and makes it compulsory for the neutral pin on three-prong plugs to be labelled "Screen" too.

Seriously, do you think the guys who gave us the "Death Switch" with its accompanying "Death Cap" really care about the safety of musicians? If the death switch is set wrong, and the death cap goes leaky, then the shield of your guitar cord is in fact connected to 120V. It makes the effects send jack look like a nuclear power plant safety system in comparison.