CRATE BV amp Ch1 prob- "Crunchy" Noises.

[Jazz P Bass]

Oh, "Cell" is in reference to it being a "photocell"
As the LED illuminates , it strikes the photocell & lowers the cells resistance.
LED off: high resistance
LED On: low resistance
A light controlled switch!
Pretty cool, huh?

[Sea Chief]

I have looked the VTL5C6 up, Ive even bookmarked where I can(not) get hold of one.

But I look at the schematic & within the OC there are two diagrams, a resisitor & a diode. (i dont see a battery, or a cell so that means Im unsure/ exctremely confused with this all of a sudden). Im sure there is a simple reason for 'cell'. But All Im asking before I consider shorting, is whether the 'cell' you say is on the schematic pictured as a resisitor. And why.

Once I get my head around the basics of what exactly Im about to test, I will go ahead with it.

[Sea Chief]

Ok I am going to say one half is 'sort of a reisistor'!

Right I understand to short across the 'cell' side then. What I want to understand beforehand though, is why. Not 'cos you wanna test it dummy!' but what I am trying to rule out. If as I -think- (i still dont know) the 'cell' is some sort of resisitor, then is the short I introduce there to assume this 'resisitor' is what..

Conducting, but not producing any resisitance at all, as it is meant to, becos its shot. Or..
Not conducting anything at all becos the 'resisitor' is severed s'how?

If the first of these, then I cannot see the point of a short if as in its broken state, it is effectively simply a short and has lost any more of its duties than that (so the noise will be the same with a jumper, no? is my point).

[Jazz P Bass]

You are searching for the cause of the noise.
When the clean channel is selected , there is noise.
When the clean channel is selected, OC 1 is in a low resistance state.
In that state, it 'may' be making noise.
By jumpering the photocell (it is a resistor that changes resistance when the LED light strikes it, hense the resistor symbol), you are eliminating any OC induced noise.
Jumper it.
Noise or no noise.
Noise, move on to the next point.
No noise, replace it.
(Remember 'divide to conquer'? You have to take the next steps, so please move forward)

[Jazz P Bass]

Conducting, but not producing any resisitance at all, as it is meant to, becos its shot. Or..
Not conducting anything at all becos the 'resisitor' is severed s'how?

If the first of these, then I cannot see the point of a short if as in its broken state, it is effectively simply a short and has lost any more of its duties than that (so the noise will be the same with a jumper, no? is my point).

If you think the OC is shorted, measure it.
Off state:100Meg ohms (yes, million)
On State: 2K ohms (thousand)

[Sea Chief]

If you think the OC is shorted, measure it.
Off state:100Meg ohms (yes, million)
On State: 2K ohms (thousand)

Ok thanks for the explantion #49. Now I understand the way this little sod works, Im happier to go poke it.

Right as to physically jumping the cell side of OC1 then. 1stly when you say measure it is there any use measuring when its off? can I not see if I get the 2k without needing to power it up, for a quickie/ start test? or is that utter balls.

[Enzo]

NEVER think up reasons not to test something. As soon as we start telling ourselves "Oh I don;t need to test THIS part because I can tell it can;t be the problem" we leave ourselves open to overlooking potential solutions.

A photocell is referred to as a cell in verbal shorthand. It is a light dependent resistor. SHine a light on it, it goes down in resistance. So you have two OCs coming to the point that feeds on into R12 and beyond. If noise is coming from one or the other OC, and it doesn;t matter which at this moment, then shorting R12 will kill the noise. If OC1 is making noise, R12 shorted will kill it. It doesn;t matter that other things also come past R12. ALL we wanted to know from the test was whether the noise was from before R12 or not. If it was, then OC1 is the likely culprit. If grounding R12 does not kill the noise, then OC1 is innocent. Simple as that. One shorting wire, 10 seconds, question answered.

It isn't about R12, R12 is simply a convenient place in the circuit to ground the signal path. R12 connects to the tube grid. If that is more convenient, ground it there and forget R12. electrically the top of R12 and the grid of the tube are the same place.


OC1 and OC2 are really just switches, in a sense, well, more like relays, I suppose. You have the two circuit paths, with the OC at the end of either before they join up. When the LED is off in each one, it has a high resistance, which effectively blocks the signal path. When the light is on in each one, the resistance goes low, so the signal path is complete. So by turning one on and not the other, we turn on that channel. The resistor inside the OC is variable. so in the on state it has low resistance. I apologize for being unclear. SHort across the resistor part of OC1. If it has a noisy resistor, this should kill the noise. I assumed we knew not to short the resistor part to the LED or across the LED. All we wanted to do was short across the resistor so it left the equation.


I don't know which pin is which on the OC, but I can figure it out. To be in that channel, we would have the LED on inside the thing, so the two leads that have a couple volts DC between them ought to be the LED. I think the LED end has a little + and - sign printed there. But ultimately we want to complete the circuit from the volume control wiper to the tube grid, so if in doubt, short the center leg of P1 to the p[in 7 of V1. That is where the ends of the OC resistor are connected.



The thing is this, regardless of how it works, OC1 is in the signal path. Signal comes in one end and out the other. Instead of concerning over just what is inside it, save that question for later. ALL we care about at the moment is whether IT is the source of the noise or not. Pretend it is a mysterious black box and no one knows what is inside it. We can still determine if it is adding noise or not. If we short its input to its output, it has a hard time adding anything. By shorting its output to ground, anything coming from it or through it will be killed. So two quick tests with a piece of wire should tell us whether it is adding the noise or not.


Unpowered all you can test on an OC is its off resistance, which will be very high. The only way to test its on resistance, which should be low, is to power the LED in it. You can do that with an external powr supply and a resistor on your bench, but it is easier to just do it in the amp circuit. Since the amp seems to change channels OK, we can assume both OCs are working. But just going to a low resistance and back does n't tell us if it is noisy. It is possible the LED inside is noisy and so the light it shines on the resistor is noisy. That doesn;t matter. If something inside the OC is making the noise, it has to be replaced, regardless of exactly what.

Once I get my head around the basics of what exactly Im about to test, I will go ahead with it.

Here is a thought: sometimes just doing the test helps make clear its purpose.

[Sea Chief]

Enzo you are a gentleman & Id ike to buy you a pint and a jolly stiff whisky for your help too (&JazzP). And boiling it down to 'mysterious black box & no one knows what is inside it' is rather comforting! (I bet youre a dad- & an ace one too no doubt).

Ok alas even with your explanation as clear as it seems, its still a different language mostly to me Im afraid. I can cope with how the OC works just. But as to why the shorting of R12 (it just looks like a 1M resistor that branches off the signal path to gnd/ I cant envision it doing anything more than 'adding a bit of relief to the highway traffic before it goes into the tunnel'/ tube) and how shorting it would give any indication of whether OC1 is good or bad.. I just cant understand. Nor can I understand if the shorting of R12 and the shorting of OC1's cell side are effectively testing for the same thing, or slightly different things. They seem to me, relativey speaking, totally unrelated.

Oh dear I am sorry after all your help. Nevertheless: I can apply the tests even w'out understanding what or why exactly Im doing (apart the bleedin obvious/ testing the OC1) .

So if these two shorting tests -are- for the same ends, am I choosing one OR the other? or am I doing the first test, then the second test? the 'easiest' for me? or is there reason for doing the R12 jump over/short before the cell-side of OC1 jump over/ short?

[Enzo]

R12 itself isn;t the point. If there is noise coming through OC1, where does it go? It comes out the OC1 at the top end, turns right along that horizontal line over to the grid of V1b, pin 7.

Forget the noise for a moment. What if we were just playing guitar and ignoring the noise. You'd have the guitar signal taking that same path. Now what happens if we ground the grid of the tube, just short it to ground? I expect that will stop my signal dead in its tracks. The amplifier has no reasoning, it cannot tell any difference between sound you want and sound you don;t want. If noise comes along, it will amplify it right along with everything else. So if noise is coming down that same path, and we ground the grid of V1b, I'd expect that to stop the noise signal dead in its tracks just as it would the guitar sound. So by grounding that point, we find out if the noise was coming into that point or is really coming in later and fooling us.

Now I don;t have a mental picture of your amp, so I don;t know how convenient it might be to get at pin 7 of that tube socket to ground it. But since the top end of R12 is connected to it, that is as good a place to ground off as the grid pin itself. And since the bottom end of R12 is grounded, that is as good a ground as any to use. SO, merely because of its location, if we short across R12, it is exactly the same as grounding pin 7 of the tube to ground. Whatever R12 happens to be is irrelevant, I am only using it because its ends are connectd to things I want to short together.


If you look at the whole signal path as a river flowing along left to right, grounding points along the path is like erecting a dam across the river. Now imagine at the end of your river we have some pollution in the water. We need to find out where along the river the pollution was entering the stream. So my river is 5 miles long, with a dam at each mile. In each dam is a gate for the water to flow through. Pollution comes out the end, so if I close the gate at the first mile and the pollution stops, then I know the sourcce of that pollution was coming from before that first dam. But if the pollution continues, then I know it is coming into my river after that first dam. Make sense? SO a mile further downstream I close the second dam's gate. Same thing? Did it stop the pollution or not.

Now imagine instead of a gate we call it a grid, and instead of a dam we call it a triode. The process is the same. By grounding off the signal path, nothing before that point can pass along. SO if we start at the first triode, then the second, then the third, etc and ground the gate at each point, we can determine if the pollution is entering the stream before or after that point. And what is noise other than sound pollution? So I'd be grounding off the signal path systematically first, to identify the segment of the circuit with the issues. Shorting across the resistive element of the optocoupler is just a way to potentially check the part itself.

[Sea Chief]

R12 itself isn;t the point. If there is noise coming through OC1, where does it go? It comes out the OC1 at the top end, turns right along that horizontal line over to the grid of V1b, pin 7.

Got that.

Forget the noise for a moment. What if we were just playing guitar and ignoring the noise. You'd have the guitar signal taking that same path. Now what happens if we ground the grid of the tube, just short it to ground? I expect that will stop my signal dead in its tracks. The amplifier has no reasoning, it cannot tell any difference between sound you want and sound you don;t want. If noise comes along, it will amplify it right along with everything else. So if noise is coming down that same path, and we ground the grid of V1b, I'd expect that to stop the noise signal dead in its tracks just as it would the guitar sound. So by grounding that point, we find out if the noise was coming into that point or is really coming in later and fooling us..

Undertand approximately, but maybe the fundamental mechanics of a resisitor (apart from the fact that it resists s'thing but still lets s'thing thru) I dont understand as the difference between its function as it is (ie simply resisiting some of the signal path but not all obvously and then shunting this to gnd) I find not dissimilar to the short we introduce.

Now I don;t have a mental picture of your amp, so I don;t know how convenient it might be to get at pin 7 of that tube socket to ground it. But since the top end of R12 is connected to it, that is as good a place to ground off as the grid pin itself. And since the bottom end of R12 is grounded, that is as good a ground as any to use. SO, merely because of its location, if we short across R12, it is exactly the same as grounding pin 7 of the tube to ground. Whatever R12 happens to be is irrelevant, I am only using it because its ends are connectd to things I want to short together.

I do understand this- great. Now i can apply this short knowing (mostly but still not all) what Im actually doing.

If you look at the whole signal path as a river flowing along left to right, grounding points along the path is like erecting a dam across the river. Now imagine at the end of your river we have some pollution in the water. We need to find out where along the river the pollution was entering the stream. So my river is 5 miles long, with a dam at each mile. In each dam is a gate for the water to flow through. Pollution comes out the end, so if I close the gate at the first mile and the pollution stops, then I know the sourcce of that pollution was coming from before that first dam. But if the pollution continues, then I know it is coming into my river after that first dam. Make sense? SO a mile further downstream I close the second dam's gate. Same thing? Did it stop the pollution or not..

Makes sense yes. But I still cant apply this to the short/ test as I cannot distinguish it from the existing "ground" albeit a resisitor in between.

Now imagine instead of a gate we call it a grid, and instead of a dam we call it a triode. The process is the same. By grounding off the signal path, nothing before that point can pass along. SO if we start at the first triode, then the second, then the third, etc and ground the gate at each point, we can determine if the pollution is entering the stream before or after that point. And what is noise other than sound pollution? So I'd be grounding off the signal path systematically first, to identify the segment of the circuit with the issues. Shorting across the resistive element of the optocoupler is just a way to potentially check the part itself.

I will need to read this about 10x. I probably still wont understand and my head might explode. Honestly: I have literally had headaches from trying to get my head around some of these replies. All be childs play to you lot, but 'now instead of a gate, a grid' is conceptual mental gymnastics for me. I now need to sleep.

[Enzo]

OK, resistor is a general term for something that has resistance. I know, I know, that is circular. It doesn't specify an amount, just being a resistor. COnsider the word "tall." What does it mean? A tall man is maybe 6 foot 2". But in professional basketball, that would be short, a tall basketball player is 7 feet tall. How about a tall drink of water - or ale, surely? That is not as tall as a basketball player.

A resistor is a part that resists the flow of current. The rating in ohms tells us how much it resists. A high resistance allows very little current to flow, while a low resistance allows a lot of current to flow, at least relatively speaking. Just like tall, resistance can be high or low depending upon the circuit. But in the amplifier we can consider 1M - one million ohms - a high resistance. That means it resists the flow of current - so not much electricity flows through it. When it is places from the signal path to ground, those million ohms in it do not allow much flow, so very little of your signal (which is after all a varying current) is sent to ground through it. A piece of wire is a very low resistance and so offers no resistance to flow, and so ALL of your signal is shunted to ground. R12 connects your signal path to ground at that point, yes, but it does so through those million ohms and so the grounding effect on the signal is minimal. That resistor is there to keep the grid from accumulating unwanted charge.

[Sea Chief]

Right ok I understand the basis of that Enzo. I will leave the fact that the resistor is not a full' barrier of resisitance, ie why leave a minimal ammount of current at all, for later.

Grid Im still not picturing, I know its a term for a section of the tube. But 'gate, now think of this as a grid' Ive been trying to decipher (is it a frame of some sort? ie some current is allowed thru section odf it like the hole in agrid? but then a gate has similar, or does my farmer leaning on his gate anaolgy not ffit the gate you refer to-!? you see how my head aches with this).

Ok so i undertsnd why choose R12 (convenient place) and shorting it will allow the signal to 'shunt' to gnd before it hits the tube. But I still dont see how even if OC1 is good the result of the shunting of R12 will be of any use, because if OC1 is bad the signal will shunt to ground/ noise will cease.. & If OC1 is good signal will shunt to gnd/ noise will cease. So whats the point of this shorting test, just to see if the noise is before the tube? (but surely i know this as ive swapped V1 for a new one). It is known is it not that the noise is before V1? I thought that has been established. Is the R12 short just for 'good procedure' or s'thing?

[Steve Conner]

I think Enzo's correspondence between a sluice gate in a river, and the control grid of a triode, isn't to be taken literally.

What he means is simply that short circuiting the grid of the tube to ground does the same thing, electrically speaking, as would closing a sluice gate in a river, or a stopcock in a water pipe or whatever. It stops the water (signal) from flowing through.

In this instance it is actually the short circuit, not the grid of the tube, that's functioning as the "gate", hence maybe the confusion.

You don't need to understand the reasoning behind the troubleshooting process right now. Just do it and report back with the results, but more importantly think about the results. That will probably be the best way of furthering your understanding.

The analogy between electricity and water can only get you so far. Radio antennas are nothing like garden sprinklers.

[Sea Chief]

OK I will do this test today. (whether a sleuce gate or farmer Giles' gate.. I cant apply it to a 'grid': a grid I know inherrantly has a 'mesh' to it, ie it has solid sections meshed with holes: I had thought that was the point of the analogy ie some part of this 'grid' allows current/ other parts stop it? anyway Im going to leave that then!!).

Ok Im still not understanding why Im doing this R12 test if the result will be the same, as I see it, (that being the signal will be shunted to gnd/ the noise WILL stop whether it being a good Oc1 -OR- a bad OC1).. but I'll do it I guess.

Do I need a GTR signal in? (the noise iirc is there w'out a GTR in FWIW). And as to the jumper wire: can I use a MM somehow as a jumper? (is this the right method even? obviously not set to 20k ohms or it will read the 1M resisitor- but another setting on thr MM?). Or if not whats the std method of attatchment either side of R12 then?

[Jazz P Bass]

For now, do not use a signal.
Any piece of wire soldered to each end of the R12 will do.
Please do what you can to move the actual troubleshooting along.
It is getting a tad tiresome.
The theory can wait for another day.