marshall 4210 jcm 800 help!

[alchemy]

well i'm at least getting a bit closer to figuring out how to use this set up. a couple of things to note: the sine wave from the generator starts to look get tall and un-sine wave like (almost flat at one part of the top) as i turn the volume of the signal up. I can get a very clean sine wave to show on the scope if the signal volume is right. Is this just because of distortion as the signal gets louder? also, while taking measurements in different spots, i would sometimes have to switch the settings on the scope to get it clear. is this just due to probing diffeerent points in the circuit that have different levels of gain?

so on the input jack, i get a nice clear wave. the b+ looks like a saw tooth, but is clean looking. there does not appear to be a sine wave riding the saw tooth wave. i noticed probing the grid of v1 it looked good, but starting at v2 there are small lines (like a little dash) perpendicular to the wave that "rides it" up and down the hills and valleys. could this be what i'm looking for?

a few other things that seem strange. running a cd or the generator through the amp and speaker, i dont not hear any out of tuneness or abnormal distortion. also, i cant get over the fact that even with the dirt channel turned off (jumpered), the volume and gain knob still add that wierd "super bass" when they are both turned up high. Seems to me they should be completely isolated from the clean channel.

thanks for the help and holding my hand while i stumble through trying to learn how to use this scope.

[Enzo]

Look at the schematic. The two volume controls are essentially connected together through a couple resistors. And the dirt channel may be grounded off, but that happens way over on the far side of two tubes from the volume control.

Let's say you are on clean channel. The signal leaves the clean volume control, through the 100k with cap and on to the grid of V3a. But that grid is also connected to the wiper of the dirt volume through a 220k with its own little cap. The dirt volume is a 1meg. SO the grid of V3a has a resistor to ground that varies from 220k to 1,220k as the dirt volume is turned. Not only that, ther is the parallel 220pf cap. When dirt vol is all the way down, that 220pf is wired from grid of V3a to ground. Turn the durt volume all the way up and that cap now has 1meg between it and ground. SO in a sense that cap is in a circuit similar to the tome control ina guitar. That is how these things can interact.

And keep in mind that grounding off those couple points in the dirt channel doesn't make it dead or disappeared. All it does is ground off the signal path at a couple points. All the circuits are still there.

Your scope. It has triggered sweep. That means it is designed so it CAN wait to trigger a sweep until it sees some signal. That makes it easier to get a stable display on a repetitive waveform. The trigger mode on most scopes has Normal and Auto as selections. Normal is the one where it waits. When selected, you can get no trace until the signal is present, or sometimes just a dot of static is enough to trigger a sweep, and you get the moving dot.

FLip that switch to Auto, and the thing should just sit there free running a trace.

The horizontal sweep frequency determines how fast the beam or dot moves across the screen. Depending on what you are doing, it may be useful for the dot to creep across, or you may want it to fly really fast. For most audio work, I want it at a speed to get me a nice clear waveform display. But here is my rule of thumb. At slow sweeps, you see the dot moving across. Turn it up a notch the spot moves faster. A little faster and you have a line, but the line flickers or blinks. One more click and the line looks steady. I usually like it best at teh slowest setting that the line doesn;t flicker. That is easiest on my eyes. That is one man's opinion.


yes, you will have to tweak the vertical sensitivity. You can scope the guitar signal at the input and it is maybe a 1/10 of a volt, but that same signal leaving the phase in verter might be 30v. SO if you leave the scope set for one of these, then the other makes a poor display.

And your test wave strats top flatten at the top? That is clipping in action. That is what it looks like.

[alchemy]

Let's say you are on clean channel. The signal leaves the clean volume control, through the 100k with cap and on to the grid of V3a. But that grid is also connected to the wiper of the dirt volume through a 220k with its own little cap. The dirt volume is a 1meg. SO the grid of V3a has a resistor to ground that varies from 220k to 1,220k as the dirt volume is turned. Not only that, ther is the parallel 220pf cap. When dirt vol is all the way down, that 220pf is wired from grid of V3a to ground. Turn the durt volume all the way up and that cap now has 1meg between it and ground. SO in a sense that cap is in a circuit similar to the tome control ina guitar. That is how these things can interact.

Guess that just goes to show i've got a lot to learn. I looked at the schematic for a long time and I actually saw that, but decided that it couldn't work that way. Oops. Thanks for clarifying. Now why the hell would they make it interactive like that? The amp sounds a lot better on the clean channel (if dialed in right) with those 2 knobs turned up. But then when you switch you are stuck with high gain and low master (unless playing somewhere it can be at full volume, not my house).

And keep in mind that grounding off those couple points in the dirt channel doesn't make it dead or disappeared. All it does is ground off the signal path at a couple points. All the circuits are still there.

that much at least, i did figure out.

Your scope. It has triggered sweep. That means it is designed so it CAN wait to trigger a sweep until it sees some signal. That makes it easier to get a stable display on a repetitive waveform. The trigger mode on most scopes has Normal and Auto as selections. Normal is the one where it waits. When selected, you can get no trace until the signal is present, or sometimes just a dot of static is enough to trigger a sweep, and you get the moving dot.

FLip that switch to Auto, and the thing should just sit there free running a trace.

I'll give the trigger function a try.

The horizontal sweep frequency determines how fast the beam or dot moves across the screen. Depending on what you are doing, it may be useful for the dot to creep across, or you may want it to fly really fast. For most audio work, I want it at a speed to get me a nice clear waveform display. But here is my rule of thumb. At slow sweeps, you see the dot moving across. Turn it up a notch the spot moves faster. A little faster and you have a line, but the line flickers or blinks. One more click and the line looks steady. I usually like it best at teh slowest setting that the line doesn;t flicker. That is easiest on my eyes. That is one man's opinion.

Thats good to hear. least i'm doing something right! I just kept messing with the scope until I could get a good waveform, and that was how I ended up setting it up. Seemed to work best that way.

yes, you will have to tweak the vertical sensitivity. You can scope the guitar signal at the input and it is maybe a 1/10 of a volt, but that same signal leaving the phase in verter might be 30v. SO if you leave the scope set for one of these, then the other makes a poor display.

And your test wave strats top flatten at the top? That is clipping in action. That is what it looks like.

[/QUOTE]
Cool. These i had guessed as well, so I feel a bit better that I'm starting to absorb some of it. I know using a scope isn't that complicated, but it is a bit daunting having neveer done it and not having someone run me through it once. You alls help is invaluable. Thanks.

So what next do you think? I did not see any sine wave riding the saw toothed b+, so i'm guessing that's not where it's coming from. But what about the little lines I saw riding the sinewave from v2 on. Could that be the culprit, or just a product of a crappy generator being amplified? Should I put shielded wire (one end grounded) on the other grids that dont have it? How high can I make the grid resistors on the power tubes? Bumping them up to 10k seemed to help, just wondering if i can take them higher to see if it blocks it more. Change the rest of the coupling caps that haven't been changed? WHat about removing the diodes? Is it possible that part of the signal is coming back the ungrounded side of the dirt volume pot and getting to ground through the diodes? Any reason I can't just pull em out to test it? Shit , for that matter I guess i could just pull the 220p cap and 220k resistor and see if that gets rid of the noise. That otta turn the damn channel off!!

[alchemy]

If i want to scope the output of the amp, should i scope the OT secondary or the plates of the power tubes, or does it matter? I just didn't know if the scope would add an extra impedance to the OT, essentially putting it in parallell. Just wanted to make sure....

[Enzo]

Your scope, like ANYTHING you connect to a circuit, has its own impedance. SO it can intereact with the circuit. But the scope input impedance is sometjhing on the order of 10MEG, so it will have a vanishingly small effect on most things you work with. SO think of it as a 10 meg resistor with a view screen.

And where do you look? ANywhere you want. The whole point of a scope is that you can look at what is happening all over a circuit. Like a wire - there is signal at one end. is it also at the other?

[alchemy]

so i tried playing it for a while today, very loud, and with an EQ pedal ran through the effects loop, just to see if some of the shittyness could be EQ'd out and because I'd never played the amp that loud for that long. By cranking the amp and using an EQ, I can almost get a usable sound, but it obviously still has problems.
After about 10 minutes, smoke started coming out, it looked like from the screen grid resistor. After shutting it off and letting it cool, and testing the tubes ,I plugged back in and took some measurements. All the screens and plates are very close, about 470v, and the screens are about equal with the plates. I was under the impression that the screen needs to be at least a volt or two below the plate voltage.
Since it does not appear that the tube in particular is drawing more current than the other (i'm only assuming this due to the fact that I would expect the voltage to drop if the current went up), then does this yet again point me towards an oscillation? Could this invisible signal cause the screen resistor to smoke?
Gonna change the resistors just for piece of mind (they are original), and try scoping the output, just to see if I can see any funny waveforms.
Should I try to lower the screen voltage, or is the fact that it is equal to the plate voltage a sign of another problem?

[alchemy]

Almost fixed. A few other ???'s

Well I think I found the problem. This was actually a bunch of problems that slowly got fixed one by one, but the last big one was the bias somehow drifted down very very low. I had checked it repeatedly during this whole process and it was dead on, but somehow in the last few days since checking it drifted down to -46v, which was something like 26ma. After setting it again the amp sounded pretty good, though it could still use a few tweaks. I just put new sprague atoms in the bias supply and swapped resistors when I got the amp and converted to el34's. And I didn't think I had changed any components that would affect the bias. The other big thing that made a difference was cutting the little rubber bands and moving the grid wires away from everything else, and i think i shortened a few.




So, hopefully my last set of questions:

1)What would cause the bias to drift?
2)What are some common solutions to add a bit more (and tighter) low end to these amps? Presence resistor? Caps in tone section?
3) Can anything be done about the vol/gain/220p/220k path to ground when the dirt channel is off? This only matters if I'm trying to play with the gain and vol all the way up on the dirt channel, and then switch to the clean channel. It makes it unplayable when you switch.
4) As for the channel bleed, it's still there, but not too big a deal now that the parasitic is gone. But can anything be done about it? Is the problem simply from the circuit design and the transistors are not hefty enough to completely turn the channel off? Would a different transistor make a difference?

Again, thanks you all so much for the help (especially Enzo). Walking me through the oscilloscope was invaluable, and will benefit me for years to come.