Seem to have found something interesting, need explanation

[The Dude]

If you have a dual trace scope, you need two probes to use the dual trace function. Even then, you will mostly just use one probe. The dual trace function is for comparing two waveforms. The scope displays two traces simultaneously. Handy, but not a necessity for most audio work. Even if you have a dual trace scope, you can still use a single probe to display a single waveform.

I'm not sure which exact probe you are talking about, but the switch is likely a X10 switch meaning that the scope will "receive" the waveform at one tenth it's actual voltage. This is done because all scopes have a maximum voltage limit and if you exceed this voltage you can damage it. With the X10 probe, you can look at much larger waveforms without damage to the scope. Some scopes "assume" you are using an X10 probe and automatically calculate the volts/division measurement/display. Others (mostly older and/or cheaper scopes) do not and you have to keep that in mind when calculating the amplitude of the waveform.

[daz]

If you have a dual trace scope, you need two probes to use the dual trace function. Even then, you will mostly just use one probe. The dual trace function is for comparing two waveforms. The scope displays two traces simultaneously. Handy, but not a necessity for most audio work. Even if you have a dual trace scope, you can still use a single probe to display a single waveform.

I'm not sure which exact probe you are talking about, but the switch is likely a X10 switch meaning that the scope will "receive" the waveform at one tenth it's actual voltage. This is done because all scopes have a maximum voltage limit and if you exceed this voltage you can damage it. With the X10 probe, you can look at much larger waveforms without damage to the scope. Some scopes "assume" you are using an X10 probe and automatically calculate the volts/division measurement/display. Others (mostly older and/or cheaper scopes) do not and you have to keep that in mind when calculating the amplitude of the waveform.

So if i only need one probe for most things, how does a single probe get information....doesn't the unit have to be grounded to the amp? I can't see how a single connection to the scope would work. What am i missing here?

[The Dude]

The scope probe will have a ground wire coming off of the side to ground it like the one in the picture below, which brings up another point. DO NOT ever use the scope to troubleshoot a switching power supply unless you are using an isolation transformer. Most all switching supplies use a ground other than chassis ground!

[Enzo]

Look up a photo of a scope probe, it isn;t like a set of voltmeter probes.

A scope probe is a thing shielded cable with a probe body on the end the tip is usually a point, but most have a little hook thing that slips onto the end to turn it into a hook clip. The shield of the cable is also its ground, and there will be a little short clip wire from the probe for you to clip to chassis or whatever your ground point might be. Most scopes also have a ground binding post on the scope panel, so you can use a separate long ground clip wire to chassis and then remove the short clip from the probe itself. So your scope probe is both probe hot and probe ground in one.


Here: watch this... It is just the first one I found, you may find others you like more.

How to use an oscilloscope / What is an oscilloscope / Oscilloscope tutorial - YouTube

OR

Basic 1X and 10X Oscilloscope Probe tutorial - YouTube


And while you look at that, notice all the other scope related videos along the right side? There are ZILLIONS of tutorial things about scopes on the interwebs.



You need a probe for each display channel you wish to use, that is for each point in your circuit you wish to monitor. Most of the time I use just one probe and one channel. In my shop, the dual trace scope - two channels - has a regular probe in one channel, the othr channel has a input cable over to my speaker/load patch panel. So if I am working on an amp, I can connect the other scope channel to the amp output and can switch v=over to that any time I want to compare the amp output with some other point. And of course anytime i want that second channel for probing, I simply unplug my cable from the patch bay and connect a regular probe.


Most scopes these days are dual trace, and have a variety of triggering options. The scope will do things you cant dream of yet. I can connect channel B to some point and use that to trigger the other channel's view of something. That allows me to see timing relationships and sych. But you don't need to know any of that to get 90% of the value of a scope. It is like learning the guitar, you don;t need to know a diminished suspension modulating to a 9th to make good rock and roll.

[Enzo]

Daz, do you have any friends with scopes? ANyone into electronics to some extent? That you could visit them and ask just to see the basics of scoping. really, just doing that would answer so many of your basic questions.

[daz]

Daz, do you have any friends with scopes? ANyone into electronics to some extent? That you could visit them and ask just to see the basics of scoping. really, just doing that would answer so many of your basic questions.

No, I don't, but i will watch that youtube link tomorrow. I watched a bit just now and it's a lot better than a couple i looked at earlier today that left me scratching my head. This one seems pretty clear. I will go look at that gold star again this weekend if they still have it, which i'm sure they will. Now that i know the probe has a ground connection point and know it's comple i'll buy it as long as they can plug it in so i can see it works. Thanks.

By the way, back to the original subject, i got home and put some play time in. No doubt that whatever that 2k's worth of cathode resistors did, w/o a doubt it was a major find. It fixed a lot of things that have always felt were wrong and the amp now sounds great. Off the top of my head, both guitar volume pot and picking strength are considerably more dynamic, (#1 on my list of most important details) tone is more articulate, master is no longer an on/off switch, louder volumes are smoother and less harsh, turning the gain down gets the amp much cleaner than before, and the overall tone is just better as tho the proper harmonics are being generated. I have no doubt that extra gain i have been fighting forever was happening at the PI. Not a huge amount, but it messed things up enough to keep me from getting it just right. A serious improvement. Then i thought i'd put the closest resistor to what i had there instead of leaving 2 in series. I had lifted one end of the 470R and added a 1.5k in series with it to get about 2k when i 1st tried this. So i put a 2.2k in thier place and the tone wasn't quite as good. Took that out and tried about 3k....it got tinny and sounded bad. So it seems i threw a dart at the dartboard blindfolded and hit the bullseye. So i put the 2 resistors back.

EDIT: changed the 10k tail to 22k and it's even better. There is a quality to the tone i can't put my finger on, but as soon as i played it i knew it was a very important missing component that was always present in my marshall amps that i really liked. I don't mean when i changed the tail, but that just added more of that goodness to what the 2k cathode did. Part of it that i CAN put my finger on is a degree of focus that this amp has never had. Makes playing soooo much easier. All wooliness is gone and it's as articulate as i could ever want and dynamic as hell, even tho it's all preamp OD ! W/O a doubt this was a major major improvement. It does worry me that the cathode at 2k is higher than any amp I've seen because it makes me think like chuck said something won't be right at some point....maybe at stage volumes something won't be right or such. But i can't imagine that as good as it is right now. I do see a lot of amps, many highly revered ones use 1.2k and larger tails too. Nothing more than 1.2k for the cathode tho.

[jazbo8]

Do you have Merlin Blencowe's book? The subject of the bias level and the tail for the LTP and their effects on tone/sound is covered.

[Chuck H]

This is exactly where a scope comes in handy. It would allow you to see exactly what's happening to the wave form at each stage so you know what changed. Then, if you DO find that something is a little hinky you can attempt to recreate the good part of the change without the hinky because you know what you're looking for. No more stumbling around in the dark. As in "I made a change and it sounded great but now when I turn it up it doesn't sound like it did" or whatever. You have a few posts like that. If you can SEE what's good and bad you have an easier target when making adjustments. The scope will show you things like whether or not the gain or balance of the PI changed. It will also tell show if the new colder bias condition is driving the power tubes into cutoff and saturation ahead of the PI (as it should). It will show you how your ideal wave form is changing at different stages as you alter your master volume allowing you to adjust and idealize for all playing conditions. So, in short, the scope will show you what it is that you discovered sounds good and also let you know if the amp is within correct operating parameters. What more could a guy like you want. So it seems that this side step into a scope discussion IS on topic. After all, how can we comment of what your changes did if we don't have the above information that the scope provides? Since you are using a pre PI master and altering the PI bias it's really hard to know what you're clipping and what you're not and how the stages are interacting.

Sans scope I will take a shot in the dark on your original issue. Since you may not be clipping the PI it's possible that altering the bias has given the PI a tad more input signal headroom. That is, the PI will take more signal before it clips. But that's not all. You have also skewed the drive balance. The colder bias starts to favor the non inverted side a little because of your different plate resistors. Adding the 22k tail would push that a little further. It could be that the imbalance is what you like. Asymmetry in a push/pull circuit cuts power a tad but also reduces the cancellation of 2nd order harmonics. With your pre PI master it's likely that the added PI input headroom and increased 2nd harmonic are what you're liking about the change. Unfortunately it's very likely some of that will get lost as the PI or power tubes start to clip or interact. And with the cold bias on the PI it's hard to say just what will happen at that point without looking at it on a scope.

[daz]

This is exactly where a scope comes in handy. It would allow you to see exactly what's happening to the wave form at each stage so you know what changed. Then, if you DO find that something is a little hinky you can attempt to recreate the good part of the change without the hinky because you know what you're looking for. No more stumbling around in the dark. As in "I made a change and it sounded great but now when I turn it up it doesn't sound like it did" or whatever. You have a few posts like that. If you can SEE what's good and bad you have an easier target when making adjustments. The scope will show you things like whether or not the gain or balance of the PI changed. It will also tell show if the new colder bias condition is driving the power tubes into cutoff and saturation ahead of the PI (as it should). It will show you how your ideal wave form is changing at different stages as you alter your master volume allowing you to adjust and idealize for all playing conditions. So, in short, the scope will show you what it is that you discovered sounds good and also let you know if the amp is within correct operating parameters. What more could a guy like you want. So it seems that this side step into a scope discussion IS on topic. After all, how can we comment of what your changes did if we don't have the above information that the scope provides? Since you are using a pre PI master and altering the PI bias it's really hard to know what you're clipping and what you're not and how the stages are interacting.

Sans scope I will take a shot in the dark on your original issue. Since you may not be clipping the PI it's possible that altering the bias has given the PI a tad more input signal headroom. That is, the PI will take more signal before it clips. But that's not all. You have also skewed the drive balance. The colder bias starts to favor the non inverted side a little because of your different plate resistors. Adding the 22k tail would push that a little further. It could be that the imbalance is what you like. Asymmetry in a push/pull circuit cuts power a tad but also reduces the cancellation of 2nd order harmonics. With your pre PI master it's likely that the added PI input headroom and increased 2nd harmonic are what you're liking about the change. Unfortunately it's very likely some of that will get lost as the PI or power tubes start to clip or interact. And with the cold bias on the PI it's hard to say just what will happen at that point without looking at it on a scope.

Well, heres the big question. Doing this without the shadow of a doubt had a major effect EVEN ON THE PREAMP. I bypassed the gridstopper you recently told me to use and no cutoff or oscillations ! Tried a 470k as the load there in place of the 220k you told me to use, same thing....no issues. I put them back to your values because they sounded best./ But the point i;m trying to make is twofold. 1-Somehow, and it makes no sense to me, the OD has dropped in the preamp to normal levels. How the PI DOWNSTREAM fro the preamp did that is well beyond my understanding unless maybe the preamps degree of signal was too high for the PI as was and it was happening at the PI instead of the pre. I dunno, i only know the results. crazy huh? And 2-how in gods name do other amps use the same PI and not have this issue? Thats not a rhetorical Q, I really wanna know. The PI values and config are just like many amps, but i dunno thier voltages. Could it be the voltages or filtering? I have 20uf there and the plates are around 230 or thereabouts, one of course being different by a few volts. Then again, many amps DO have larger values if not AS large on the cathode. Many have the 1.2k/47k cathode/tail combo. maybe i should drop the cathode to 1.2k.

Now anything i try with the preamp gives me much more obvious results with no surprises. I think i nailed the preamp tone years ago but this has always kept it from reaching full potential. I will go see if thats scope is available this weekend.

[daz]

Well, heres the big question. Doing this without the shadow of a doubt had a major effect EVEN ON THE PREAMP. I bypassed the gridstopper you recently told me to use and no cutoff or oscillations ! Tried a 470k as the load there in place of the 220k you told me to use, same thing....no issues. I put them back to your values because they sounded best./ But the point i;m trying to make is twofold. 1-Somehow, and it makes no sense to me, the OD has dropped in the preamp to normal levels. How the PI DOWNSTREAM fro the preamp did that is well beyond my understanding unless maybe the preamps degree of signal was too high for the PI as was and it was happening at the PI instead of the pre. I dunno, i only know the results. crazy huh? And 2-how in gods name do other amps use the same PI and not have this issue? Thats not a rhetorical Q, I really wanna know. The PI values and config are just like many amps, but i dunno thier voltages. Could it be the voltages or filtering? I have 20uf there and the plates are around 230 or thereabouts, one of course being different by a few volts. Then again, many amps DO have larger values if not AS large on the cathode. Many have the 1.2k/47k cathode/tail combo. maybe i should drop the cathode to 1.2k.

Now anything i try with the preamp gives me much more obvious results with no surprises. I think i nailed the preamp tone years ago but this has always kept it from reaching full potential. I will go see if thats scope is available this weekend.

EDIT: for the record, i put the tail back to 10k. I tried going back and fourth a/b'ing both the tail and cathode till i was sure of the results. I was wrong on the tail, it's better at 10k. The cathode remains at 2k because when i a/b'd that, the difference is still a great benefit over the 470R.