scope waveforms?

[daz]

Do not use the amp for the probe adjustment. When you touch the probe to that calibration point there should be a 1kHz square wave present. It may be such a pure square wave that it looks more like two dashed lines with little or no rise and descent lines visible.

Ok, i tried to post and couldn't before you posted. But i figured it out after re-reading loud thud's post and yours and i DID calibrate it touching the probe to the cal on the scope and made the dashes flat and level adjusting the screw on the probe. Waveform still looks as in the pic i posted. But it changes a lot with tone stack adjustment. I have the controls where i play it.

[JoeM]

You will see the waveform change with tone control adjustment. If you have a square wave generator, you'll be surprised how much the controls can change the shape of the waveform.

[daz]

Yeah, but it makes me wonder....would chuck have said the waveform looks good if i adjusted the tones so it was much different from that? Hmmmm...

Oh, and chuck.....i tried checking with 470R and 2k for the PI cathode. The wav looked good with 2k, but with 470 the lines got fuzzy. the shape of the wav didn't change much, but the actual green lines looked undefined and fuzzy, and it was different along the length of it. Some areas looked cleaner, some more fuzzy. Forgot to take a pic, but I've been trying to do many things at once here.

[Enzo]

Part of learning what the waveforms look like is losing the preconceptions. Your nice clear picture looks like it looks, but you were EXPECTING it to look more smooth and round, right? But that expectation is born of the idea that a pleasing tone ought to have a pleasing appearance on the screen. But that is often not the case. Those little hooks on the edges might just add the sparkle or edge you want to hear. SO if the amp sounds great, then any waveform you see should be what you want, regardless of what you were thinking it would look like ahead of time.

[Chuck H]

Bad 470k resistor? Possibly the lower PI output the 1.5k in series is preventing an oscillation or some induced crosstalk? Get a pic up of the 470k vs. 2k wave forms at the power tube grids whenever you can. It sounds interesting.

I do recognize the wave form as it relates to EQ. You're using more top end than I do It's the shape of the dip and recovery along with the amount of asymmetry that I was looking at in your pic.

[Jazz P Bass]

Scope Shots

Here are two shots that I took of a Peavey Musician Mark III.
The first one is at 1K input, 100mv's, with the gain at half way.
The amp has a nice gritty sound.

The second one is at 800 Hz with the gain full on.
This is full blown, raunchy distortion.

[daz]

Part of learning what the waveforms look like is losing the preconceptions. Your nice clear picture looks like it looks, but you were EXPECTING it to look more smooth and round, right? But that expectation is born of the idea that a pleasing tone ought to have a pleasing appearance on the screen. But that is often not the case. Those little hooks on the edges might just add the sparkle or edge you want to hear. SO if the amp sounds great, then any waveform you see should be what you want, regardless of what you were thinking it would look like ahead of time.

I get that. But I believe there are waveforms that would universally considered bad for one reason or another. I wasn't looking for a pleasing looking one, just looking at shapes i thought i recall as being bad from all the ones i've seen posted over the years. Apparently my memory wasn't serving, but it definately wasn't a matter of looking for a rounded WF. I just figure that probably means a cleaner tone.

Bad 470k resistor? Possibly the lower PI output the 1.5k in series is preventing an oscillation or some induced crosstalk? Get a pic up of the 470k vs. 2k wave forms at the power tube grids whenever you can. It sounds interesting.

I'll try and remember to do that. I don't think it's a bad resistor because it's always sounded the way it did before i added the 1.5k. If i remember correctly, i think i was probing at the speaker out when i checked the cathode with 470R vs 2k.

I do recognize the wave form as it relates to EQ. You're using more top end than I do It's the shape of the dip and recovery along with the amount of asymmetry that I was looking at in your pic.

I don't use a lot actually. Don't forget the speaker, whether i'm using NFB or presence or what. Anyways, my tone is generally quite warm and i only add top till it's got enough clarity to keep t from being be too smooth and mushy. But the amp's top is very smooth so it never sounds thin or harsh.

[Chuck H]

I don't use a lot actually. Don't forget the speaker, whether i'm using NFB or presence or what. Anyways, my tone is generally quite warm and i only add top till it's got enough clarity to keep t from being be too smooth and mushy. But the amp's top is very smooth so it never sounds thin or harsh.

Ahhh!!! But you DO have a lot of top end in your EQ other than the tone stack. Your preamp bypass caps are both .68uf, you have a 250pf bright cap across the gain control and a 500pf jumper over a 470k resistor between stages 1&2 and 2&3. That's WAYYY more HF accentuation than any amp I'm familiar with. I think anyone here that's done this stuff for awhile would agree that it's a very bright preamp. It may be part of the mojo you attribute to your amp though. You tone down the top end after that with the 33k slope resistor. After that, if your OT and speaker are somewhat dark in tone that has allowed you to pump up the top end and add clarity to the front end of the amp giving bottom end definition that some Marshalls lack! It's all tone soup. There's a lot of different recipes.

[daz]

Ahhh!!! But you DO have a lot of top end in your EQ other than the tone stack. Your preamp bypass caps are both .68uf, you have a 250pf bright cap across the gain control and a 500pf jumper over a 470k resistor between stages 1&2 and 2&3. That's WAYYY more HF accentuation than any amp I'm familiar with. I think anyone here that's done this stuff for awhile would agree that it's a very bright preamp. It may be part of the mojo you attribute to your amp though. You tone down the top end after that with the 33k slope resistor. After that, if your OT and speaker are somewhat dark in tone that has allowed you to pump up the top end and add clarity to the front end of the amp giving bottom end definition that some Marshalls lack! It's all tone soup. There's a lot of different recipes.

Thats basically it.....tone it down later on. The point i was making was that the end result isn't anything like you might think looking at the preamp. I did it that way because it's what it took for this amp to get the gain stages to have a clear tight/focused tone without flab and wooliness. I have tried and tried but nothing i did that eliminated any of the high end accentuation in the pre ever worked. Remove just one thing, any of the treble peakers or the bypass caps and the tone loses what makes it special and the feel as tho it plays itself. Thats why when people say "just remove the bypass cap" or some such thing when trying to tell me how to get less gain or whatever i'm trying to do, i tell them i can't. They don't get it because they aren't here to listen to what happens w/o it, and if i tell them they won't believe me unless they hear for themselves. You've said it before yourself....paraphrasing: things don't always sound like you would think going by a schematic. I've spent years wondering why my amp isn't bright as hell since my pre has so much hi end accentuation compared to others. I still don't know, but it isn't bright as it looks. Thats what it took to not be a mud fest and have focus. Oh, and i have lower filtering than most marshalls of this type on the PI and 2nd stage. That keeps things more juicy and less cutting too.

[Chuck H]

Lowering the filtering to soften it up a little with a bright preamp is pretty smart actually. And I know that most of the work you've done on this amp has been trial and error so that's a long haul. You might consider building an identical clone at this point and keep it somewhere else so if anything tragic ever happens (house fire, running from the law, etc.) you'll still have the amp

[daz]

Trust me chuck, after all this time if anything happened to it I could build another from memory even w/o a schematic.

[Enzo]

Universally bad.

COntext is everything. The reason guys overdrive amps in the first place is that waveforms coming through the amp are so distorted - which is "bad" in the context of making a clean amp. But still "good" because we like the sound of it. Some seriously asymmetrically clipped signal might be just the sound you want. But if it is the output of a power amp driven alone, I'd say not good.

[daz]

Universally bad.

COntext is everything. The reason guys overdrive amps in the first place is that waveforms coming through the amp are so distorted - which is "bad" in the context of making a clean amp. But still "good" because we like the sound of it. Some seriously asymmetrically clipped signal might be just the sound you want. But if it is the output of a power amp driven alone, I'd say not good.

Well, for example, parasitic oscillation or crossover distortion when biasing with a scope are a couple things that i think would be considered bad by 99.9% of people. Point being, knowing nothing about scopes i was looking at what i thought i'd seen being described as bad at some point. Not going by what looked pleasing or not, thats all i'm saying.
By the way, can anyone show me a wavform of what parasitic oscillation looks like?

[Enzo]

You are right, a parasitic would almost always be unwanted. And you are also right that it is a waveform. When you were talking about waveforms, I had in mind signal shape itself, which excludes what you had in mind. Here is a basic article, and down the pafe a way is a picture of a parasitic. It is drawn, not a photo, but it gives the idea. You may find the rest of it interesting as it discusses waveforms and harmonics.
http://www.r-type.org/articles/art-125.htm

[Enzo]

You are right, a parasitic would almost always be unwanted. And you are also right that it is a waveform. When you were talking about waveforms, I had in mind signal shape itself, which excludes what you had in mind. Here is a basic article, and down the page a way is a picture of a parasitic. It is drawn, not a photo, but it gives the idea. You may find the rest of it interesting as it discusses waveforms and harmonics.
http://www.r-type.org/articles/art-125.htm