Scope image interpretation

[pdf64]

I suspect that the operating conditions typically used for tube MI amps tend not to have a particularly linear transfer function.
So with the typically low degree of negative feedback, high power sine waves may look a bit 'off'.

[Enzo]

Guitar amps are not now, or were they ever, designed to be hifi amps. They are designed to have limited and shaped frequency response, and clean reproduction of something was not the goal. Plug your guitar into an input on your PA system, and THAT is hifi reproduction of the guitar. SOunds like crap.

A hifi amp is a reproducer of sound. A guitar amp is a primary producer of sound - it is part of your instrument. That is why they all sound different. Guys tell you they play a Fender or a Marshall.

[g1]

I was also thinking it was biased hot like Stan mentioned. At full power like that I expect to see at least some "hint" of crossover notch.

[nsubulysses]

I'm sorry I should have maybe noted that the last scope image I posted was a Yamaha G100. A boiled meat example.

However here is a tube amp doing nearly the same shape, Orange Dual Terror


I get a lot of old Ampegs. Maybe I am somewhat brainwashed to expect to see a very accurate reproduction. Have you ever used a V4 as a stereo? It sounds really good through a 2x12. And you can hear it a block away

[nsubulysses]

You can see a lot more information if you can view frequency domain as well as time domain. The scope is the tool for time domain but a spectrum analyzer is the tool for frequency domain, so you can see the harmonics and their absolute amplitude.

You know, I was kind of wondering how you knew what you did just from the scope image alone. thanks for mentioning that. Now you're making me want a spectrum analyzer. Or I wonder if it would be cheaper to get a modern MDO that can work as one.

[Chuck H]

One of the el84 amps I build looks about like that. Symmetrical at the 'just starting to flatten' level. I never liked the clean tone of that design And they ARE biased a little hot. That design is more of an overdrive design though. Get it humpin' and all bets for symmetry and absence of crossover distortion are off. Not surprised the Dual Terror looks like that. At the 2009 NAMM show it stopped me in my tracks because it sounded so similar to my amp. Hmmm.?.

EDIT: I agree with Enzo on the ideology and purpose of guitar amps. Regardless of their awkward way of making distortions analogous to power I've always sort of thought of them as "signal generators" rather than any sort of reference amplifier.

[Jazz P Bass]

I'm sorry I should have maybe noted that the last scope image I posted was a Yamaha G100. A boiled meat example.

However here is a tube amp doing nearly the same shape, Orange Dual Terror

Next time, set the horizontal position to line up the reticles with the sign wave peaks.
Key information is to be had there.

The first pick, from post 1, should sound good because of the lack of symmetry.
(the top is narrower than the bottom)
(maybe turn it down a tad to elimate the notch)
The second pick, with the square corners, will sound raspy. (odd harmonics)
The third pick is going to sound tubier because of the rounded corners. (even harmonics)

[Tom Phillips]

The scope shots in posts #13 & #19 don't include the absolute amplitude information. Therefore, we don't know the key information regarding at what point the amp produces those waveforms. It makes a significant difference if the waveform was produced near full power output, all the time or under some other condition. I think that the comments all assume that the the waveforms were produced at or near full power but that was never stated. The rate of the transition from clean to clipped is also important as is the touch sensitivity to amplitude change during the transition. Lots of stuff to consider.

[g1]

Have you ever used a V4 as a stereo? It sounds really good through a 2x12. And you can hear it a block away

No, but I have a pair of them I sometimes use in stereo . Out here "around the block" is 4 miles.

[Enzo]

Old Ampegs are an exception, Mr. Hull indeed WAS trying for a hifi sound, that is why an old B15 sounds so great with music coming through it, can't do that with a Marshall.

[km6xz]

The last waveform is not preamp or PI distortion, none of the characteristics since class stages have different cut off and saturation slopes so I assumed it was output voltage limited....clipping. It is probably an excellent reproduction amplifier with reasonable compression with a stiff power supply(does not have too hard at 15-30 watts). If it is going through a preamp, it is probably set completely flat. A preamp has typical Class A and its own distortion that could make that combination pretty sweet, with lots of even order harmonics. Preamp distortion has a different signature and different spectrum than pushpull outout does. But can it get dirty? The volume was probably at 10 and gain at some low value. It would be interesting to see what the opposite would produce. It the pre has enough gain, it probably sounds pretty rich in harmonics generated in the preamp and reproduced cleanly in the power amp. Overall, not having experience with that amp I would venture that its clean is almost too clean and dirty is full of second harmonic

[Mick Bailey]

When scoping a guitar amp you also have to consider what the tone stack is doing. Sometimes the signal is asymmetric going into the stack and then with a little bit of clipping and tone-shaping there can be a pretty odd-looking waveform coming out of the amp. It's always worthwhile adjusting the gain and tone controls to get the best looking waveform and flattest response to see the best representation of what the amp is doing.

[km6xz]

Old Ampegs are an exception, Mr. Hull indeed WAS trying for a hifi sound, that is why an old B15 sounds so great with music coming through it, can't do that with a Marshall.

Yes, it doubled as PA for some groups with vocals feeding it. Pretty nice sound for a small venue.

[Leo_Gnardo]

Yes, it doubled as PA for some groups with vocals feeding it. Pretty nice sound for a small venue.

One of my clients used an early 70's 50W Marshall head as PA for his folk-music duet, drove a pair of speakers-on-sticks (Community CSX35). Hundreds of shows over the course of 10 years or so. Sounded great, no complaints. But it did look a little funny, the guy-and-gal duo strumming acoustic guitars with that Marshall on stage. By looking you would think it would suck, but it didn't, so ignore the amp & dig Mr & Mrs Folkie Goodvibes. It's the music that counts innit?

[Enzo]

My first gig, we needed monitor power, and all we had was a Fender Showman head for the job. It was a lot of work to get good sound out of it without lots of feedback freq peaks.