Guitar Amplifier Overdrive

[Malcolm Irving]

Ulrich's website:
https://sites.google.com/site/stringsan ... -overdrive
includes a free supplement to the book, as well as other interesting material.

[roberto]

Thanks Malcolm,

I've one question concerning the description in the link you just posted:



It is clear the purpose of having different input impedances at different frequencies, so higher impedances at low frequancies to easily ovderdrive them and create harmonics from them, while having low impedances at high frequencies in order to better drive them and reduce the highest order of harmonics. Generally speaking, less high frequency overdrive, so less IMD on high order harmonics (more definition on the notes) and less harsh overdrive.

The samples posted on the site are all very low gain, something like an high volume small Fender clean can do.
Have you tried this on high gain amps (imagine a Soldano SLO100 lead channel), trying to avoid the CF/SF to overdrive instead of the downstream stage?

I would like to have a more in depth analysis of how you play with impedances and frequencies on grid, cathode and anode, in a future book

[Malcolm Irving]

Forwarded from Ulrich:

>The samples posted on the site are all very low gain, something like an high volume small Fender clean can do.
>Have you tried this on high gain amps (imagine a Soldano SLO100 lead channel), trying to avoid the CF/SF to overdrive instead of the downstream stage?

Yes - there are many variations to OD stages, I've only followed one thread of options in that article. It's meant to illustrate a way of thinking about and controlling what an OD stage does. Roberto is correct about how the low and high freq's are treated differently. That's part of the point. In fact the example stage behaves very different from "standard" practice. It's standard practice to boost highs and cut lows before an OD stage so that HFs contribute more to the OD distortion. The LIO stage does the opposite - it keeps the HF's clean and allows the lows and mids to create more of the OD tone. The result is very different in character - and again, it's just an example of what's possible and how to think about OD stages.

As for high gain configurations - I did not explore that in the posted example. I am doing some experiments with a cold clipper and CF in a new amp, but I'm not ready to post about that yet. That's a very different configuration and different issues are at play. A cold clipper stage doesn't distort due to grid conduction, for example.

>I would like to have a more in depth analysis of how you play with impedances and frequencies on grid, cathode and anode, in a future book

I appreciate your comments and interest in the topic. I do think OD stage design is an area that's neglected in books, but I have no plans at the moment for attempting one. It seems difficult to even define the scope of a book on the topic.

[roberto]

Thanks for the prompt reply and happy new year.

IMHO the topic could be integrated in a second edition, in the same chapter where you analyse low-normal-high plated warm and cold stages, integrating some FFT and frequency shapes of multiple signals to show IMD difference and overall eq freedom.

What I see it's a good solution for high gain (4 stages plus CF) on the 4th stage, my main worry being not to overload the CF/SF that is needed for a low impedance load of the 4th stage (where grid conduction is more common, whilst cold clipping is more a cutoff).

Anyway it's an interesting approach also to modify the Marshall 6100. Do you think that was the original purpose, or just casuality?

[Malcolm Irving]

Thanks for the prompt reply and happy new year.
...
Anyway it's an interesting approach also to modify the Marshall 6100. Do you think that was the original purpose, or just casuality?

Happy New Year.
Did you mean 'coincidence' rather than 'casuality'?

[uneumann55]

Well - might as well sign on here...

IMHO the topic could be integrated in a second edition, in the same chapter where you analyse low-normal-high plated warm and cold stages, integrating some FFT and frequency shapes of multiple signals to show IMD difference and overall eq freedom.

What I see it's a good solution for high gain (4 stages plus CF) on the 4th stage, my main worry being not to overload the CF/SF that is needed for a low impedance load of the 4th stage (where grid conduction is more common, whilst cold clipping is more a cutoff).

Anyway it's an interesting approach also to modify the Marshall 6100. Do you think that was the original purpose, or just casuality?

Perhaps you could take a stab at it based on your suggestion. My view is that there are so many variations of stages and clipping methods, it's hard to be comprehensive - especially once you also consider SS approaches (diodes, opamps, etc).

Maybe you can do a study of the 4 stage+CF options and post something about what you find. You probably would find an interested audience.

As for the 6100, if you're asking if I had that in mind - no, I was just considering how to add some OD to a simple 1st stage for Fender (or most amps) when I did the example study.

[roberto]

Hi Malcolm, I meant that Marshall designers could have approached something similar without going so much in depth.

Hi uneumann55, I’ve done some thousands of posts on 4 and 5 stage leads on other forums and there’s audience there, but I don’t care about audience, I care about sharing and learning.

[frankdrebin]

Thanks Malcolm,

I've one question concerning the description in the link you just posted:



It is clear the purpose of having different input impedances at different frequencies, so higher impedances at low frequancies to easily ovderdrive them and create harmonics from them, while having low impedances at high frequencies in order to better drive them and reduce the highest order of harmonics. Generally speaking, less high frequency overdrive, so less IMD on high order harmonics (more definition on the notes) and less harsh overdrive.

The samples posted on the site are all very low gain, something like an high volume small Fender clean can do.
Have you tried this on high gain amps (imagine a Soldano SLO100 lead channel), trying to avoid the CF/SF to overdrive instead of the downstream stage?

I would like to have a more in depth analysis of how you play with impedances and frequencies on grid, cathode and anode, in a future book

i wonder if mr Dumble did the same reasoning around the 220k/470p network on his second stage,or if its just a byproduct of the grid/anode negative feedback.

[Murrayatuptown]

Two years later...

I add some books to my 'library' (a box) every year or so.

I have GAO & read portions of it occasionally. It was written by two PhD's, and not as easily digested as others. California & British English, not translated. Probably targeted to a different audience than Robinette's writing which has hands-on bench-tech/experienced-DIY language.

Blencowe's books, to me, and maybe I read his description somewhere, bridge the gap between 'how to build this amp' writing and theory. More practical and approachable by more readers who build.

Richard Kuenel describes his books as targeting (IIRC) engineers, designers, manufacturers. Tougher reading, some interesting analysis tutorials, but not getting very close to DIY language the majority of builders find usable to build from. One book (...Systems Design...?) has two complete SE designs (5W & 24W?) that share a significantly similar design and analysis, part of a philosophy he supports that requires less return to zero for a new design. Each stage is analyzed & compared to a 'budget' that controls the function of each stage to make it meet targets. Based on assuming a signal level from certain pickups, he can predict what pot settings the amplifier's thresholds for breakup & sometimes 2nd & 3rd harmonic production are. I didn't absorb all of the books and I am abbreviating (for me, anyway).

I also acquired Ulrich's Guitar Amplifier Design, which he basically says was written for himself. It offers the interested reader an 'over the shoulder view' of what he was thinking & why. There is a lot of discussion of things tried & sometimes abandoned along the path of designing a hybrid MOSFET, JFET, vacuum triode, bipolar transistor with AOT amp on a single circuit board with switchmode power supplies.

He acknowledges many people question many of those choices, but they are his. He has designed, redesigned & built many fully-tubed amps with sockets, iron & wiring & wanted to do something different.

I bought the book to learn more. I finish books quicker than amp projects. I is what I is.

GAD is easier to read than GAO. Closer to Blencowe in technical language readability with a lot more thought process explained. Some may like this, some may not. I did. I had NO intention of building the amp the book documents (somewhat scalable between 50 and 100-120 W based on speaker impedance and current limiting in the output stage power supply).

I really liked the book and decided I might as well get the circuit board from him...thinking to study the layout & maybe apply his MOSFET/JFET cascode in other projects. Apparently I bought the first board!

Next thing I know I'm looking at the cost of parts & what kind of artistic chassis I could put such a small lightweight amp into. Another distraction.

Nice guy, too.

Unless there was another Dr. Malcolm Irving, it's my understanding he passed away approximately 6 months or so ago. R.I.P., and I apologize to this audience if it was a different Dr. Irving.

I have more amp books than amps (even counting incomplete projects) and probably more guitar parts than guitars. Gotta have a herd of hobbies, right?

[pdf64]

Yes, I recently looked into Malcolm’s inactivity of late, and discovered that he died suddenly, back in May 2019. See https://music-electronics-forum.com/for ... irving-rip

[Murrayatuptown]

I can't view that page for some reason.
I just found this...

https://in-memory.theairambulanceservice.org.uk/

[roberto]

What a sad news...

[Littlewyan]

Very sad news indeed

[Mark]

That is sad news indeed, I’m sorry to hear of his passing.