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Chapter 19 Addendum additional algorithms
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May 9, 2022 - 7:57 am
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I am new to the fascinating world of DSP programming, but have had my nose deep in my tube amps for years, adding/removing gains stages, modifying many components between the input "bright cap" to the output negative feedback. I have some purely theoretical questions.

1. How hard would it be to simulate/model the cathode follower, which feeds the tone stack in many amps (starting with Fender and copied by Marshall)? The DC coupled cathode follower creates lots of even harmonics and contributes to the tone of a Marshall amp.

2. What about modeling the phase inverter? The output stage can be driven harder or softer by the PI, and again contribute to a certain tone.

3. And last question... what about the negative feedback circuit? I would say a somewhat subtle but yet powerful way to shape the tone.

4. Sorry, one more question came to mind... In an amp, each gain stage has a different combination of cathode bypass resistor and capacitor. Those actually do play a greater role in shaping the tone, since they are early in the circuit.

Are all those hard to model with algorithms? Would they be computationally too expensive?

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May 9, 2022 - 9:13 am
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I am currently working as a SW engineer at a company that does tube amp modeling so I can't really say much more about this right now without violating my agreement with them - the Chapter 19 addendum which is about 300 pages of tube theory and modeling, will need to be my final words on this for the foreseeable future. 

That said, I would recommend getting a good SPICE simulator and working with the concepts you outline above and then translate them into code. 

I use MicroCap 12 which is now 100% free myself.

Will Pirkle

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May 9, 2022 - 11:40 am
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Got it, I understand. I see you work for Audio Media Research. I like Revalver a lot, but that was a few crashed computers ago and I never installed it back. I haven't kept up with what happened lately, but I'll look into it again.

It's exciting to hear you work on such projects, looking forward to what you guys come up with.

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May 10, 2022 - 1:51 pm
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ReValver is very cool and there are several ways you can use it for your own experiments (the free version comes with one amp/cab to play with - so you can still try it out).

First, the ReValver models are extremely accurate, and implement essentially everything on your list. You can load ReValver into a machine and then probe it with test signals, basically treat it like an amp and stimulate it with your different tests - this requires either a second computer, or Parallels with two OS's and two audio adapters running (I actually do this all the time with Parallels, using RackAFX to probe and measure plugins running on the MacOS side).

Second, you can tweak the internal schematic for any of the tube amps and play with the plate voltage and resistances, different transformers, and other portions of the circuits (you can't edit every single component, but you can edit a lot of the meaningful ones). You can play with the output transformers and speaker impedance transfer Then, you can probe it and run tests to work on your own modeling techniques. You have to be able to measure this stuff anyway if you want to verify your design, and sometimes measuring the device characteristics will lead to insights on how to model the variations you see. 

When cool stuff is happening on the ReValver and other fronts, I will post something in the Forum about it. 

Will 

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May 10, 2022 - 3:52 pm
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I just tried the BluesMaker and it sounds absolutely great! I don't know much about any other tube amp besides Marshall. Those I studied, built, modified and played for many years and I'm still not bored of them. I obviously listen to music made with other gear, but I have no interest in owning anything else. You could say I have a little obsession with Marshall tones... The BluesMaker is a fine tone machine, and the entire cab/mic setup also offers great tone variety.

When you say "you can tweak the internal schematic for any of the tube amps and play with the plate voltage and resistances" I guess you mean MicroCap, right?

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May 10, 2022 - 6:18 pm
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ReValver allows editing of the actual circuits. Right click on the amp and choose "Tweak Schematic"

rv1Image Enlarger

Then you get this - anything in the little green boxes are tweak-able (depends on the amp which items you get to play with). For this one, you can change the tone-stack, adjust the bias hot/cold, change the plate voltages (Variac), output transformer, and more:

rv2Image Enlarger

It's actually one of the coolest things about ReValver - with only one amp model, you can spin off tons of your own variations, and even make custom modded amps for yourself. You can also tame down some of the higher gain amps, and mod-up some of the lower gain ones. 

Will 

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May 10, 2022 - 9:40 pm
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Wow! The tone alone sold me on the Revalver, but this is something else... I wasn't aware of this feature. I see some mentioned it on the forums in the past, but it kinda got lost in the overall online noise.

By far my favorite Marshall plug in has been in S-Gear, but the last couple of hours I've been AB'ing the heck out of S-Gear and BluesMaker and I genuinely like the BluesMaker better. I'm a believer lol I'll do my best to spread the word.

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May 11, 2022 - 10:36 am
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Yeah, that feature often gets overlooked in part because most people don't know anything about the analog circuits involved and so editing the schematic is just a crap-shoot. In addition, many of the mods can create relatively subtle changes (as with the actual hardware) but these add nuances that people who play and record tube amps can easily hear, while others may expect more severe changes. 

That you can do this is also a testament to the fact that ReValver models the tube amp circuits literally, down to the components. Of course I am heavily biased, but I've not run into anything that comes close IMO.

It's also a great educational tool - when you combine it with SPICE analysis, it can really help lead to deeper insights in how components interact as well as approaches for modeling. Almost all of the ReValver modeled tube amps have schematics that are available on the internet, so you really can make valid comparisons. 

Have fun,

Will 

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May 11, 2022 - 11:45 am
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I agree, some think that modifying things like bypass caps and resistors or negative feedback will make it sound like it has an additional gain stage.

Could you tell me some more on how you run the Parallels testing? You play a .wav file in RackAFX into another computer that has Revalver on it?

How do you connect the two? Headphones out from the RAFX computer into microphone input on the Revalver computer? I assume MicroCap runs on the Revalver computer. Thank you

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