High Gain Modern amp schematics?

[Darkbluemurder]

Kevin O'Connor discusses high gain preamps in The Ultimate Tone 5 in detail. Among the keys for an articulate high gain tone is to limit bass and too many highs and to limit the signal going into the next grid. I guess that's why most of the 4 gain stage preamps use a lot of interstage voltage dividers, at least one small coupling cap and large grid stoppers. To some extent you find limiting of bass response also in HAD's preamps, although in the overdrive section only. This to me is the major difference to e.g. Bogner Shiva, SLO, Crate Blue Voodoo and Marshall where the first gain stage has a reduced cathode bypass cap. Thus they can never have the full clean tone of a Dumble style amp or BF Fender for that matter.
Another difference is that Bogner and Soldano use a clipping stage with unbypassed 10k respectively 39k cathode resistors. I have not seen such clipping stage in a D-style preamp. I doubt that the clipping stage is necessary for a smooth lead tone but obviously helps for getting an aggressive tone.
Finally both the Bogner and the SLO have cathode followers before the tone stack but since they do have unity gain or slightly less they are not really necessary to achieve a high gain tone.

[ODwan]

I agree that small CC/large cathode bypass cap (CBC) sounds different than large CC/small CBC. To me a small CBC creates a high/mid boost whereas a small CC limits bass/low mids.

Hi,
The sound difference between large CC/small CBC and small CC/large CBC is how they effect the frequency response curve. Large CC/small CBC creates a "gain step" in the curve, whereas the small CC/large CBC creates a continously falling slope. That does really sound different!
Just my .02€
Greetings,
Timo

[David Root]

That's very interesting. Which Y-axis is solid line and which is dashed? And what is the right hand Y-axis indicating? Phase shift?

[ODwan]

That's very interesting. Which Y-axis is solid line and which is dashed? And what is the right hand Y-axis indicating? Phase shift?

The right hand Y-axis is phase shift in degrees and corresponds to the dotted line. I did this plot with LTSpice.
Timo

[drz400]

This to me is the major difference to e.g. Bogner Shiva, SLO, Crate Blue Voodoo and Marshall where the first gain stage has a reduced cathode bypass cap. Thus they can never have the full clean tone of a Dumble style amp or BF Fender for that matter.

Bogner Shiva has an unused 12ax7 stage which could have been used to add punch to the clean channel and not share the first stage with the lead channel, this is my gripe on my Shiva.

Egnater uses a large cathode cap and small coupling caps so that is his work around.

The CAE stuff uses an independant signal path for clean and dirty, that is the way to go IMO, there are no shared stages. The Dumble still looses low end punch in the clean with the 5uf cathodes. I still dont feel the Dumble clean stages compete with a BF Super or any really nice Fender

So never say never

[drz400]

That does really sound different!
Just my .02€
Greetings,
Timo

Interesting ...so the small cathode cap larger coupling cap has less phase shift. No wonder I like it better

[Darkbluemurder]

That does really sound different!
Just my .02€
Greetings,
Timo

Interesting ...so the small cathode cap larger coupling cap has less phase shift. No wonder I like it better

I really wonder whether this is what Mesa Boogie had in mind when they changed the phase inverter input cap/resistor values from 0.001uF/1meg (Mark I, 0.003uF/1meg in the Mark I RI) to 0.1uF/100k in the Mark II to Mark IV. That leaves the Mark II-IV amps with coupling caps of nothing smaller than 0.047uF through the signal chain.