Marshall 2204 preamp pedal with Baxandall EQ

[nandrewjackson]

Up and running and sounding good.


My only complaint is that the treb and bass controls really need to be linear taper, all I had was audio taper.  There are spots in the travel of the pot with large changes in tone with only small changes in pot rotation.


Notice the Baxandall is totally NFB , with only 1M to ground on V2 pin 7.


The power supply is the small Torroidal from Hoffman, and wired according to his diagrams for tube pedal power supply.


I saw Baxandall tone stack in a book years ago that was totally NFB, instead of the layout like in Duncan Tone Stack Calculator.  I don't have the book anymore, it is "Valve and Transistor Audio Amplifiers:  John Linsley Hood".

[PRR]

The true Bax is all about NFB and wants Linear pots.

(Your version will be more accurate on boost if you bypass V2B cathode, 10uFd or more. Ideally you want infinite gain here. Or at least much-more than the 10X boost. You have gain like 25, which is not much-more. Bypassed gain goes near 50, which isn't fabulous, but 98% of 1962 hi-fi amps got away with it. Bax used a hot pentode.)

The "other", simmed in TSC, is sometimes called the James, and wants 10% audio taper pots and a gain of 10 to make-up losses.

[nandrewjackson]

Thanks PRR


I may try 10uF on v2b.


Heres Google drive links for pics.
The enclosure is a $8 aluminum deal from China on eBay. Very flimsy, needs an upgrade, but does essentially what it's meant to.


https://drive.google.com/file/d/1k5JTul7w1nOpaoL2wIbCa5KCIdI4yOvS/view?usp=drivesdk


https://drive.google.com/file/d/1yoWyt2sc19c2R-ei8X8zAW7FzI82F6pH/view?usp=drivesdk


https://drive.google.com/file/d/1qyhfHlFJcjEztPYyYDhymzFoGMGCbJqB/view?usp=drivesdk


https://drive.google.com/file/d/1HqbbjxbmfvxP2rTSb-HKhzj9aeIjF1QA/view?usp=drivesdk

[pdf64]

I suspect that the tone control stage will overdrive, significantly if a control is set to boost.
A stage must operate within its linear range for NFB to operate as intended, ie it's not feasible to overdrive a stage and then for any NFB around that stage to further increase the output; the gain may increase but the actual output can't.
The values of the input and output coupling caps seem rather too low to achieve full bandwidth.
As it is, it may be that the actual range of control, particularly boost, is less than would be achieved by a regular Marshall tone stack.

[nandrewjackson]

It sounds pretty good to me. The gain works like expected from the purely Marshall variation of the circuit, and the treble and bass controls definitely do what they're supposed to. Now whether or not I'm actually getting boosts in treble and bass, or if they are only functioning as treble and bass cuts, that's for minds greater than mine. 


But through the full range of the bass control, it goes from "thin" to "boomy". And through the full range of the treble control it goes from "muddy" to "glassy".


The output is so high, I've got a 33K from volume swiper to ground, this led me to believe the last triode didn't need a cathode bypass cap.  But if a bypass cap on the last triode could improve the EQ functionality, that's definitely a plus!