-I corrected the few mistakes I made. Firstly, I redid the circuit for a dual-rail supply. Secondly, I added a volume pot and a buffer before the output.

-To implement the asymmetric clipping, I added differently valued resistors in series with the diodes. These reflect the asymmetrc ratio of your load line guesstimate.

-In the first stage the negative lobe gets clipped more, in the second the positive, etc. (since the signal itself is not inverted)

-As for the respective gain of the stages, I'm not really sure what to make of them. To go by the theoretical gains of the individual tube stages (50x or 34dB) would probably yield too high signals and too much clipping.

-As for now: I've finished the schematic and a layout with it. It's time to stop the work for today (I'm getting hungry). In the next days I'll go over it a few times to check for basic feasibility. If the basic topology is satisfactory, I'll fab a pcb next week and put sockets in place of the ambiguous components, e.g. feedback resistors, diode-resistors

As a closing note; I will wholeheartedly and directly admit that I'm not an electronic engineer, nor do I have extensive knowledge of tube amplifiers. Therefore I sometimes revert to the build-and-tweak approach. Something I can afford because our local hackerspace had an entire component library donated, and the local polytechnique grants me acces to their PCB-facilities just because.
I'll advance in Micro-Increments
 

Dear Readers,
This will be my first post on the SSGuitar-forum. I've been happily reading many a thread from a visitor-perspective. Forgive me the audacity not to introduce myself in the appropriate subsection and jump straight to the matter. Firstly I'll list my build goals. Secondly I'll list the constraints I'll subject myself to. Thirdly, I'll propose a method of approach.

1. The goal is to design, prototype and build a solid-state preamp that sounds like the Orange OR15.

2.
-I'd like to limit myself to the use of op-amp gain stages. Although -- and ROG keeps proving this point-- JFETs have proven to work well in solid-state adaptations of valve topologies, J201's are hard to come by in my region. The project will hence serve as an attempted proof-of-concept that op-amps can be used functionally to mimic valve topologies.

-The preamp should preferably work off a single 9V supply, although if nescessary, a dual-rail supply can be constructed and implemented

3. I've spent some time analyzing the OR15-schematic. (I'll appendicize it) and used a calculator to calculate the gain of the first 4 preamp tubes. Negative feedback of the op-amp stages will be set accordingly. I presumed the filters between the stages could be copied   one-on-one. To incorporate soft-clipping characteristics , antiparallel diode pairs are placed in the feedback loops. To adjust the clipping threshold, a resistor has been placed in series with the diode pair. (These will require tweaking. Asymmetric clipping in one or more stages might be desirable)

Now I'd like to ask some questions and place some notes:
-In the feedback-loops are radio-frequency  pF-value caps. How would I choose appropriate values?
-There are DC-blocking capacitors from the inverting input to ground. How would I choose these values? (They should not have a filtering function, since filtering is done through coupling capacitors and roll-off capacitors)

Of course, feel free to vent your thoughts

Berto


I'll add both my proposed schematic and the OR15-schematic.