To get asymmetric clipping like that makes me wonder if the input to the OP-Amp is biased at 1/2 voltage.
Can you measure the voltage across pin 4 to 3 on IC1, also across pin 4 to 7 and pin 3-7, all with no guitar plugged in.
If it is not set at 1/2 you might be able to tweak the value of R16 and R17 until you get exactly 1/2v at the input at pin 3 on IC1.
You might even have to change the bias slightly off to make IC1 clip evenly.

This is just my thinking and i could be wrong, I think the bias on the input to the OP-Amp determines how the output hits the power rails and flattens out.


If you have a spare 20k pot (or bigger) then connect it to 0v-22v and lift the leg on R20 that goes to the +11v and connect it to the viper on the pot, then you can adjust the bias voltage you have created to see what happens to the clipping on the input stage. (If you can understand my crazy way of explaining it)

Rail to rail clipping is used to produce distortion in some amps just like people use diodes or leds to clip the waveform to produce distortion, it might have been intentional on the amp just to make the sound you don't like but the designer wanted.

EDIT: I did a quick trace of the preamp to see what I got, I used the values already quoted and also the references.(I haven't double checked it or anything)
It looks like a basic baxandall tone stack for Bass & Treble and then the Mid is just like the active Mid in a Lab Series L4 Bass Amp.


Cheers
Mick

I think it might be a line out since it is not variable, the one marked Pre Out has a volume pot.
Headphones would also need a level/volume pot.
The output might also be for the VU-leds so you can see if you are over-driving the gain stages before the master volume ?

I can't find any info on the old Randall R-412 DXT cab neither, are there any more markings on the speakers ?
Look at the back of the speaker cones and on the metal frames, sometimes there is information with speaker dates and builds etc.

The amp is 300 watts output but it uses 500 watts to make the 300 watts for the speaker. Some valve amps are much worse then that.

I would place the effect in/out between the Post Gain pot and C20.
It might be easiest to remove C20 and run 2 wires to the new jacks from there, place a C20 cap between the return jack and PCB. (Solder one leg to the jack and the other to the wire)

Looking at a WT300 PCB (R74, R75) it shows that location as a 22k and 47k resistor at the 12AX7, bottom left of the WT300 schematic.
Looking at the WT400 schematic it shows the same thing just different component numbers.(R81, R82)

I presume they used a WT400 Preamp PCB to build that WT300 amp.

You might not need a heatsink for that module unless you are going to run it hard, just try it and see what happens.
You can get stick on heatsinks from a lot of places, you will have to see how much room you have and order one that fits if you want/need one. I think you will just have to peel the tape of and press the heatsink down to attach it, they are really easy to use.
Google stick on heatsink and you will find lots of them, and they are cheap to buy as well.

Read the spec sheet on the 3118 chip and see what it says.

Personally I would get a MeanWell 24V power supply that is rated around 100VA to get plenty of headroom for one of those modules.

The biggest question would be what size heatsink is sitting on the TPA3118 chips, and are they 3118 or 3118-D2 chips on those modules of yours.

To run at full power the chips should have a heatsink on them, but most often they don't have any installed on the Ebay amps.

I went with the TPA3116-D2 mono 100 watt boards from Ebay a long time ago when I tried them, they had a nice heatsink on the modules so I got them.
I think I used a 24V 120VA MeanWell supply to run one for a home practice amp, it worked fine but it never got run hard.

The ICE power modules are well priced for what you get, you can get cheaper modules from China but often you have to buy a power supply and an amplifier and put them together.(And most likely not run them anywhere near the advertised power without dropping out from overheating)

The  Straight Ahead Amplification power amps seems like someone is just putting pre-bought modules into a cheap box and selling them from home, I could be wrong but I would not buy one unless they have been fully certified for public use like that.

If you want and need a really powerful amp then it is best to buy a good quality made one as it will be certified with all the various agencies required nowadays.
I don't think that it would be worth the effort to build an amp from scratch if you want to truly be able to gig with it, I believe some venues might not allow non-certified gear to be used due to insurance reasons.
If an arena was to burn down because of a musos homemade amp caught fire there could be serious problems with the insurance companies as well as with the legal implications if someone got hurt or worse.

If you want your own sound then build a preamp in a stomp-box and run that into the return jack on a commercial amplifier, proper Class-D Bass amps are not that expensive and deliver a lot of power. Certification  can cost $20,000-$50,000 to get through for a single amplifier model and that has to be included in the final cost of a proper amp you can buy in the shop.

If you just want it for home use then get/make whatever you want and go for it.
Have you read Teemu Kyttälä's Book about solid-state guitar amplifiers ?
There is more in it then most people will ever need.

https://www.ssguitar.com/index.php?topic=711.0

I would not recommend plugging the effect send into the input of another amp, however if you measure the voltage from the guitar itself and then turn the volume of the amp down until the send signal is the same or lower level as from the bare guitar it might be ok.
The output of a guitar is very low so you do not want any signal you put in to be much higher then that or you risk damaging the pre-amp of the other amp.

Make sure you turn the level of both channels right down if you are going to try this.

Schematic:

I will take a wild guess that it is there as a delay so there is no big pop as the amp is turned on.
The 1uF cap C25 is slowly charged up through the 10M resistor R45 until it turns the  Jfet on and allows the signal through. But I could be wrong.

If you are looking at that much power I would like to suggest you only look at the ICE power modules, the ICE power modules can be found in several professional Bass amplifiers.
I can't remember if it was the ASX models or the AS models you can find inside several brands of Bass Amps, but I am 99% sure it is one of them.
If you get Chinese made ones of Ebay then I would de-rate them by at least 50% to be safe, bass amps need to be very well made.
https://shop.icepoweraudio.com/

If you are wondering if you can use a hifi power amp in a guitar amp build then I would say yes, but you would be advised to use bigger heatsinks and cooling.
A larger power supply might be good as well.

If you want to use a stereo to play guitar through then build one of the many Pre-Amp pedals out there and run it into the line in on the stereo, it will sound a lot better if you have a dedicated guitar cab connected up to the stereo as well.

Just remember to only use half of the power available on the stereo as a safety margin.

If you start to look at many modern solid state guitar amp schematics you will find a lot of them use the same type of power amp chip that gets used in stereos as well.

There are also some nice schematics around you can build for speaker emulators, it makes the combo amp sound more like other cabs Ie like a 4x12 Marshall cab with Greenback speakers in it.
From memory Bajaman made a schematic and veroboard layout over on the Freestompboxes site, https://www.freestompboxes.org/viewtopic.php?p=226100&hilit=Baja+cab+emulator#p226100
"Phatt" has posted his Phabbtone here somewhere for us to use (Thanks Phil)

Can I ask why you are asking ? Are you looking at any specific amp design ?

Cheers
Mick

I think a pedal is a great way to start, and if you are familiar with schematics then you should also consider trying out a spice program and a PCB making program.
Start simulating the pedals and small amps in a spice program and you will learn to understand what is happening in the circuit you are playing with.
If you start using a PCB program you will learn a lot by just drawing the schematic in the program and then building your own PCB will feel like a big step forward towards making cheap pedals and amp yourself.

I love using KiCad as my program to make up PCB's for myself, it is completely free and have no limits to what you can make up.
KiCad also has a version of a spice program built in, it is fully functioning but I haven't tried it. You might have to import models of parts into it.
It might be a version of NGspice I think.

You can also breadboard many circuits on small vero boards instead of making up PCB's for pedals, often you will find vero layouts for stomp boxes online.

If you want to make an amp then start with a known good sounding simple design and try it on a breadboard first, many people like the little Marshall 12 watt amps and they are very simple to make.
Start with small compact circuits so you don't waste too much money if you make mistakes, and always remember we all have to learn to crawl before we can walk.

For pedals FreeStompboxes is a good place to start, but there are many other great sites out there. https://www.freestompboxes.org

This is what I have come up with from the pictures of your RG50-112 PCB.
I copied a picture of the tracks (mirrored) into KiCad, drew up the original schematic and made up the components.
I had to give the components names in KiCad so I made them up as I went. (Ie. R1, C1 etc)
I placed the components on the picture of the PCB tracks and drew the tracks in as close as I could.

I did find some differences compared to Randalls schematic so I changed my schematic to match what was on the PCB.

I think my version might be helpful for anyone who is trying to fault-find one of these amps as the components have names on the schematic and on the PCB drawing.