I have finally gotten back home a week ago and had some time to  work on this PCB, I can't guarantee it's correct but I will order some PCB's for myself later this year to test.

I decided to squeeze in a 9V regulator for pedals as its a smaller amp and is good for practice and learning on, and it cant hurt to be able to run a pedal or two from the amp.

I have made the Gerber files for JLCPCB to make the boards, but feel free to use the project any way you want.
I am going to use the KiCad schematic to make a pedal eventually as well, without the reverb circuit (unless I can get the correct mini spring pan).

I can think of one more thing you can try, increase the taper of the pot.
place a resistor between pin 1 and pin 2 of the pot and the audio taper curve will increase.
The taper effect will be more pronounced at low volumes, you can buy audio pots with different taper curves, but the easiest way is just add the resistor.
You can read more here :http://www.geofex.com/article_folders/potsecrets/potscret.htm
It would give you even more control at low volumes.

You cannot view this attachment.

All I have on small Orange amps

Also this thread on DIY stompboxes :  https://www.diystompboxes.com/smfforum/index.php?topic=87122.0

This video shows a schematic as well : https://www.youtube.com/watch?v=7WM6HoQmDgU

Scoobynuts, please take note of the fp before the 130 on the resistor.
This might mean the resistor should be fireproof, if it fries again it will not go up in flames as long as you use a fireproof resistor.
I could be wrong, but would you chance it. Some of the more knowledgeable people here might pipe in and I and others might learn something important if they know what it stands for.
If it might get hot, make sure you leave some space under the resistor for airflow, also makes the PCB less hot.

I started to looking into building a 12 watt Marshall by mixing all the 12watt Preamp circuits I found, and also mixing the 12 watt and a 30 watt amp circuit in Marshall amps so I could make any one of the 12 watt amps with 12 to 30 watts output on the same PCB. You just had to pick the right components to make the version you wanted.
Some 12 watt amps had a Bias pot and some did not. Mk1 preamp has diode clipping and the later ones dont, etc.

I got derailed when my dad got sick, traveled from Tassie to northern NSW (Australia) and spent 4 weeks with him. Had to go back to Tassie for medical reasons and be with my family for a week, then back as dad passed away as soon as I left, stayed 5 weeks then back home for Doctor visit and now I am going back up to pack what I want to keep and sell the rest, sell houses, cars,  etc.
I am going to sort all my stuff nice and neat when I get a chance because its hell trying to go through your parents stuff when they are both gone and everything is all over the place.

Anyway, this is as far as I got on a 12 watt amp before the wheels fell off.
I was going to post the KiCad files  of the PCB when I finished and still might when or if I get back to it. I put a jumper on the board if reverb is not wanted, and a place for effects to be inserted which can be bridged if not wanted.
I made the PCB so TO3 transistors had to be wired in or TO247 can be used on PCB.

I have seen at least 3 different Marshall 12 watt amp schematics, one has what looks like clipping diodes.
And then there are the 12 watt Reverb models as well to consider.

Thanks guys for the answers, I think I will not keep the Electrolytic caps since I don't know the age of them.
Better be safe then sorry.

Have you tried making your own PCB?
I like using KiCad to make up PCB's for whatever I want to build, I get 5 PCB's from JLCPCB for the price I can order one from someone else.
If you download KiCad you should be able to check out the PCB I made for the Sunn Beta, my design is adaptable for the Bass or Lead version, just install the right components.
I guessed at the spacing of the pots so they will not match the original amp, but close enough for me.
Look up: https://www.ssguitar.com/index.php?topic=5080.msg39729#msg39729

I think I left all the files there, including the Gerber files I sent to JLCPCB when I ordered my PCB's
If you want to use my PCB just order some.
I think I have tested the Bass version of my PCB, but not completely sure, I have had a messed up year so far.

G'day all
So my father died a week ago,and my brother 3 years ago and mum 5 years ago.
Going through all the stuff in my fathers house I have found lots of Electrolytic caps,
Some are around 6800uF 65V, 2200uF 80V, etc. I know these used to be my brothers but he moved overseas about 20 years ago.
Will they still be ok to use in new amp builds or should I scrap them, they have never been used.
There are also bags full of 1uF, 10uf, 22uF etc, never used and heaps of WIMA box caps.
I have to ship them over 2000km back to my place so I only want to keep good *s!!t*.

I always wonder why someone who struggles with English don't put up where they are from, or what language they speak or understand. There is such a diverse group of people  on the forum there might be someone who could explain it in whatever language they prefer.
I can speak some Swedish, Norwegian and fluent in English, I personally don't care if someone replies in another language. I don't think there is a rule everything has to be in English, but an English translation as well would be nice if it solves the problem. Just my opinion, music is universal.

It looks to me like the 2 dots might have been a jumper wire which was removed, and then the earth re-routed with a wire, but I haven't seen the PCB in an Amp.
If you have the amp check what is there originally.
The solder pads must have been used for something.

Found a picture of what I think is the correct link on the other side, I circled the link in red.

Thanks for all those comments guys, I was only thinking of replacing the Electro-caps and the pots if the school wants them.
I just wanted to make them last as long as possible for the kids.

Thanks Joe.
I was given this amp and a Marshall G15R-CD for free, apparently the pots are noisy.
I am thinking of giving them to the local primary schools music department if they want them.
I think young kids will like them, as long as the school has some guitars.
I think I will change all the Electrolytic caps and all the pots in both amps before I ask. The amps are 23 and 28 years old I think

Caps and Pots will cost up to $13 total for both amps from Tayda, and I am putting in an order soon anyway. That's if I use the best Electros Tayda has.
I might also mount some speaker cloth across the backs to keep the little fingers out of the speaker. Or maybe a metal mesh would be better.

G'day SemiConductive
I'm fairly new to audio circuits myself, but I have heaps of experience with industrial electronics.
The way I understand the active inverting low pass filter of IC1B is as follows:
The gain is set with R4 and R6(and R5), we don't want to mess with this, unless you feel there is too much gain with the pedal engaged. then increase R5 till you are happy with max gain. (Pot ?)

So R6 is 150k, or 46.78899k with R5 in the circuit.
My understanding is that you calculate this the same as a RC low pass filter.
Capacitance is 470pF
Look at this page, the same formula for normal RC Low Pass filter as an Active Inverting Low Pass Filter like yours. So we can use the top calculator for frequency drop off.
https://www.learningaboutelectronics.com/Articles/Low-pass-filter-calculator.php#answer1

With R=150k and C=470pF you have a drop off frequency of  2.26 kHz
With R=46.78899k and C=470pF you have a drop off frequency of 7.24kHz
If you want 7.2kHz with the R=150k you have to change the capacitor to C=146pF (150pF)

You can easily put in a switch to change the frequency roll off capacitance to set it were you like it, rotary switch you get heaps of otions, but a bit overboard

But maybe I misunderstood the whole thing and got it wrong, If I did can someone please give me a hard smack over the ears please, That way I will remember next time.
Michael

Quick question for someone, when you change capacitors in an old amp,
do you just change the Electrolytics ? (Preventative Maintenance)

I have only changed the electrolytics in most amp I have come across.
Should I change greencaps and ceramics as well ?

In the 45 years I have been an electrician I never replaced capacitors just because they were old, and I mainly worked in maintenance, Industrial electronics mainly.
Lots of huge DC drives for DC motors, PLC's, big stuff mainly. I also did instrumentation  calibration and testing as well.
Just never any Audio stuff, until now.
Thanks