I don't think that green wire under the PCB is at fault, they just relocated the wire from the relay to the Master Volume pot.
The wire goes from the relay to a point after the Gain Pot in the Distortion Channel.
It goes to a junction between a 1k resistor, JFET, 22pF capacitor (20pF).
I am still tracing it slowly for you.

Nice song Phil, very relaxing.
It's nice she got to hear it.

Quote from: phatt on December 20, 2023, 03:00:47 AMthe rest was me learning how NOT to waste time building land fill.

Have you figured out Phil ?
No more landfill ?
Please tell me the secret, I have been doing it since the mid 1970's and I am still working on landfill all the time.

Don't know if it helps but here is a photo of a 3210 PCB I found on the net some time in the past,and schematics 2 & 7.

If you want to mess with the amp then you should leave the power amp section alone.
The pre-amp is where the tone shaping happens.
The closest you should go to the power amp is the 100nF cap going into the power amp, that 100nF cap with the 100k to earth is a HPF filter that drops any frequencies below 15.9Hz. Change to a 10nF cap and the frequency is 159Hz, experiment if you like.
I would concentrate on the tone stack, changes there will affect your tone more.

Are you sure it is not the speaker itself that is dark sounding ?

G'day and welcome to the forum.
In my opinion the Drive-55 will not have enough output to drive the power amplifier properly, you will still need a pre-amplifier as well.
You can incorporate the Drive-55 in it like a dirty channel but it wount have enough gain by itself. (In my opinion)

PedalPCB have a lot of Preamp pedals and many of them will most likely drive that amp.

There is the Honey Amp project designed by people on this site (Thanks JoeCool) that is a great starter amp, and you can use the Drive-55 pedal with it as well.
Just look at the top right of the page for the link to the shop.

BAJAMAN on the FSB forum designed many preamps that sound like nice amps for stompboxes, and there are vero board layouts for them as well.They should be able to drive the amp.
https://www.freestompboxes.org

Aion effects have the Sunn Beta and the Lab Series L5 which are nice.https://aionfx.com/projects/

The Preamp pedals are usually able to plug into the return jack on a normal guitar amp and totally bypassing the preamp stage in the guitar amp itself, basically making it a different guitar amplifier.
Hope I make sense, audio circuits are not my area neither, I am a retired electrician myself but I have extensive electronic experience in industry.DC drives,AC drives PLC interfacing etc on machinery that had up to a hundred motors and pneumatic and hydraulics all tied together.

The SX300 preamp on page 25 in the manual has a very similar input circuit as the SX50, the SX3000 uses 2N5457 fet's so I think we can say that Q21, Q22 are 2N5457 FET's

This might make a nice pedal from the overdrive channel, 2 FET's and 4 dual OP amps is all, 5 pots and 1 switch as well

That is one of the better pictures I have seen of the track side of a PCB, it really helps to see the shadow of the components through the board.
I often use a torch to shine through the board when I take the photo.
Would you be able to take some good clear shots of the component side, it helps to see where the tracks go to.

Perhaps you can open your 150 amp and see if the same link is on the bottom of that PCB ?
It seems to be the same amp basically and the PCB seems to be the same type.
If you do that then you might as well take the photos of that amp.

I think you would have to make your own PCB board for that, my favorite FREE program is KiCad as it has no restrictions at all on what you can do with it.

You can get the PCB made from many different places and they are not that expensive if you shop around.

The first problem will be to work out all the components in the schematic like I mentioned above, if you could open your amp up and find Q21 and Q22 and see what they are would be a good start.

I can't find any values for P1 which is a dual gang pot, see if you can see a number on it in your amp please.

If you take the lid off and take some good photos of the PCB top and bottom it would be a great help so we can work this out with you, especially if the components are easy to read.

Thanks for the Reverb Tank info, I am definitely putting this amp on my to-do build list.
If you have the lid of the amp you could measure the voltage across the 12v Zener diode, I think it's the one behind the big resistor so it might be a bit tight to get at.
You can also tap some components in that area with a plastic or wood chopstick to try to make the LED's flicker on, it is an easy way to look for bad connections.
Make sure the chopstick is plastic or wood only and it should be safe, plastic is the preferred type of course.

Do you have a SX-50 still ?
I am wondering as we will have to identify some transistors (FET's) as they don't seem to be marked on the schematic.
Q21 and Q22 are most likely JFET's in a TO-92 casing so we need to identify them.
The LED's D1, D3 and D5 are most likely red, but best to make sure, they should be on the PCB as well.

I was looking at the schematics as well and we really need to know what model we are talking about, there are differences between the different size amps.
We might also need help identifying some of the components as they might not be marked the schematic.
I was pleasantly surprised when I found  the schematics on Carvins website.
https://carvinaudio.com/pages/guitar-amp-schematics

Would you be able to check the reverb tank for the model ?
I might build myself one of these and the reverb tank information is not on the schematic.
If you could provide all information you can see on the tank I should be able to work it out.
Thanks
Mick

I use KiCad to make schematics, I use it mainly to make up PCB's for stomp boxes and amps i want to build for myself. It is freeware so it costs nothing and is really good and easy to use.

I know the basics of audio circuits but nowhere near as much as many others on this forum, I'm just a retired electrician so I know more about industrial electronics.
cheers
Mick

I redrew the schematic around the channel swiching so you can see it all in one place.
I included the red wire so i put 2 connectors in the circuit.
It might be easier to follow from the switches to +58v