From memory I found a connection diagram for that PA system, PA systems do not use a speaker output but they use a voltage output that feeds into a transformer at each speaker.
It is done that way so you can have really long cable runs with less losses at the speakers.
If you look for audio transformers you will find the correct type of transformers to use since they are rated at 100V, 70V, etc on the input and the output is rated at watts (usually for an 8 ohm speaker)
I have 2 small transformers like that that I can use as an output transformer in a 1-2 watt tube amp, even though they are not meant for that.
Here is a link to the ones Mouser have for sale:

https://au.mouser.com/c/power/transformers/audio-signal-transformers/?m=Visaton&secondary%20impedance=4%20Ohms~~8%20Ohms%2C%204%20Ohms&rp=power%2Ftransformers%2Faudio-signal-transformers%7C~Secondary%20Impedance

The selector switch is most likely so they could send the sound to the right department, office, workshop, dispatch canteen etc.
they could also select the different wattage taps on the speaker transformers so it suited that area, I imagine the office would be less noisy then the workshop floor.

I think they could have used that unit almost anywhere since they were/are common in factories, shopping centres, train stations, clubs, and many more places.

Check any wire to earth before you do anything, as in the metal of the transformer.
Do some ohm readings and you should be able to work it out, if you have a variac then connect it and slowly bring the voltage up while you measure the other end of the transformer.

Some transformers have 2 primary windings at 110V each, you can run them in series for 220V or in parallel for 110V.
Some have a primary with multiple voltage taps so you can select the correct voltage for where you live. (100V, 110V, 120V, 220V, 230V, 240V)

I have never seen a schematic for a crate MX10 amp, I have seen them for the MX15R and some other low watt models.
See if these will help you work it out:

I have not had any problems with it, I just havent used it since I blew up the last laptop power supply, I think the amp is still fine.
If you want to see an even smaller one there is a company selling tiny 30 watt poweramps that are 25mm x 25mm x 100mm.
Even their 50 watt pedal is nice with a VU meter on it.
https://svisound.com/index.php/products/amps-cabs

Sounds like you need a multi tap transformer with several outputs.
0V
6V
12V
24V

I cam across this today looking for something else so I took a picture.
I remember now that there were a trimpot on the input of the amp module so I removed it and put a pot on the front of the box

Maybe a little toothpaste on a finger can clean up the rust on the transistors, toothpaste is like a cutting compound that will scrub away whatever you use it on.
If you have some cutting compound for your old cars paintwork then that will be fine as well, just rub a tiny bit with the finger tip and it might reveal what it is, rub hard and the writing will be gone as well.
I usually heat the solder up with the vacuum sucker right there ready at the solder, when the solder melts I push the Teflon tip right over the solder hole as I release the plunger.
Sometimes the solder will not work with you and you end up with a big mess, mostly you get it all up in one hit.
I sometimes have to add some new solder to the board if it really fight me, suck, solder, suck, solder, suck, swear, solder, suck and win or suck and swear and suck on a beer for a while before I retry.
I should get myself a new one since mine must be 30 years old and I can't get any tips for it anymore, I would look for a heated one if I used it a lot but I don't use mine much at all.
Cheers
Mick
 

If it is speaker cable then it will not be shielded so it can pick up hum very easily.
The old amps used the same plug for signal cables and power cables to the speakers, you should never swap them.
Signal cable can melt if used as speaker cable, then it can blow your amp up.
Modern Bass amps are using speakon connectors more and more as they are made to handle the high loads that modern amps can push out.

When I mounted a fan in my pedal amp I had the fan blowing into the pedal, I did not make any other ventilation holes so I placed a spring-washer between the back lid and the box on the screws to make a small gap all the way around.

Since there is no heatsink on the module it might be a chip that dissipates the heat through the bottom of the chip, look at the back of the PCB and see what is under the 3118 IC.
Some PCB's have a lot of vias (small holes) in the PCB to transfer the heat to the bottom of the PCB to act like a heatsink, the back of the PCB becomes the heatsink in that case.
 Make sure you mount the board with enough space under the board for airflow to get through since this might be the side that dissipates the heat from the amp.

I was lucky I got the size close enough the first time I did it, I looked up what size photos you could get printed and then I made up a template that size so I could see what would fit and what would not.
The graphics just came from free art files of the web and I just copied and pasted them into the template, did a bit of blending in any lines and  then sent it off like it was a photo to be printed.
I am sure it was a lot cheaper then buying good quality paper and then printing it at home, photos are so cheap now I don't know how they do it.

The Sunn Beta pedal was one of the first and that did not get anything fancy, but that is the way I like the Sunn Beta. I made the PCB for that one myself.
I bought a corner cutter after I had done a few, used the UHU glue stick and a black marker for the edges of the photos, glue on and spray clear coats a few times.
I only made them for home use but they are staying in good condition, at least last time I saw them since my daughter have most of them and I think a few are with her friends who gig a lot.

Later on I started to make a thin PCB's for fascia panels, especially on amps that fit in a 12" x 3" x 7" Hammond box, on one I placed SMD leds under the knobs so they can be lit up.

A long time ago I used to make up silly graphics for my stompboxes in the same size as photos, then I would send them off to the local photo shop (BigW) for printing on photo paper.
Cut the pictures out, glue them on and a few coats of clearcoat and that was it.
I think I paid about AU 10 cents a photo. 

Quote from: Miyagi_83 on October 24, 2024, 02:23:34 AMThank you, sir

Quote from: Tassieviking on October 23, 2024, 12:20:13 PMThat amp will fit into a 125A stompbox

Do you mean a 1590A enclosure? (Search results for 125A spit out a tube amp output transformer)
In any case (pun intended ), the 1590A is awesomely small, which is exactly what I need on my pedalboard 

QuoteI cranked the volume right up when I hooked up a bass guitar so I made it work hard until it did so no more.

I understand that you pushed your power amp into clipping. How did that sound? Or did you use some sort of input signal clipping?

I meant to say 125B box, I have been looking at a lot of tube amp transformers lately so I can make a small tube amp in a pedal format so the 125A transformer must have been on my mind.
I think I overloaded the power supply with my pedal, I was using a HP laptop supply rated at 120 watts but I was playing loud testing a Bass speaker I had built.
I used a mono TPA3116D2 board with a heatsink on it and 2 top mounted insulated jacks for in and out sockets.
Power was a GX12 2 pin socket, I made leads with alligator clips so I could use it on a car battery if I wanted to.
A small 5V fan that ran of a L7805 regulator was sitting at the opposite end from the jacks.

https://www.taydaelectronics.com/hardware/enclosures/1590b-style-1.html
https://www.taydaelectronics.com/hardware/6-35mm-1-4-plugs-jacks/6-35mm-1-4-stereo-insulated-socket-jack-solder-lug.html
https://www.taydaelectronics.com/dc-brushless-fan-5vdc-1-2-inch.html
https://www.taydaelectronics.com/lm7805-l7805-7805-voltage-regulator-ic-5v-1-5a.html
https://www.taydaelectronics.com/connectors-sockets/2-pins-male-circular-connector-gx12.html

They are inductors.
That amp will fit into a 125A stompbox with a small fan at the end, at least my one did and I used a 100 watt mono amp with a heatsink.
I got a small 5V fan from Tayda and a L7805 regulator to run the fan and that worked great.
I kept blowing up laptop power supplies on mine, but I used it to test stompboxes and I cranked the volume right up when I hooked up a bass guitar so I made it work hard until it did so no more.

One day I will get a better power supply and resurrect it again.

Good luck with it.

I presume you are measuring either the positive or negative side of the amp, one 0R6 resistor is on the positive rail and one is on the negative rail.
If you use a 2 channel cro and run the cro on both resistors you would find one half sinewave on each resistor.

What sort of preamp are you wanting to use on that amp ?
Are you looking for a clean tone or distorted, Fender, Marshall, Vox ?
If you want to build a different type of pedal then have a look at the C2CE range of PCB's.
They are all Tube driven at high voltage but run on a 9V-13V pedal supply, although your power supply has to be able to deliver plenty of power. 0.5A to 1A.
https://c2celectronics.com/
Nathan of C2CE & Sushi Box also sells ready made pedals if you don't feel safe dealing with high voltages in a pedal, his other shop Sushi Box is here :
https://www.sushiboxfx.com/

It would be easy to make your own PCB's for his designs but since he is a small company I just buy his PCB's to make sure he keeps the business going, and also because he has my respect for developing the PCB's for us all to build.

I would first check what sort of signal is going into C215 which goes into the last OP amp in the pre-amp circuit, or the signal between the pre-amp and the power-amp (C).
Then figure where the best place is to add an effect loop send / return.
Then just use a nice pre-amp pedal straight into the effects return.
To fix that amp you might have to replace both speakers and all the electronics to get a nice sounding amp, and what are you going to fill all those pot spaces with ? You would need to get a new front panel if you change the pre-amp.

You cannot view this attachment.

https://elektrotanya.com/vermona_regent_1040_guitar_audio_amplifier_mixer_sch.pdf/download.html