I used to use carbon paper to transfer the tracks onto the PCB's and then "borrow" some nail polish from mum to paint the tracks onto the copper.
It felt like watching paint dry when you waited for the acid to etch the boards, then a little "borrowed" acetone to clean the nail polish off with.
Some of my early PCB's were a bit rough but I don't remember any not working.

Possibly these

X and Y will show you the voltage one way and a time scale the other way, you have to change the time scale if you measure 50Hz and then change to a 5000Hz signal
Nearly all will show volts up and down and time scale left to right.

I think you are learning things faster then I am forgetting things, 5 years from now I will be asking you for help.

g1 has it 110% correct, you should never apply a higher voltage to a transformer then what it is rated at, the only exception is if you have the full specs and  it states what class the insulation is on the winding. ( max volts it can handle )
Often the primary windings will have a different class of insulation then the secondary windings, they do that to save money as the higher the voltage the more or better quality the insulation has to be.

That solid copper wire is varnished with an insulating varnish, you can tell by the orange colour it has for starters.

Varnished copper wire is used to wind transformers and electric motors among other things, if it was not insulated that way a transformer or motor would just short out in the windings.
If it was insulated with a plastic sheath it would become too thick to wind.

It is also common practice to dip a wound transformer into more varnish to prevent them vibrating and making a 50 or 60Hz hum.

Just look at your variac and imagine the shorted out windings if there was no varnish on the wrie, usually the area on a variac where the viper brushes over for the mid output has been sanded down so the viper can touch a bare area of the windings.

I would try using the 240v as the primary tap and then measure the 24V and see what you have.
It should be safe to put 110V on the 240V primary winding, you will just get a lot less on the secondary side.

Transformers work on a ratio so drop the primary voltage by half you should get about half the secondary out.
Just don't put a higher voltage on a transformer then what it is rated at.

The other way is to use the variac to supply the transformer so you could regulate it that way
Cheers
Mick

Is that the first time you pick up the 50Hz signal ?
What about on the input jacks, both sides of the 10k resistors and also both sides of the 100nF capacitor.
Also check both sides of that 470k resistor to Ch2, I like to measure on the components leg if possible since the signal could be picked up in a component or a track.

If you can't isolate it that way then probe with a wire to 0V ( ground ) on these areas and see what happens with the signal.

Components sometimes work like a great antenna for the magnetic field around that transformer.

I would like to add that some heat transfer paste is conductive and will cause shorts if used with heatsinks and mica washers.
The conductive heat transfer paste is often used in computers for the main processor heatsink, it makes no difference in there since there are no electrical terminals close by.

Make sure you only get non conductive heatsink paste if you get some, a small tube of really good expensive paste will last forever and ever so don't get anything cheap and nasty.

That is the wrong type of jack to use in that amp.
The ring should not be connected to the chassis, it is not at 0V.
You need a jack with a plastic insulated mounting ring that does not connect the ring to the chassis.

The KiCad schematic was supposedly traced out by someone from their amp, I would trust the other 2 more as well.
If you cant see the schematics in full scale then right click on the small picture and open in a new tab or window and it should come out larger.

Try these

Love it, its going into my to do list, I might even make a tiny PCB up for it.
I might even make one in a stompbox and use it as an effect pedal.

PBTL is just 2 amplifiers working opposite to each other, one puts out a positive signal while the other one a negative and then it reverses during each sinewave.

Usually the speaker negative is connected to ground and the positive to the sinewave, the ground is say 0V and the other +-12V so you get a swing of 12v on the speaker coil.

In PBTL when the positive on the speaker gets +12V the speaker negative gets -12V, and then the reverse in each sine wave, thats a full 24V swing across the speaker coil causing more movement and power.

The load on a PBTL is worked out different then just a single amp module because both amp modules share the load between them.

Where did you get the cassette adaptor ?
Whats the brand and model of it ?
A nice picture speaks a thousand words.
I really like to try this as I have some old cassette players somewhere.

If I have to remove any components I usually cut all the legs off first and then I can remove them one by one.
Components are cheap and unless I have to remove an expensive part just for a test the legs get snipped.

Why do I always get a cold shiver when I hear or say snipped?
It must be a guy thing I guess.