If you are talking about the capacitor between the FET and the volume pot then that might be your problem.
When you use a capacitor and a resistor together you end up with a filter, in this case it is a High Pass filter that only lets frequency's above the set filter frequency through.
The 47nF cap and the 10k pot makes a high pass filter at 339Hz
The 22nF cap and the 10k pot makes a high pass filter at 724Hz
Nothing below those frequencies will be heard, you eliminate them.
If I ever made that amp I would use at least 220nF to allow bass frequencies through, but that is just me, it is set a such a silly high HP filter setting to play through a crap speaker.

Have a play with this calculator to see what I mean:
http://www.learningaboutelectronics.com/Articles/High-pass-filter-calculator.php#answer1

The efficiency of speakers are not talked about that much for guitars but when you get to bass guitars it often comes up.
A good efficient speaker and cab can double the loudness of your amp.

My first tube "anything" did not have a standby switch, but it took about 5 minutes before any noise came out of my 1957 VW Beetles all tube radio.
It freaked some people out how the radio came on all by itself, it just needed time to warm up.

I think that PCB is not well designed, you will need a good heatsink on the power transistors but the transistors are right in the middle of the PCB, right behind the transistors where you should mount the heatsink bracket are 2 small footprints for 7 watt resistors.
Have I been asleep for centuries and 7 watt resistors have now shrunk right down in size ?
I would throw that PCB in the bin, I would never try to build that amp since it will undoubtedly lead to heartache.
You can buy a 100 watt class D amp for less then the cost of the components you need to complete it, and the transformer will be expensive as well. There are so many better and cheaper options out there.

Closest I have

You could place a HP filter on it, or a preamp with tone controls.
A simple High Pass filter with a drop off slope of 6dB is just a capacitor and a resistor, I have read somewhere that those small amps have an input impedance of 50k ohms so just a capacitor on the input might be enough.

Try placing a 22nF capacitor between the guitar and the amp and see what happens, you will have to find a value you like by experimenting.
The smaller the capacitor the more bass you cut off.

Try using this simple calculator to work out different values:
http://www.learningaboutelectronics.com/Articles/High-pass-filter-calculator.php#answer1

Are you plugging the guitar straight into it? No other preamp with tone controls at all or is there a pedal inbetween.

NO
do not connect them in parallel because you will get 2 ohms and destroy the amp.
You want them connected in series/parallel to get 8 ohms.

2 - 8ohm speakers in series is 16 ohm (make up 2 of this)
2 - 16 ohm in parallel is 8 ohm

Look at example 4 on this page:
https://www.amplifiedparts.com/tech-articles/speaker-impedance-power-handling-and-wiring

I imagine it's some sort of 2 part epoxy glue like Araldite.
I would have used a couple of screws myself.
Maybe they glued it down so it got some thermal contact to use the case as a heatsink, but I would think a small 30 watt class D amp would not generate much heat anyway.

The module is only 22mm x 26mm and under $10, 20 watts at 12V @ 4 ohm and 10 watts at 12V @ 8 ohm.
That is plenty for a practice amp, 10 watts into 8 ohm most likely means you run it easily on a wall wart if not a pedal supply.
Want an amp when you go camping ?, you could run it on a small solar panel with some rechargeable batteries.

Quote from: Tassieviking on November 05, 2024, 09:42:19 AMI 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

I found a picture of this tiny amp, it is just a basic 30watt amplifier module glued inside the metal box.
It would be really easy to make one the same.

I just had another look at that power supply you have, there are 2 switches on it.

IND would mean the 2 outputs are separated from each other, not connected to each other at all.

SER would mean in series, I presume that this links the 2 outputs so you have a +,0V, - output.
SER will give you  double the voltage of one output but no more amps.

PAR would mean parallel and the two outputs will be joined up with + to + and - to -, you get double the amps compared to one channel but also only the volts of one channel

Try it with the switches set to SER and then measure the voltages of the 2 outputs, I imagine the plus on one side is equal to the minus of the other side.( they will be linked by the switches internally)

If you have two 9V batteries and you want 18V you connect the plus of on battery to the minus of the other battery, result is 18 Volts.
Where the 2 batteries meet is what we normally call ground, and then you have plus 9 volts and minus 9 volts on either end compared to the ground.
From the minus 9V to the plus 9V will be 18V

On some schematics they just say 0V, 9V, 18V instead of -9v, ground, +9V.

There are so many variations but after some time you just know what they are with a quick glance.

I find having a positive and a negative supply with a ground much easier to trace and understand  a circuit because sine waves have positive and negative sweeps, just think of that oscilloscope waveform where the middle is 0V or ground.

Here is another calculator design tool place thingy, it can be a little bit harder to use:
http://sim.okawa-denshi.jp/en/

You most likely know about the tone stack calculator:
https://www.guitarscience.net/tsc/eseries.htm#RIN=1300&R2=220k&R3=100k&RB=1M&R5=220k&RT=1M&C1=10n&C2=250p&C3=100n&C4=5n&RB_pot=LogA&RT_pot=LogA

If you like to play with tubes here is one I like to use for simple stuff:
https://www.ampbooks.com/mobile/amplifier-calculators/
https://www.vtadiy.com/loadline-calculators/loadline-calculator/

The 100pF cap with the Gain pot form a Low Pass filter.
The 15nF with the 27k resistor form  a High Pass filter at 393 Hz
The 470nF with the 120k resistor for a High Pass filter at 2.8Hz
When 2 filters overlap the fall or rise of the filters change.
( as far as I remember)

This is a good link to read up for you:
https://www.electrosmash.com/proco-rat
I like the calculators on this site because I'm lazy:
http://www.learningaboutelectronics.com/Articles/High-pass-filter-calculator.php

Is this the right schematic ?
It looks very simple to make a pedal from that.