Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Yes, the unit must be plugged into the wall outlet.
Power switch On.

Keep in mind you are measuring dc voltages.
So your meter must be set to read Volts dc.

Hi Glen,
         Yes hard to know where to start but *JazzBass* means disconnect the speaker,,  the Amp drives a load which is the speaker. Load is tek talk for the speaker.
Assuming you have a meter,, set it to the highest DC voltage.

With speaker disconnected probe the output speaker terminals/wires and it should be very close to Zero VDC less than 0.5VDC.
Now with Ground probe on chassis the first DC supply is found at the output of the Bridge rectifier. (Marked G)  The rectifier is those 4 diodes marked D102 to D105.

(J) is common or ground and often also connected to Chassis but not always.

The voltages are on the schematic,, if not sure where they are on the circuit then often the Electrolytic caps will give you a clue as to where to find those voltage nodes.
they will not read Exactly the same but as long as they read close it's all good.

Some old amp layouts are quite dangerous so be aware of the mains input wires it can KILL you. 
 I Cover any bare terminals that hold mains voltages when I have to work on that stuff.
Phil.

Be aware as there are caveats on the full power option. 
The novice might miss the limitations of using 30-0-30VDC supply rails.
You will likely blow the chip if you use a 4 Ohm speaker. (it's written in the specs)

This is a small chip with small surface area and the laws of fizzy stuff tells you it cannot dissipate the heat required to run 4 Ohms at high rail voltages.
so 25 Volt rails is a good safe option.
If you want more grunt then you need a bigger Amp that can dissipate the heat.
Phil.

I'm not sure where the +38Vdc rail is, and how do I remove the speaker load?  I'm not an electrician, but I want to learn more about these things.

Sounds like a fun project.

Although it will take a lot more than simple potentiometer changes to get useful presets.