Quote:"lower than it should be for a 100W amp. "

Can you quantify this statement?

@ Max volume, what is the AC voltage at the (speaker) when running a 100 mv test signal through the amp.

(preferably you would want to run the amp output into an 8 ohm dummy load for this test, instead of the speaker. To preserve your hearing.)

100 watts will equate to roughly 28 Vac output.

Awesome!

That amp was made when Wegner bought out the original owners. (1985?)
Very little info on the web about the amp itself let alone a schematic.

Maybe you could try calling Acoustic.

"We offer phone support and can be reached at (855) 272-1090 from Monday – Friday 8:00 a.m. – 5:00 p.m. MST."

My take on it is it is a 'rod thermistor'.

Here is the schematic for the RG50T.

It shows a fuse on the AC mains inlet.

You will have to trace out the path of where the component goes.

The "Ultra" circuit simply lifts the tone control circuit (V2b) out of the signal path.
My feeling is that the tone control is mitigating the hiss.
If that is the case then the issue (if any) is in V1a/b and or V2a.

150 mv AC (depending on the frequency) sounds like hum to me.

Or the output is oscillating.
Again, depends on what the frequency is.

From the pic I can see a few solder joints that appear 'iffy'.

Not too sure what soldering iron or skill set the OP has.

Can you upload a pic?

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.

"So you will have to muddle through and possibly trace out and draw sections that are in question."

Ok. The lamp is lit.
That means you have voltage on the secondary of the power transformer.
Your next step is to verify the dc supplies.
there are 5 of them.
I would start at the +38Vdc rail.

A word of caution.
Being that this is a direct coupled power amplifier, I would advise removing the speaker load until you get things sorted out.

Sounds like a fun project.

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

Can you help out a bit and define 'nothing'?

No lamp light, no sound?

It would be a nice gesture to include the schematic when asking for assistance.
That said, the schematic itself is only a road map.
What is typically not provided is the board layout.

So you will have to muddle through and possibly trace out and draw sections that are in question.
The first place to start is the power supply.
Are all supplies up & running?

One question.
WHY?